ANTHROPOLOGICAL RECORDS 17:3 EROSION MORPHOLOGY AND OCCUPATION HISTORY IN WESTERN MEXICO BY S. F. COOK UNIVERSITY OF CALIFORNIA PRESS BERKELEY AND LOS ANGELES 1963 EROSION MORPHOLOGY AND OCCUPATION HISTORY IN WESTERN MEXICO BY S. F. COOK ANTHROPOLOGICAL RECORDS Vol. I7, No. 3 UNIVERSITY OF CALIFORNIA PUBLICATIONS ANTHROPOLOGICAL RECORDS Editors (Berkeley): J. H. Rowe, J. D. Clark, D. H. Hymes Volume 17, No. 3. pp. 281-334, 16 figures in text Submitted by editors April 24, 1962 Issued March 15, 1963 Price, $1.50 University of California Press Berkeley and Los Angeles California Cambridge University Press London, England Manufactured in the United States of America CONTENTS Page I. Introduction . . . . . . . . . . . . . . . . a281 II. Erosion Morphology and its Interpretation in Central and Western Mexico . . . . . . . . 283 Factors in erosion patterns . . . . ... . . . . . . . . . . . . . . . . . . . . 283 The process of erosion and recovery . . . . . . . . . . . . . . . . . . . . . . . 283 Documentary references to land conditions, and the presence of artifacts. . . . . . . 286 Secondary and subsequent erosion cycles . .. . . . . . . . . . . . . . . . .. . 286 III. Profile Analysis as an Index to Occupation: General Theory and Methods . . . . . . . . 289 Field criteria for profile analysis . . . . . . . . . . . . . . . . . . . . . . . . 289 Laboratory criteria for profile analysis . . . . . . . . . . .. . . .. . . . . . 290 IV. Topsoil Depth and Occupation Intensity: Central Sinaloa . . . . . . . . . . . . . . . . 293 Nonhuman factors . . ....... ........... ................. . . . ....... 293 Human factors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 294 Observations of topsoil depths . . . . ...... . .... ..... .. .. . 295 V. Profile Analysis of Habitation Sites on the Coast of Sinaloa and Nayarit . . . . . . . . . 299 Series 1 .299 Series 2 .299 Sum m ary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 302 VI. Profile Analysis in Southern Hidalgo . . . . . . . . .3. . . . . . . . . . . . . . . . 303 Profile I . . ...... .......... .... ..... . . . . . . . . 303 Profile II . . ........... .... ..... .... ..... . .. . . 303 VII. Profile Analysis and Erosion Patterns on the Tepic Plateau . . . . . . . . . . . . . . 305 Results of profile analysis .305 Results of field observations . . . . . . . . . . . . . . . . . . . . . . . . . . 311 Concluding discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 316 VIII. Profile Analysis and Erosion Morphology in the Vicinity of Guadalajara .317 The Tonala region . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 317 The area west and southwest of Guadalajara . . . . . . . . . . . . . . . . . . . . 321 IX. Discussion and Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 330 Arroyo patterns . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 330 Visible topsoils . .. .... .. .... .. .. .. .. .. . . .. .... . 331 Weathering rates .332 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 334 [iii] EROSION MORPHOLOGY AND OCCUPATION HISTORY IN WESTERN MEXICO BY S. F. COOK I. INTRODUCIION Some years ago I discussed erosion patterns in surface series or a profile in depth, brings to light tral Mexico as indicators of the past history of anomalies of distribution, which in turn may furnish ulations inhabiting an area and the intensity with clues to past events and conditions. The results of ch they used the land (1949 a, b). This purely both field and laboratory tests may be evaluated in the litative field approach proved suitable for the in- light of facts derived from formal archaeological ct evaluation of prehistoric and protohistoric study, if there are any such facts, or in their absence ogy and demography. Thus, in a study of the may serve as guideposts for more intensive archae- otlalpan region of southern Hidalgo, it was possible ological investigation. establish a three-cycle erosion pattern with super- Field descriptions and sample analyses are devel- ed soil profiles. This pattern in turn could be oped and carried out by conventional techniques ociated with three principal occupations, or borrowed from soil science and agriculture and may be ures: that centering around Tula several centuries reduced to a few very simple procedures available to ore the conquest, the Otomi-Aztec dominion of the any biological laboratory. Since the purpose is pri- enth and early sixteenth centuries, and the marily comparative and descriptive there need be no nish-Mexican occupation extending to the present. attempt to study mechanisms of soil formation and as also possible in other areas to segregate clearly weathering, or to consider the structure and chemistry land alteration caused by the introduction of of the soil as a medium for the growth of plants or the opean cattle and agriculture from the soil destruc- support of animals. In other words the examination of and partial recovery, associated with the huge soil features is a means to an end, not an end in itself. lations and advanced cultures which preceded the In a certain locality some particular field or labo- nish conque st. ratory method may be deemed appropriate, at other tnow appears that other modes of attack upon the places a combination may be possible. Each area of e basic problem are feasible, modes of attack historical or archaeological interest thus constitutes erging widely from the more conventional archae- an individual problem, although certain basic prin- ical approach which rests essentially upon ciples of soil formation and destruction apply to all of orous analysis of mortuary and cultural remains, them. This monograph, therefore, consists of a series well as upon an intricate stratigraphy in the field. of more or less distinct studies, or essays, each methods here explored embrace a combination of dealing with a different subject, and each approached ervation of soils in the field with physical and from a somewhat different point of view. mical analysis of adequate sample series in the Subsequent to the introduction, the first section ratory. recapitulates briefly the general theory of land use and Field observation will include the inspection of the soil erosion dynamics as it applies to the study of past th and appearance of the topsoil as it is exposed by cultures and populations at large, but with particular am cutting, excavating, and road construction. reference to conditions as they appear to have existed depth of the topsoil, set against the background of in central and western Mexico. Nothing new is brought profiles which have developed normally in the same out here but the guide lines are established for ap- ion, is an indicator of the presence or absence of plication to specific situations. The next section and to a great extent of the intensity of his activi- states briefly some of the field and analytical methods , Analysis of the soil constituents, either in a which have been found suitable. The section numbered IV then explores the problem of whether it is possible to employ topsoil depths along a linear transect (or any other spatial pattern) as a means of distinguishing those local areas which may The author wishes to acknowledge with thanks the have been subjected to more than average intensity of cial assistance provided for field work by grants occupation. If such localities can be thus detected in the Associates in Tropical. Biogeography, the first instance, then other historical and archae- ersity of California, Berkeley, and for laboratory ological methods can be brought to bear in order to ysis by Grant No. G-161 12 from the National confirm and extend the initial findings. nce Foundation. He is also deeply grateful to Section V tests the hypothesis that laboratory fessor Clement Meighan, Department of Anthro- analysis of adequate profile sample series taken from igy, University of California at Los Angeles for the a well-recognized habitation site can provide informa- ies of column samples taken during his excavations tion consistent with that already obtained or easily napa, Nayarit. obtainable by conventional excavation and descriptive [2811 282 ANTHROPOLOGICAL RECORDS analysis of cultural remains. If this hypothesis is ponents. The two groups of data were then considered valid then it might become possible to utilize soil data in their mutual relationship. Over most of these at known or suspected sites where the application of regions there is little if any valid archaeological other methods was not desirable or feasible. In the evidence available, or at least known to me, although present instance most of the samples were taken by there is a good deal of historical information extending Dr. Clement Meighan in the course of an extensive back to the Spanish conquest. Where such evidence is inve stigation. extant it has been used to supplement the soil data, as Sections VI, VII, and VIII select three areas without at Ixtetepe near Guadalajara, and of course the pre- - reference to local,detailed archaeology. The first, sence of potsherds or other artifacts is always a quite restricted, is south of Actopan in southern valuable guide to former human occupation. Neverthe Hidalgo, the second and third, both of considerable less the primary aim of the survey of large areas has territorial extent, center respectively around the been to test the value of soil data, as such, in detect-, cities of Tepic, in Nayarit, and Guadalajara, in ing and assessing the extent of human occupation. Jalisco. The purpose was to examine the terrain This work is, as it always will be, unfinished. intensively with respect to the erosion morphology, Certain omissions will occur to any archaeologist, including not only topsoil depths, but also gullying, such as chemical examination of soil for phosphate ar sheet erosion, alluviation, buried profiles, and visible the trace elements. With regard to these, work is evidence of recovery from previous denudation. In still going on and the omission is deliberate. The sQ spots which appeared to have critical value, both scientists will want a far more sophisticated treatme surface and profile samples were taken, brought back of the soils in the laboratory. This is needed but is'. to Berkeley, and analyzed for a few significant com- for the future. II. EROSION MORPHOLOGY AND ITS INTERPRETATION IN CENTRAL AND WESTERN MEXICO The Central Mexican plateau and adjacent escarp- occurrence, but is seen well developed west of Morelia, ts constitute a unique area for the study of changes Michoacan, near Yanhuitlan, Oaxaca, and in central d surface induced by human occupation. Nowhere Nayarit. in North America, and rarely in other parts of the When these two soil types are exposed to the action Id, do such a variety of conditions exist within such of running water the former tends to exhibit sheet ero- all territorial compass. The substratum includes sion, or slope wash, whereas the latter produces highly y types of volcanic and sedimentary rock, from re- dissected gullying. From the standpoint of erosion lava flows to ancient granites. The altitude ranges morphology, therefore, the constitution of the parent sea level to well over 15, 000 feet. The climate material here is of less significance than the physical s all gradations from humid tropical coast to prac- consistency of the soil developed, that is whether there ly rainless desert. The soils embrace nearly is a hardpan produced or a deeply weathered, soft, y type known to pedology. Human occupation on an clayey, red earth. ced cultural level has existed at some points for Secondary only to parent material is climate. The three millenia, at other points only for approxi- mean annual temperature is an index to both the rate of ly four centuries, and at other points scarcely at weathering and the density of vegetation. The rate of It is not surprising that the interaction between weathering is important particularly in the reconstruc- and his environment has here produced a series of tion of devastated soils, and recovery may be many responses and allied phenomena which are bewild- times more rapid in the hot country, such as the pied- g in their complexity. mont of the escarpment, or the coastal plains, than in the tierra fria of the high plateau itself. The density of vegetation, to be sure, is determined by moisture as FACTORS IN EROSION PATTERNS well as temperature, but given comparable rainfall, the hotter countries tend to show a more rapidly growing he pattern imposed upon a land surface by erosion vegetation, with plants closer together, than do the termined initially by the nature of the underlying colder areas. The effect of greater density is twofold: or, in terms of soil formation, the parent mate- recovery of denuded soil, with humus formation,is much By this is meant the raw stuff, which through the accelerated, and the visible surface destruction caused ence of external factors becomes converted by by previous erosion is rapidly blanketed and rendered ering into soil. The great diversity of these mate- difficult of observation. in Mexico precludes any attempt to describe soil Moisture exerts much the same influence as temper- in detail but there are two general categories ature. The higher the rainfall the more rapid is weath- h are outstandingly conspicuous, extensive, and ering and the denser is the plant cover. Consequently esentative. all soil processes are accentuated and the traces of e first of these is a soil rich in organic matter former land destruction are quickly concealed. It is at maturity forms a deep black surface horizon. probably for these reasons that the marks of ancient o several feet under the surface there is commonly erosion are so clear in the highlands and so nearly ated a heavy hardpan, which, when denuded of the absent on the coasts. ying material and dried out, assumes the consist- of soft rock and is called "caliche" or "tepetate." a hardpan may be formed over a limestone in a THE PROCESS OF EROSION AND RECOVERY ina type soil, or may appear under a prairie soil, ert soil, or a chernozem. Caliches are found pre- In turning to the erosion process itself, it should be antly in regions characterized by moderate or low reiterated at the outset that we are not dealing with 11. The second type consists of a pink, light red, geological erosion, which continues under all conditions ck-red profile of great thickness, with little or no at a relatively slow rate, but with human erosion, which an development, the properties of which resemble appears only at the site of human intervention with of the classical terra rossa, or latosol. At matu- soils and which may become extremely rapid. It is ere is formation of recognizable dark and light true that in some parts of the world one must be on the "A" and "B" horizons over a bright red "C" lookout for short-term effects of nonhuman origin. n which may extend many feet and grade into par- Thus sudden and drastic climatic changes, especially weathered and then into unweathered bedrock. in marginal areas, have been known to induce changes tter is frequently basalt of volcanic origin but in the rapidity of degradation and aggradation which e sedimentary rock of various sorts. Since we simulate the influence of human culture. However, in t dealing necessarily with terra rossa or latosols Mexico and the Pacific Coast generally, it is doubtful hnically defined, the noncommittal expression whether there have occurred within the past millenium arth" may be used to indicate this class of soil. any climatic shifts of sufficient magnitude to interfere rst category mentioned is encountered widely, for with the evaluation of soil conditions produced by man. le, in the Teotlalpan of Hidalgo, the Valley of The first step must always be the removal of the o , central Puebla, the Mixteca Alta in Oaxaca, natural plant cover and the exposure of the naked soil r oughout Queretaro, Guanajuato, Zacatecas, and to the action of water falling as rain. Such exposure iS Potosi. The second is more restricted in usually occurs in conjunction with one or more of four [283] 284 ANTHROPOLOGICAL RECORDS well-recognized cultural activities: (1) clearing for If it is untouched territory and has suffered no damage' habitation and dwelling construction; (2) clearing, or a section through the new arroyo will show the normal burning, and ploughing for the planting of crops; (3) profile, with the A, B, and C horizons in position. If cutting or burning of forests; (4) ranging of livestock. the area has been subjected to serious wear prior to When such an event takes place in virgin territory we cutting of the gully, as frequently happens toward the have the inception of a primary erosion cycle, active heads of old or mature arroyos, then the A or An uncomplicated primary cycle is characterized by or both horizons may be lacking. In extreme instan four aspects or stages. The first is removal of natural the arroyo may reach and even cut deeply into the bd cover, as pointed out above. The second is degradation, rock, depending upon the consistency and organizati the third aggradation, the fourth recovery. The second of the latter (see figs. 1-3). and third are simultaneous in time, preceded by the first and followed by the fourth. It is frequently, but by no means always, possible A to identify the process whereby ancient erosion was B initiated. A good deal depends upon the length of time which has subsequently elapsed. In Central Mexico G certain areas may demonstrate erosion which on his- C torical, documentary, or other grounds surely antedates the Spanish conquest-thus precluding livestock as an bed rock agent. The general nature of the vegetation and the terrain, plus archaeological evidence, then, will fur- nish clues to the originating process. In the valleys, grass or brush is likely to have been cleared for the building of villages and the planting of crops. In the high mountains, forests may have been destroyed for timber, charcoal, or other products, with no subse- C C quent attempt to cultivate the land. Still other areas have been subjected to damage only within recent cen- turies, and here the active agent has frequently been livestock, although agriculture (corn, bananas, sugar bed rock cane, citrus fruit), lumbering, burning, and urban construction have been universal contemporary causes of erosion. Figure 2. Degradation takes two forms. In the first, running water cuts a channel through the soft upper layers of the soil and forms a valley, or trough. Such a channel is known variously to students in different fields of G knowledge, and in different parts of the world. In the United States it is customarily referred to as a gully.bed rock In Latin America it is known widely as an arroyo. Either term will be used here, and restricted technical definitions will be ignored. The form of the cross section of a gully depends upon the physical structure of the terrain, the force and vol- Figure 3. ume of the water, and the age of the channel. In soft ground, water cuts rapidly down and more slowly later- ally, giving the cross section a V-shape, with sloping Since the gradient tends to steepen toward the up sides and a very narrow floor, or bottom. Underneath end of a gully we find that it normally grows by ext a hardpan, or other stratum of tough consistency, the ing its length backward, or headward. Thus the old sides tend to be perpendicular and the bottom wide and portion is toward the foot, the youngest toward the flat. Thus the cross section is almost rectangular. A Occasionally cutting may have ceased entirely at the similar rectangular form is produced in desert regions foot while the head is still actively extending. A ve when powerful currents following very heavy rain cut fine example of this phenomenon may be found on th their way with great force through silts, sands, or open, grassy plains in the northern part of the Tol gravels which are easily washed away. Kirk Bryan Valley. (1925) would limit the term arroyo to this type of wash, Arroyos are disposed to branch. Lateral stream a type frequently seen in northern Mexico and the south- cut channels into the main gully. The active heads western United States. When a gully becomes mature and split. Eventually a highly ramified system res and is no longer being actively incised, the floor may and when the original, or parent arroyos are numer fill with debris and the banks may tend to slump or to and close together the final result may be an extrao be washed downward. There results a U-shaped cross narily dissected badland, such as dismays the trav section, or often a condition in which the tops of the U at Yanhuitlan or Tzintzuntzan. are bent outward so as to produce a reversed curve like It has been suggested that arroyo formation is m the cross section of bowl. Such a figure is almost highly developed on soft soils where local unevennel invariably evidence of senescence and recovery in a conduces to effective concentration of water volume gully. rapid deepening of its channels. In central and wes The soil profile exposed to view by the cutting of an Mexico this process is best observed in the red ear arroyo depends in detail upon the condition of the area. areas. Where the soil is underlain either by horize COOK: EROSION MORPHOLOGY AND OCCUPATION HISTORY 285 ata of solid rock such as limestone, sandstone, or the long axis of which runs parallel to the contours of e, or more often by a thick hardpan or caliche, the slope above. With a simple hillside merging into a ying is certainly present. It occurs locally on a plain a section across the slope would be as represented 11 scale while the soft, mature, virgin soil is being in figure 4. When two slopes meet to form a valley the troyed, and generally on a large scale when pro- latter receives soil from both and may become filled ed exposure to erosion has permitted channels to be so as to present a level surface (figure 5). avated in weak spots in the native rock or the caliche. Following exposure and active erosion, processes re deep barrancas are found which may be of great of recovery become established which characterize the but which rarely present a high degree of ramifica- fourth aspect of the cycle and which tend ultimately to or digitation. restore the land to its original state. As a matter of 'here the subsoil structure is resistant the second fact we are dealing with relative rates. During phases of erosion is the more conspicuous and destructive. two and three destruction is outrunning recovery, al- s is sheet erosion, or slope wash, as it may be more though the tendency to recover is always present and liarly termed. The water, instead of being confined may even as sert itself locally while destruction is continu- eparate channels, moves in a layer, or sheet down ing over the area at large. If, however, the initiating hill. The surface remains relatively smooth, but factors-habitation, ploughing, forest removal, or soil is carried away in successive increments. The grazing-are at any time suspended, then recovery ct is first the removal of the A horizon, followed by proceeds more rapidly than further denudation and if B and the C horizons, and if the process continues permittedto do so continues to completion. enough all the loose material is displaced, and the Recovery assumes many forms, some of which are d, underlying structure is laid bare. Spectacular susceptible to direct observation in the field. First and ples of ruin by sheet erosion are the dazzling white most important is weathering. Weathering occurs on etches of tepetate to be seen from the air over north- slopes which have been stripped of topsoil and there Oaxaca and southern Puebla. subsoil, or even bedrock, are exposed to the air. It here are many modifications of sheet erosion. If also proceeds on alluvial deposits of all kinds, for the effect is relatively mild only the topsoil may be re- latter consist of detritus in which there is predomi- ed, leaving perhaps a truncated B-or C horizon nantly random chemical and physical organization. In ch is still capable of supporting agriculture. If the cool, dry climates weathering is a very slow process ent material consists of hard rock the finer particles indeed; in the hot and humid tropics it is accomplished be washed out leaving behind the larger, unweath- with great rapidity. The zone of alteration is at first rock fragments to form a layer, often many inches extremely thin, then thicker, and finally becomes p on the surface. Such a layer may be called a differentiated into true soil horizons. In the meantime ck mantle" and is found widely along the west coast translocation and accumulation of solutes may result Colima to Sonora. in the new formation of hardpans. The presence of any he clay, sand, and gravel which is carried from its degree of weathering on denuded or alluvial surfaces tion by moving water during degradation of the hill may therefore always be taken to indicate a substantial es must eventually come to rest in the lowlands, the period of quiescence, or absence of the factors which s, and valleys. The latter thus undergo aggrada- conduce to erosion. The morphology of aggradation depends upon the A second manifestation of recovery is the filling in, e and distribution of the water in which the mate- or healing, of arroyos formed during the degradation is suspended. In a small arroyo the soil brought phase. It has been pointed out that a young, active from the head may be deposited at the foot. A gully may be V-shaped, or sharply rectangular in cross k or a river spreads its load over its bed or its section, whereas in maturity or senescence the section plain. With light precipitation and moderate assumes a curved, or shallow bowl shape, owing to the am volume, watercourses all aggrade the area washing in of the banks and filling of the floor. The e to their normal bed. With heavy rainfall, how- latter condition consequently may be regarded as prima these streams may flood and deposit relatively facie evidence that the constructive processes are sur- e sheets of soil far into the surrounding country. passing the destructive. ed in the arid regions of central and northern A third factor, which is associated with recovery, ico it is the heavy floods which cause the important although not necessarily a part of it, is the recrudes- ges, the great alluvial deposits which now cover so cence of vegetation. A new cover of grass, or shrubs, y of the river basins and former lake beds. or perhaps forest on wasted land facilitates repair in special case is presented by sheet erosion. The two ways: it binds the loose earth and prevents further laced soil is not channelized and carried long dis- es,but is moved only to the foot of the slope from it originated. Here it accumulates as a deposit, | > =~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~1 --denuded slope *_ accmultin original surTace - ~- l 3 ~Figure 4. Figure 5. 286 ANTHROPOLOGICAL RECORDS loss from water flow, and it provides a source of or- occurrence and location of human residues are of the ganic matter, a necessity in the formation of new soil. greatest significance. Such residues almost invaria The time element in the erosion cycle is of great take the form of relatively imperishable detritus or importance, but unfortunately is extremely difficult to artifacts, of which in central Mexico the most fre- evaluate quantitatively. The initial impetus is likely to quent are obsidian chips and potsherds. Occasionally be sudden, the clearing and exposure of the land. fragments of animal bone or of charcoal may also a) Nevertheless, if the causal agent is livestock, exposure pear. However these substances must be unequivoca may be relatively slow and uneven, taking decades in- attributed to the activity of man in order to be of us deed to become of significance. Degradation and We must also except from consideration most ceremo aggradation may proceed Alowly, with steady wearing nial structures and burials, since these rarely bear away of cultivated lands and increasing size and com- significant relation to the phenomena of erosion. W plexity of arroyos. Simultaneously the soil which has we are really preoccupied with, consequently, is the* been washed out will accumulate elsewhere layer by rubbish, the day-to-day accumulation of debris which layer. On the other hand, areas are well known, es- is the inevitable sequel to human living. This debris pecially in arid climates, where catastrophic storms cast at random over the scene of habitation and there and floods have induced more destruction in a few hours becomes part of the soil itself. than was caused by normal weather conditions through- The refuse first accumulates on the surface. M out many years. There are no fixed rules governing of itdmay remain in situ indefinitely, as the surroun rates of erosion and no empirical formulas. An assess- ing finer soil is washed away. Hence the presence of ment must always be made in the individual instance, appreciable quantities of such material on an eroded based upon all known contributing facts. surface is immediate proof of occupation and destruc Recovery is normally much slower than destruction; tion of the area by man. The cultural affiliations of centuries or perhaps millenia are consumed in the the artifacts are direct clues to the period of active complete reconstitution of mature soil profiles. De- occupation, for deposits of this type may remain for pending upon mean rainfall and temperature, weather- very long time without removal. ing of raw rock or mixed alluvium may proceed at rates Artifacts on an eroded surface also may be an in varying from imperceptible to a few inches per century. cation of population density. In a general way, the In central Mexico, at least, the existence of a new, higher the density of potsherds or obsidian, the gre substantial, weathered zone is evidence of the lapse of the number of people who contributed the refuse. T considerable time. Differentiation into horizons, or sites of religious centers, villages, or even citiesa the presence of incipient hardpans, requires a minimum thus detectable. Furthermore it is an empirical po of several centuries. late of real validity that where heavy artifact accum Vegetation is often helpful as an indicator. Its entire lations occur one is likely to find at least the scars absence in an area where prolific growth is possible ultrasevere erosion. The converse is frequently b denotes little or no recovery. Rather it demonstrates by no means always true. Extreme sheet erosion or that active erosion is still in progress. A fair-to- highly dissected arroyo system, is always to be re- dense cover on washed slopes, on young alluvial de- garded as a possible site of ancient human activity, posits, in healing arroyos, and the like is evidence of they do not necessarily yield high densities of artifa a time interval sufficient to establish that cover. The and the intensity of habitation cannot be expressed minimum interval naturally depends for its value upon numerically as a function of refuse distribution. On the type of vegetation. A growth of thin grass requires the other hand, the presence of any artifacts at all only a few years, a thick sod may mean from several speaks some degree of occupation. to many decades. In temperate regions a young-to- Many pieces of refuse, once thrown upon the sur- mature forest needs from fifty to two hundred years face of the ground, do not, as assumed above, rem for its establishment. The rain forest of the lower in place while the dirt and gravel is washed away plateau escarpments, however, renews itself within around them. Numerous fragments are carried off the memory of a single man. A case of special interest the flow of water, like any other small pieces of ro is that in which a large tree of great age is found grow- and are dropped in the stream beds and valleys. Th ing in the bed of a stream. Since the bed had to be in may then be exposed and observed when for any rea existence prior to the sprouting of the seed, the stream the alluvium is opened up. The discovery of such must have been in its present location at least as many artifacts in water-borne deposits is of primary val years as the tree is old. The soils through which the in dating human habitation, for two conclusions are stream has cut must, therefore, be still older. immediately warranted: (1) Man must have occupi the area from which the new soil was derived. (2) occupation must have predated the degradation phas DOCUMENTARY REFERENCES TO LAND CONDITIONS, which ultimately resulted in the stream deposit co AND THE PRESENCE OF ARTIFACTS ing the artifacts. If, now, the alluvium can be sho to have a certain age, a minimum antiquity is estab As a further point with regard to time it may be lished for the culture which produced the artifacts. mentioned that occasionally direct references to land conditions are to be found in documentary sources. Where such occur they are of the utmost value, pro- SECONDARY AND SUBSEQUENT EROSION CYCL vided the source is reliable. Not only is the history of a local area established, but also such an area, once The preceding discussion has concerned itself wi known in detail, can be cross-checked with other areas the characteristics of a primary erosion cycle. Co and knowledge of the latter thus indirectly obtained. plications are introduced when secondary and subsz For the purpose of associating land changes with the quent cycles have been present. presence and disappearance of extinct populations the The duration of a primary cycle is indeterminate COOK: EROSION MORPHOLOGY AND OCCUPATION HISTORY 287 ressed in years. It is contingent only upon the. the primary cut. If so there is no means of segregating stence and completion of the four phases mentioned the two cycles at the particular point observed. the outset. Even this statement is subject to mod- Recovery from degradation may occur in the sec- cation'because the final aspect, recovery, very ondary cycle. If so the secondary cut will be filled and rely brings about full restoration of the original in turn will become U-shaped or bowl-shaped in cross ditions. Almost always a partial restoration is section. Vegetation which grew on the primary fill will hieved, which may or may not closely approximate be dislocated or washed out where the secondary cut original state. Therefore we are in practice impinges upon it. Otherwise it will remain in place ced to regard the primary cycle as fulfilled if the and during recovery will spread to the secondary fill. covery is reasonably advanced and is clearly recog- 2. Secondary erosion cycles very often result in ble through morphological criteria as recovery. carving new stream channels or arroyos across flood The admission of this qualification is mandatory, plains and alluvial deposits so as to expose fresh pro- cause, with the special case of sheet erosion, de- files. A characteristic feature of such exposures is the adation on open slopes may cease for long periods appearance of a compound, or "reversed" profile, in hout the appearance of any noticeable change in the which the original soil is seen to be overlain by mixed rain. If, thereafter, another period of degradation material from the primary cycle. The buried soil may gins, the effect will be directly additive. We can still show its normal horizons, or it may be truncated sider the second period of active wear only as a (see figures 7 and 8). Moreover, if weathering follow- ayed continuation of the first, with no clear segre- ing the primary cycle was of long duration before the on of cycles. Consequently no satisfactory onset of the secondary cycle, incipient stratification uence of events can be established in time, and may be observed in the upper layers. Thus figure 7 m the denuded area itself no deductions can be may be modified to give figure 9. de. 3. The phenomena just outlined represent degrada- Where recovery takes the form not of a mere sus- tion in the secondary cycle acting upon aggradation sion of all soil activity, but of a positive process, structures of the primary cycle. However, the sec- matter how slight, the situation is different, for ondary cycle may also aggrade upon material laid down we have criteria for observation and measurement. in the primary cycle. It is true, of course, that the noted previously recovery is manifested most resulting structure will later have to be exposed to tinctly in (1) the filling or healing of arroyos, and view either by artificial cuts, such as road excavations, the weathering of water-borne deposits either or by stream action during a tertiary cycle. Assuming ally in stream fills or generally on outwash plains. such exposure, three superposed aggregates will be hird manifestation is frequently observed, the detectable: the original soil, the deposit of the primary thering of denuded surfaces, usually on slopes. A cycle, and the deposit of the secondary cycle. Each of requisite here, however, is that the weathered these may show layering due to weathering and ad- izon not be removed by sheet erosion during the vanced profile formation, in differing degree, the erval between two cycles. Let us suppose that, the initiating cause of the ary cycle having been liquidated and recovery having n accomplished, a new attack is made upon the land primary cycle deposit its human occupants. Active degradation, associated aggradation, will follow. In their mechanics the A o cesses of this secondary cycle will resemble those Boriginal the primary cycle. But the substratum upon which soil y act will be different, that is, not a virgin soil but C already altered by the primary cycle. Certain racteristic features are developed, some of which rit brief description. Figure 7. 1. A primary arroyo, upon cessation of erosion, s to fill, as described previously, and assumes road U-shaped, or shallow bowl-shaped cross tion. If subsequently a new cycle begins, the cutting primary cycle deposit umes, but now the incision is made into the new fill c original er than the native soil. A section resembles the ram in figure 6. The extent of cutting varies. Usu- bed rsoi y the two cycles can be distinguished, as in figure but occasionally the action of the water is strong ugh to wipe out the fill entirely and begin enlarging Figure 8. weathered zone primary cycle deposit V f i ,F unchunged t ~~~~~~~~~~original soil Fiuron6ry cut.uncgu Figure 6. Figure 9.~ 288 ANTHROPOLOGICAL RECORDS variants being indefinitely great in number and not from all periods prior to its formation. The distribu. requiring detailed description here. tion of artifacts according to age, however, will tend The occurrence of artifacts follows the same pattern to show those of a certain period predominating in the in secondary and tertiary cycles as in the primary cycle. corresponding stratum. Actually, thorough statistic Any human residues left on the surface of the ground analysis of potsherd occurrence has never been made prior to the formation of a fill or stream deposit and in such a case. Hence only the general principle can capable of being washed in the proper direction, may enunciated. occur in the deposit. Furthermore potsherds and Tertiary cycles have been found in central Mexico. obsidian may be washed out of an old deposit and incor- Possibly quaternary cycles exist, but if so their inter' porated in a new one. Hence it follows that each incre- pretation would be very difficult and would be of little ment in a series of deposits may contain relict material significance to archaeologists and prehistorians. III. PROFILE ANALYSIS AS AN INDEX TO OCCUPATION: GENERAL THEORY AND METHODS Every parent material which goes to make up a soil, instance just what is to be regarded as "soil, " in ether igneous or sedimentary rock, water-borne or order that valid comparisons may be made. md-blown deposit, if allowed to be acted upon by the Since the distinctions between "A, " "B, " and "C" encies of the physical and chemical environment, horizons are likely to be highly technical, and since dergoes the process of weathering. Ultimately we are dealing actually with a single parameter, total ere arises a distinctive distribution of weathering depth, it is useful to employ the empirical category of oducts to form the horizons characteristic of a topsoil. By this term we mean those upper horizons of ature soil. Such a distribution is usually referred a profile which can be distinguished by visible criteria as a profile. Of these there exists a vast array of from the underlying unaltered, or imperceptibly cognized and described types, differing one from altered subsoil, or parent material. The criteria have other according to the nature of the parent material to be texture, or consistency, and color. For each d its response to the varying factors of the environ- area under study a decision has to be made just where ent. Under human occupation these profiles are the line is to be drawn. The method becomes some- tered, particularly as the upper horizons are de- what arbitrary but if systematically applied yields royed or modified by habitation, agriculture, and valid data. estock. Both the total quantity of material and the Since the method requires the direct observation of ttern of distribution of component substances may numerous profile exposures it is conveniently employed dergo wide changes. It is reasonable to hypothesize, by means of a linear transect. Such transects at the erefore, that if these changes are recognizable and present time are best selected along predetermined onsistent, an association might be established strips of recently constructed highways in uneven or tween soil composition and human -occupancy which hilly country where frequent road cuts, borrow pits, ould permit conclusions concerning the past history and casual excavations are to be found. The pertinent the land and the people on it. information is then recorded in as quantitative a There are numerous criteria which may be em- manner as possible for subsequent graphic or oyed, both physical and chemical. They fall rather numerical treatment. Finally the human occupation, turally into two groups, those based upon visual past and present, is associated with the observations, servation in the field and those requiring the anal- and the degree of correspondence evaluated. sis of samples in the laboratory. The field methods In order to detect a correlation between topsoil ill be considered first. depth and the activity of man a reasonably large number of observations must be made. Local varia- tion in the physical and biotic conditions, together with the difficulty in achieving a high precision of FIELD CRITERIA FOR PROFILE ANALYSIS measurement, necessitates considerable tolerance in estimate. Thus within a horizontal distance of a few In the preceding section attention was paid to the feet the topsoil depth may vary from none at all to vere slope denudation and gully formation, as well several inches or to a foot or more. Consequently a i to the extensive stream deposition so characteristic large array of measurements or estimates is required. erosion in the old inhabited regions of the Mexican The limit will usually be imposed by the frequency of teau. Such conspicuous results of land use are, available exposures. Many other factors must be ever, not always present, despite the fact that controlled. Some of these are discussed in section IV ttlements have been in existence for many years and below, where an illustrative study from central Sinaloa e land has suffered perceptibly. In their absence, is presented, and to which reference may be made for ere are other phenomena which might serve as further detail. iteria and which deserve to be explored. One of Apart from the total quantity, or depth of the top- ese is the comparative depth of the soil itself. soil, it is often feasible, in the field, to evaluate its When a mature soil is subjected to disturbance by differentiation into layers, or horizons. In a mature n, one effect widely observed is the removal of the soil, distinctions can usually be perceived by eye; if pper layers of the soil. The magnitude of the effect not, and if the profile is important, samples can be highly variable, ranging from barely perceptible taken for laboratory analysis. As surface erosion arification to the complete denudation which often progresses horizons are successively obliterated until ccompanies extreme sheet erosion. Normally the only the subsoil remains. Such complete truncation of srt result is the loss of the so-called "A" horizon, the normal profile is usually discernible. .llowed by a progressive lowering of the surface If recovery replaces destruction, weathering first vel. The lower limit may be a matter of judgment produces disintegration of the coarser particles with d definition. Thus the zone of removal may reach formation of finer material and very likely some clay. yond the conventional "A" and "B" horizons into the Simultaneously in all but the most arid climates plants "horizon. This may consist of a stratum of more seed in, grow, die and decompose to produce humus. r less weathered and altered bed rock, or it may be The organic matter thus formed accumulates and omposed of the unaltered rock or other parent becomes visibly manifest in a darkening or even aterial itself. Since conditions vary widely from blackening of the surface layer. Further development egion to region it is necessary to define in each exhibits a deepening of the dark horizon, and finally in [289] 290 ANTHROPOLOGICAL RECORDS most soils a segregation of zones according to color of comment that little attempt has been made as yet to or consistency, which actually are the outward reflec- explore the use of these outstandingly important tion of ionic and molecular reactions and translocation. materials, either individually or as a group, for If this chain of events is completed a new, mature soil detecting soil alterations referable to human inter- has been produced. However, in areas settled by man, vention. degradation may not come to a complete halt with the The principles underlying the formation and fate of end of the intense erosion cycle, but may continue at clay and organic matter are well understood by a reduced rate and thus conflict with or counteract the pedologists, and are exhaustively set forth in many forces of recovery so as to produce some kind of texts and monographs on soil technology. Clay is intermediate condition. Thus there may be established formed at the weathering surface of any rock or soil. an equilibrium state wherein weathering and accumu- As the soil ages the clay tends to move downward lation of organic matter may proceed at a reduced rate. until it accumulates in a lower, often the "B" horizon. Within the weathering zone differentiation between "A" The depth of this zone of accumulation may be very and "B" horizons may or may not take place. great. Organic matter will likewise take its inception This all means in practice that a sure diagnosis of at the surface, through the growth, death and decom- soil history can be made only under restricted circum- position of vegetation. Hence the upper levels of any stances, and even then preferably when confirmed by soil, save those most arid and those most sterile, other evidence. In any area selected for study by this contain organic matter. This material, consisting of method, a deep, mature, well-differentiated profile plant residues, largely cellulose, is reworked by should first be sought in order to secure a basis of microorganisms almost as fast as it is formed, with comparison with eroded profiles of the same type. the consequent appearance of humus and other products" Mature, undisturbed profiles are rare in Mexico, but Thus even a few years of plant growth establishes a when found they may be accepted as having remained zone ranging from light gray to black at the surface. unchanged since very ancient times, and thus may As time goes on the lower boundary of this horizon serve as a model of the type. moves deeper, until, in ancient, mature soils, it A truncated profile, with any of the expected upper reaches a condition in which the humification processe horizons reduced in depth or missing, must be regard- are balanced by the oxidative removal of the organic ed as either (1) very recently eroded, or (2) eroded at matte r. a past time but prevented from recovering by sub- Any interference by humans with these processes, sequent continued land use, for example, for grazing. through habitation, land burning, agriculture, de- A profile showing one to many inches of weathering structive logging, or stock grazing, will distort the and humus accumulation directly in contact with un- normal distribution of clay and organic matter through' altered or partially decomposed parent material the soil in depth. Such distortion, in turn, when found indicates a period of former erosion followed by some by analysis should be a clue to the time and intensity degree of recovery. However, the time element is of occupation. indeterminate unless the recent and present rate of The absolute values of soil components will of soil depletion can be estimated. course depend upon the type of soil. Thus a pure As a result of the shortcomings mentioned, direct silicious sand, or a porous volcanic ash deposit might field description of horizon differentiation, by itself be expected at maturity to show a lower clay content alone, will be inadequate for the full definition of past than a compact, deeply weathered basalt. The climat land use, and must be supplemented and reinforced by within past centuries is also highly significant. The other types of evidence. With this reservation, how- degree of weathering and the profuseness of vegetative ever, the existence, the loss, and the gain of individual growth is dependent upon moisture and temperature. soil horizons, by whatever names they may be called, Hence dislocation due to activity of man must be con- are of great assistance in reconstructing the history of sidered within the framework of the normal soil land use and human occupation. resulting from specified parent material and edaphic factors. Within these limits some formulation of pas history may be attempted. If a surface soil upon analysis shows a relatively LABORATORY CRITERIA FOR PROFILE ANALYSIS low clay content then it must be considered the upper. horizon of an old soil from which the clay has been The second group of criteria available to profile displaced downward, or an early stage in the forma- analysis includes a series of physical and chemical tion of a new soil from material which has hitherto tests made in the laboratory on samples taken in the been only slightly weathered. In the first instance the field. The data provide information concerning the lower levels should have a high clay content, owing to quantity and the vertical or horizontal distribution of the downward movement of clay formed in the super- various substances which are thought to be associated ficial horizons. In the second instance the lower in different ways with the activities of man. levels should contain little clay, since there has been Progress has been reported by several investi- only feeble weathering at the surface and no signifi- gators in following the level of phosphate in the soil, cant downward movement. A special case of this sor and also of trace metals such as copper and zinc. is frequently observed on stream deposits and slope However the simplest and most direct diagnostic washes caused by rapid erosion of the soils above. approach to soil changes would be to evaluate the The clay content may be small, but where the eroded formation and distribution of clay, the universal soils were themselves mature and rich, it may be product of weathering, and of the omnipresent organic surprisingly great. Nevertheless only little weathers matter derived from plants and microorganisms. To ing on the deposit need be assumed provided that the; these might be added the hydrogen ion concentration, clay content is approximately constant throughout its. and, where applicable, the lime content. It is worthy depth. COOK: EROSION MORPHOLOGY AND OCCUPATION HISTORY 291 On an old or original land surface, if the super- which are not applicable to a general soil investigation, ficial horizon shows a high clay content then we have and which are not pertinent here. either the former lower level of an old soil which has Phosphate and the trace elements appear to ac- been eroded away, or an intermediate stage in new cumulate as the result of occupation of land by man or soil formation. In the latter case the underlying the domestic animals. This accumulation is probably trata should also show a high clay content. In the referable to the excretion of these substances, after rmer there should be a rapid reduction of clay with their concentration in the body of the organism. Hence creasing depth. habitation sites should demonstrate on analysis larger If the clay distribution indicates a mature soil then amounts than the adjacent open land, and conversely e organic matter should be high at the surface and the discovery of an abnormally high level of phosphate hould extend downward, diminishing slowly, for a or copper would be taken as evidence of human or onsiderable distance. If we have the initial stages of animal occupation. However, although the analysis ew soil formation the organic matter might be high at for trace substances has real promise for the study te surface but should diminish rapidly with depth. If of prehistory, the deposition of these elements is not e are dealing with the badly eroded surface of an old an integral component of the process of erosion and oil the organic matter might appear in small quantity soil reconstitution, the subject of the present dis- roughout, but if we have an intermediate stage in new cussion. Hence the soil dynamics of phosphate and oil formation it might be present in large quantity copper are best left for future, separate consideration. oth at the surface and in the lower levels. The actual laboratory methods enmployed in this The fact should not be neglected that, whereas the study are adaptations of well known, conventional ormation, distribution, and destruction of organic procedures. Most have been taken directly from the atter, that is, of humus, is relatively rapid, that of recent monograph of Jackson (1958) on soil analysis. lay is very slow. Hence there is no necessary cor- They have been selected for simplicity rather than elation between the profile of humus and that of clay, refinement, for the error introduced by the variables rticularly when detritus or severely truncated older inherent in the material itself is of a much greater oils are being weathered or reconstituted. Each order of magnitude than that referable to experimental rocess must be studied individually. It must also be method. orne in mind that the general precepts offered above Determinations of pH are made with the standard re to be taken as a broad guide. Individual situations Beckman pH-meter on suspensions of 10 grams of soil re likely to show sharp deviations from any categori- in 30 milliliters of water. The nitrogen is estimated 1 formula. by the micro-Kjeldahl procedure. Organic carbon is A subsidiary criterion with respect to organic determined by a modified Walkley-Black technique, atter is provided by the carbon-to-nitrogen ratio. If with dichromate and titration against ferrous sulphate. e C/N ratio is high at the surface and diminishes The heat is mild and the yield is approximately 80 per apidly below, the exposure is unstable, since most of cent. On the other hand the results are closely repro- e plant residue is in the form of cellulose or derived ducible and consistent. These qualities render the arbohydrates which have not yet had time to be con- method highly suitable for a study the objective of erted to the protein of microorganisms. On the other which is to secure reliable and accurate comparative and, if the C/N ratio persists at a more or less results rather than a precise estimate of total carbon onstant and relatively low value it indicates that as such. Furthermore, the fact that charcoal is not eworking by soil organisms has been proceeding for included in the determination, as it would be with more long period without disturbance. By inference, drastic methods such as dry combustion, is a distinct erefore, the soil is old. advantage in dealing with soils which have been ex- This principle applies only to such horizons as posed to numerous fires of human origin. rovide a comparatively high level of organic matter. en the anlyses for carbon and nitrogen show very Calcium carbonate is determined as CO2 released all quantities of these elements, the margin of by acid. The gas is collected in an absorbent and lytical error is great, and the ratios become weighed. This analysis is performed only with ,ctremely erratic. If the nitrogen falls below 0. 03 samples previously found to have a pH value of more r cent and the carbon below 0. 3 per cent, the C/N than 7. 0. atio has little or no significance and may best be The clay has been estimated by a decantation isregarded. method, wherein a column 270 millimeters deep is The presence of specific elementary and ionic soil allowed to settle 24 hours and then the supernatant omponents may or may not be contingent upon human liquid poured off. The soil suspension is rendered ccupation or activity. The hydrogen ion concentration slightly alkaline after each decantation with weak determined primarily by the chemical nature of the ammonia. The supernatant liquid is poured into 0. 1 il itself, although it may be modified by agriculture N HCI for flocculation. The clay fraction is finally r habitation. The outstanding exception is the matrix washed, dried, and weighed. This method is some- accumulation deposited as a result of long continued what crude and arbitrary and would not be suitable for ccupation of a restricted area by significant numbers separation of a series of soil fractions according to persons. This is the so-called habitation midden predetermined particle size. For present purposes, requently found associated with archaeological sites. however, it is quite adequate, for it furnishes a :very often tends to be moderately or highly alkaline simple, rapid method for isolating the finest compo- ven in areas where the pH of the country soil is low. nent with an arbitrary, but constant upper limit of fidden constitutes a special type of material, since it particle size. This component may be regarded as oes not arise through the normal processes of soil being completely synonymous with, and indicative of, ormation. For its study methods have been developed the clay fraction of the soil. 292 ANTHROPOLOGICAL RECORDS By these methods soils have been examined from observation of soil profiles and erosion patterns. The. several areas in Mexico. For some of these areas it degree to which these quite diverse methods reinforce is not only possible to draw conclusions from the each other or conflict with each other may be judged chemical approach but also possible to combine the from the detailed descriptions and discussions which analytical results with those derived from field follow. IV. TOPSOIL DEPTH AND OCCUPATION INTENSITY: CENTRAL SINALOA The linear transect which formed the subject of this disintegrating rock is rarely like a knife edge in study was a fifty-mile stretch along National Highway sharpness, and that there is usually an intermediate No. 15, as it existed in 1958. The road runs south band of transition between "soil" and "weathered rock." from Culiacan through level country for several miles, Nevertheless a reasonable estimate may be made by then traverses nearly one hundred miles of rough, low eye of the lower limit of the soil. Soil depth, there- hills, finally to emerge on the coastal plain near fore, includes all material above this limit, regardless Mazatlan. Near the half-way point the rivers Elota of possible subdivision into narrower horizons. nd Piaxtla are crossed. The segment extending from Parent material. In this area under the prevailing l0 miles north of the Rio Elota to 20 miles south of the environmental conditions of great heat and moderate Rio Piaxtla has the merit of relative uniformity of moisture all types of native rock tend to disintegrate trrain, underlying rock, climate, and vegetation. Its with considerable rapidity. Hence soils approach a recent human use has been confined almost wholly to common level with respect to mass and extent, although stock ranging, with the exception of the area near the differences in the parent material are reflected no io Elota which has contained small villages for doubt in chemical differences between soils. It follows centuries. that as far as depth is concerned, the importance of Certain factors must always be recognized and if parent material becomes secondary to that of the other ossible controlled. Some of these are now discussed soil forming factors, and with a few exceptions may be ith particular reference to the area of interest. neglected. Topography. Local land formation is of critical significance in the evaluation of comparative soil depth. NONHUMAN FACTORS The extent of soil development within a small, re - stricted area varies widely depending upon slope and The soil layer measured. In this region of uneven, exposure. Consistently, in the Sinaloa coastal area as omnewhat hilly landscape a characteristic profile dis- elsewhere, the thickness is at a minimum at the top of ys a true soil of variable depth. It is reddish to knolls, hills, and ridges, increases greatly as the rk brown in color, and with few exceptions shows no slopes are descended, and reaches a maximum at the sible segregation of horizons. Under favorable foot of the slopes and in low spots such as meadows, onditions, such as in creek beds or damp depressions, stream beds, river bottoms, etc. It is essential, ere may appear in addition a surface layer of dark therefore, if we wish to use depth of soil as an index rown to black color which represents an accumulation to some other factor, such as human interference, that organic matter or humus. This horizon where the physiographic position be described, and if possible resent might be regarded as an "A" horizon, in which that it be kept uniform. se the usual brown soil would have to be designated Now in following a highway it is common observation "B" horizon. There is doubt, however, whether that the cuts cross high points and that in depressions ese terms are appropriate, since the distinction is and on flats no profiles are exposed. Consequently it sed only upon color and the presence of humus. Let is not only convenient, but almost obligatory, if a s then for simplicity and convenience of reference large number of examples is to be recorded, that the note either or both of these horizons collectively as cuts through high points be utilized. In this study, as e topsoil without commitment as to the physical or a consequence, a consistent selection has been made emical nature of the color zones. of soil depths at the summit of each exposure. Rarely With the exception of a few sand and gravel deposits does a road cut cross the top of an isolated hill. the river bottoms all the soils here observed are Usually it traverses a ridge, or, in an irregularly rived from sedimentary or metamorphic rock. hilly region, a spur of the slope of a hill, mountain, or espective of its intrinsic composition, this rock range. The soil is observed, as a result, at the hill, oughout the entire area has undergone very pro- ridge, or spur crest. In most instances the cross d decomposition, in some instances reaching section of the hill or ridge is bounded by a curved line, pths of twenty feet or more. Consequently nowhere thus, N , rather than by straight lines and an we find the topsoil resting directly upon, or even angle, thus, . The summit consequently is ose to unaltered bed rock, but always upon an inter- relatively flat, and the surface of the ground relatively ediate zone of weathered, decaying, disintegrated level for a distance of at least several feet and fre- ck. According to one point of view this zone might quently several meters. As a result there will be regarded as a "C" horizon and part of the soil itself. microvariations in the depth of the soil horizon and ,so the unaltered rock at the base would be considered the value of the latter will have to be expressed as a parent material from which the complete profile range rather than as a single figure. s derived. For present purposes, however, this In addition to the lateral slopes to each side of a troversy is irrelevant, since only the uppermost ridge there may be a longitudinal slope parallel to the ers, not the intermediate, partially weathered zone, axis of the formation. If the latter slope is steep, as Ehave been affected by human culture. By topsoil, on many side hills, it must be taken into consideration, lirefore, we mean only the strata above the zone of or the area discarded altogether, for it is clear that tcomposing rock. It is true, to be sure, that the line soil may wash downward along a gradient parallel to tseparation between the topsoil and the region of the axis of the ridge or spur as well as at right angles [293] 294 ANTHROPOLOGICAL RECORDS to it. This factor is not of significance for the stations ing, "Plain about the Lower Piaxtla River, " but upon which this survey is based. presumably refers also to the lower Elota River. Climate. Precise data for rainfall and temperature Kelly says the Tacuichamona culture embraced the are not available for this portion of Sinaloa. However, lower courses of these rivers but did not herself certain derived facts are reasonably certain. Since investigate them. the area lies essentially on the coastal plain to the The earliest historical mention is in the documents seaward of the Sierra Madre Occidental, the mean relating to the 1530 expedition of Nu'no de Guzman. annual temperature is high and, furthermore, probably These are described with much commentary by Sauer does not vary signific.antly over the fifty-mile stretch and Brand (1932, pages 46-47). After leaving the concerned. The rainfall, which occurs wholly in the region near Mazatlan "The expedition thereupon en- summer, is moderate. At the same time the total countered the lean country of the Piaxtla.... Lopez, moisture probably increases from north to south. This scouting for the army, complained of the lack of food possibility is supported by the figures cited by Sauer and even of water. The army marched for about a (1932, page 7) of 700 millimeters annual precipitation week through little pueblos, perhaps reaching the for Mazatlan, and 550 for Culiacan. Such a trend Piaxtla about Ixpalino. [Ixpalino is several miles would be reflected in deeper soils at the southern end above the modern highway crossing.] The river was of the transect. settled to the sea on both sides. '' But there appear to Vegetation. At the northern end of the transect the have been here no towns of real consequence. vegetation is that characteristic of the semiarid ex- Shortly beyond the Piaxtla, Guzman's party reached tension of the Sonora desert. It consists of many the Rio Elota, "least of the streams crossing the species of Leguminosae, together with some Cacti and coastal plain, but far superior to the Piaxtla in grass in open areas. On the whole it presents a dense alluvial land". Here were found several large towns. thicketlike appearance. In the larger depressions and Sauer and Brand identify two of these: Bayla, or stream bottoms tall deciduous trees may be found. Baila, and Abuya, which still persist. They mention Toward the south the spiny Leguminosae and Cacti are (page 47, footnote 47) as existing at the end of the gradually replaced by an arboreal vegetation with dense sixteenth century: "Elota, Apacha (Baila?), Vinapa, undergrowth of the type, although not of the extensive Avuya, and Tabala. " development, of the so-called rain forest. From The next document of importance is the Suma de direct observation, without any detailed taxonomic Visitas, compiled in about 1548 (see Borah and Cook, study, it is evident that many species disappear and 1960). Here are mentioned several villages in the others appear during the traverse of fifty miles. area concerned: The transition seems to follow a shift from the zone Abucho, with 53 tributaries (No. 78); Vayla, with designated by Leopold (1950) as Thorn Forest to that 131 tributaries (No. 789); Elota, with 39 tributaries; he called Tropical Deciduous Forest. Sauer and Brand Cololo, Casalne, Patino, and Apomia (with 58 Indians), (1932, page 7 and footnote 6) are in essential agree- and Cabaa with 24 Indians--all No. 279. Several of ment although they use a different terminology. They these are shown on the Ortelius map of 1579 (reprinted say: "Climatically, the area is tropical savanna (Aw by Kelly, 1945, plate 12): Abuya, northeast of of Koeppen) at the south, steppe at the north (BSh), the Culiacan; Bayla, northeast of Tebuchamana; Tavala, central portion being transitional. " Also: "For the the San Lorenzo River near Tebuchamana: Elota, present, by inference from changes in vegetation, the Cosaluc (Cosalne), Parmo (Patino) and Aponia, all on Piaxtla may serve as dividing line between drier north the right bank of the Elota River half way from the and more humid south until more data are at hand. " coast to the hills; Cololo and Cabaa, both on the left (For a good description of the plant environment bank of this river. reference may be made to Sauer and Brand, 1932, Turning to modern maps we find some of the ancient page 8. ) villages. Tebuchamana (now Tacuichamona) is given The vegetational change, both in species and in on the state map of Sinaloa (1: 1, 000, 000) as about 8 density, is undoubtedly associated with the climatic miles south southeast of San Lorenzo. Tavala is near- shift, since the southern portion would receive a higher by, according to Ortelius. These towns are consider- rainfall than the northern. The progressive increase ably outside the present area of interest. Abuya is in plant density, apart from speciation, might be shown on the Culiacan sheet of the 1: 500, 000 map as expected to be correlated with heavier soil develop- in Lat. 240 14' N, Long. 1070 2' W, and about 27 mu ment in the south. northwest of Elota. Baila (formerly Vayla) is in Lat. 240 10' N, Long. 1060 58' W, and 22 miles northwest of Elota. These towns are near but not on Mexican HUMAN FACTORS Highway No. 15, although the old wagon road passed through them. Elota is in Lat. 230 57' N, Long. 106? When we turn to the human factor we find that 431 W, on the Rio Elota and very close to the highway. Spanish and Mexican occupation has been continuous The other villages on the same river, mentioned in the since the invasion of the west coast by Nu7no de Guzman Suma, and on the map of Ortelius, do not appear on an shortly after the conquest. Even prior to this time modern map and have probably passed out of existence there was occupation by aboriginal tribes. With respect to the Piaxtla basin numerous settle- The archaeology of the region has been studied by ments are shown on the Ortelius map along the upper Sauer and Brand (1932) and to a lesser extent by Kelly course of the river, and four along the lower river. (1945). Sauer and Brand found very little evidence of Between these groups, where the present highway run extensive occupation. They say (page 27): "The there are none. The descriptions in the Suma are in Piaxtla country in general gave only small, obscure general conformity with the Ortelius map and with the sites, because we think this meager land held only statements found in the NuZno de Guzman documents. settlements. " This sentence is found under the head- Summarizing the data for the sixteenth century we COOK: EROSION MORPHOLOGY AND OCCUPATION HISTORY 295 may say that at the time of the Guzman conquest there The graph shows depths of soil at points from 18 existed a relatively numerous native population from a miles north (left) to 30 miles south (right) of the Rio point some 20 miles northwest of Elota on the Rio San Elota. Each line represents the range of the depth Lorenzo. The valley of the Rio Elota itself contained observed at a specific locality where the distance at or near the highway crossing four or five villages traversed along the highway is not more than approxi- ith a fairly dense population. The latter diminished mately 100 feet. The blocks, or tied lines, show the rapidly southward, and at the point where the highway average conditions, for several such localities, ex- crosses the Rio Piaxtla there was no habitation of tending along stretches of highway of from several consequence. For the next thirty miles through the hundred feet to as much as two miles. w hill country toward Mazatlan no occupation is 16. 3 miles N. Crest of a long ridge crossing the ecorded. highway. Bedrock shale. Soil 3-12 inches over the As a result of the devastation of the early sixteenth zone of weathered rock. entury the aboric-inal population was drastically re- 16. 1 miles N. Another exposure of the same ridge uced. Some vill,ges were wiped out completely. A crest. Bedrock shale. Soil 3-12 inches over the zone w such as Abuya, Baila, and Elota survived and are of weathered rock. The latter is soft, friable, and ill inhabited, although whether upon the ancient sites decomposed to a depth of 5-8 feet, grading impercep- s difficult to say. tibly into unaltered rock. The land was taken over by the Spaniards who used 15. 8 miles N. A 5-degree slope to the south. Bed- through the big ranches, primarily for stock raising. rock shale. Soil 3-12 inches as previously. s Sauer and Brand (1932, page 8) put it: "For at least 15. 1 miles N. Crest of a ridge which crosses high- ree and a half centuries the monte was heavily way. Bedrock shale. Soil 3-12 inches as previously. tcked with cattle, which, browsing on certain plants 14. 5 miles N. Ridge crest. Bedrock a fine con- d avoiding others, must gradually have altered the glomerate. Soil 0-12 inches over 3-4 feet of weathered mposition of the vegetation. " The matter of plant and disintegrated rock. In places soil as such com- ecies is beyond the scope of this study, but it is pletely absent and the underlying, unaltered material rtinent to note that soil changes would be expected to exposed. company any profound shift in the vegetation. 14. 1 miles N. Crest of a low ridge which crosses urthermore the continuous beating by the hoofs of highway. Bedrock shale. Soil 0-12 inches. ters and goats and the close cropping of grass by 13. 7 miles N. Crest of a low ridge, similar to the eep would tend to disturb and cause the removal of preceding. Bedrock sandstone. There are 0-12 inches e upper soil horizons throughout the entire area, and of mixed soil and rock fragments over 2-5 feet of rticularly on exposed spots such as the crests of weathered rock. iges and spurs. We may therefore be looking at the 10. 5 miles N. At this point the highway is cut il picture not at all as it was aboriginally. On the through a local elevation near the summit of a long, er hand alteration due to this factor would be uni- gentle (5-degree) slope to the east. Bedrock shale. rmly distributed throughout the entire region. In The soil layer is deeper than farther north: 12-24 ief, as one travels the West Coast Highway today inches. The underlying weathered zone, or "C" om 20 miles north to 30 miles south of the Rio Elota, horizon is 2-6 feet. Toward the foot of this slope the traverses a region modified as a whole by live soil layer is considerably deeper: 24-48 inches. This ock, at the northern end of which there may be some increase in depth at the foot of slopes is characteristic ripheral agricultural effect due to the settlements at of the area. However, there is no horizon differ- uya and Baila, in the middle of which there is the entiation, or recognizable layer of dark humus, or odern village of Elota on the river bank, and in the organic matter at the surface. uthern half of which there are no traces of sub- 9. 9 miles N. At a minor elevation near the summit ntial, permanent human settlement. of a 5-degree slope to the east. Soil free from frag- ments of rock, 6-24 inches over the weathered rock. Bedrock shale. OBSERVATIONS OF TOPSOIL DEPTHS 8. 7 miles N. The crest of a low ridge, itself lying in an extensive low, level area, or bottom land. Bed- Let us now consider the depth of the soil layer in a rock sandstone and conglomerate. Soil 6-24 inches nsect through this country. The data consist of a with a concentrated layer of stones on the surface, a ries of observations. These may be presented in rock mantle. This profile is found commonly through- o ways, first by a series of verbal descriptions, out this sector. The rock concentration, or mantle, cond as a graph (fig. 10). Both methods are used represents weathered fragments remaining after the e. The descriptions are given below, even though finer material has been blown or washed away, and is y are long and detailed. The graph is to be examined clear indication of recent surface erosion. te light of the descriptions. The soil layer overlies the usual several feet of Distances are given in miles north (N) or south (S) disintegrated bedrock. Cultivated fields are near this the Rio Elota. The vegetation is "monte" unless area and the vegetation at the point of observation is erwise specified (see the previous discussion of unusually immature, indicating use of the land in the etation). The topsoil depths, in inches, represent recent past. approximate range at the summit, or maximum 7. 4 miles N. The crest of a low ridge which crosses relopment, of a road cut. Descriptions in brackets the highway. There are 6-18 inches of dark soil over ply to localities where the conditions do not conform 2-6 feet of weathered rock. Bedrock sandstone. the standards established for comparison, but which 6. 6 miles N. The crest of a low ridge, the long axis eof general interest; these are not included in the of which dips at 5-10 degrees across the highway. Bed- aph. The expression soil is to be understood as rock sandstone. Soil 6-18 inches. A rock mantle on so0il as previously defined, the surface. 296 ANTHROPOLOGICAL RECORDS 48 42 36 .24 1)8 1 2 6 18 14 10 6 2 10 14 18 22 26 30 Miles from Rio Elota Figure 10. Depths of soil in the Rio Elota area. [6. 1 miles N. A road cut at the foot of a steep hill. 2. 3 miles N. Crest of ridge. Soil features the same, The soil is very thin; 0-12 inches. The underlying as in the preceding locality. weathered horizon is 6-8 feet. Bedrock hard sand- 2. 3 to 1. 3 miles N. There are several similar expo. stone. The vegetation is denser, with numerous sures at the crests of low ridges through which the deciduous high shrubs or trees.] highway is cut. These cuts are shallow and permit [5. 7 miles N. Identical with the preceding. These observation to only a few feet in depth. In this points are not included in the graph because they are locality there has been extensive clearing of the located at the foot of slopes, not at the summits.] "monte" with subsequent corn cultivation. 5. 0 miles N. The crest of a ridge, the main axis of The summits of the banks show the parent material which slopes across the highway. The soil is 0-12 of the soil to be a coarse, sandy deposit, of brownish inches, much mixed with rock fragments. Bedrock color, whichmay be either an ancient alluvial deposit shale and sandstone. Vegetation relatively dense, ass vsie in pecedin lcite.' (although no stratification is visible) or a very deeply in preceeding lo calities. and thoroughly weathered body of decomposed bedrock- 4. 1 miles N. The crest of a ridge. Bedrock a hard 4. 1 miles N. The crest of a ridge. Bedrock a hard The nature of the latter can not be determ ined because igneous or metamorphic rock, possibly gneiss or of the shallowness of the exposures. granite, which is deeply weathered and disintegrated. o A differentiation of horizons is evident: The soil, as distinguished from the highly altered "A": a dark organic or humus layer, 0-6 inches. parent material, is a light red brown horizon 0-6 "B": a fine, red brown horizon, 6-12 inches. inches thick, in many spots having vanished entirely, "C": decomposing parent rock. or remaining in small, discrete pockets, with the 2. 5 miles N. Crest of ridge. Parent rock the same crumbly, weathered underlying material exposed at as in the preceding locality. The soil however is the surface. This upper horizon has the appearance : very thin: 0-6 inches, with no horizon differentiation, of a former B2 horizon, with a residual impregnation over weathered bedrock. of iron derived from former overlying horizons. COOK: EROSION MORPHOLOGY AND OCCUPATION HISTORY 297 0. 7 mile N. Here is a highly decomposed, appar- definite trend away from the thorny mezquite and ently granitic rock, of structure similar to that of the cactus and becomes denser with an initially developed previous locality, but with the superficial red brown deciduous arboreal cover. soil horizon completely absent, leaving the upper The formation underlying this area consists of surface of the weathered zone ("C" horizon) exposed. compacted gravels, consolidated several feet below This formation is seen as far as the north bank of the the surface of the ground almost to conglomerate. The Rio Elota. black organic "A" horizon, mentioned at the previous 0. 7 mile S. Crest of a ridge which is cut through by locality, is absent or very thin, reaching a maximum the highway. Bedrock conglomerate. Soil 0-6 inches, of 6 inches. The light brown "B" horizon is present in most places none with the underlying weathered bed- (or occasionally absent), reaching a maximum thick- rock exposed. ness of 18 inches. It includes numerous unweathered [1. 3 miles S. Cut through the top of a knoll, or rock fragments. The average depth of the combined small hill. The underlying material is a sandy silt. "A" and "B" horizon, therefore ranges between zero There is a dark, humus-containing layer at the surface and 24 inches, as indicated in figure 10 for this region but no clear soil horizons can be defined.] of the highway. There is of course in addition a zone 1. 9 miles S. A cut through the top of a knoll. of weathering in the underlying gravel, which, al- Parent material gravel. There is no soil at all. The though the alteration is not profound, may extend to a unaltered, or slightly weathered gravel is exposed on depth of 2-6 feet. On the lower slopes of these small the crest of the knoll. ridges and spurs the brown "B" layer may increase in 2. 6 miles S. Crest of a ridge. Bedrock conglom- thickness to 4 feet. But even here little if any of the erate. There is no distinguishable soil whatever. The black, humus-containing surface, or "A" horizon weathering zone of the decomposing bedrock is ex- persists. posed at the surface, and the area appears as if the 14. 7 miles S. The crest of a ridge. Parent mate- soil as such had been completely removed. rial sandy silt. There is a zone of brown color at the 3. 6 - 4. 0 miles S. Several ridge crests are cut surface 12-24 inches deep, which may be regarded as through. Bedrock conglomerate or sandstone. Some corresponding to the second layer, or "B" horizon of of the crests show no soil layer. Others have oc- the preceding localities. casional spots of soil, mixed with rock fragments 15. 9 miles S. A low ridge, in a small, cultivated 0-12 inches deep. creek bottom, cut by the highway. The parent mate- 5. 5 - 6. 0 miles S. Numerous highway cuts through rial is silt. There is a dark layer of soil on the low ridges. Bedrock harder, metamorphic or igneous. surface, 6-12 inches deep. Soil layer is uniformly present, without horizon dif- 17. 8 - 18. 2 miles S. Several highway cuts through ferentiation, 6-18 inches deep, over decomposing rock crests of ridges and spurs of low hills. The bedrock zone. consists of sandstone, shale, and conglomerate. The [ 6. 8 - 7. 2 miles S. At this point there is a swale, soil layer is variable, from absent to a depth of 24 or bottom land between two ridges, in which there are inches. The vegetation is thorn brush and shrub, with silt, sand, and gravel deposits. The latter are stream relatively little forest. laid or may be wash from the adjacent slopes. There 19. 3 miles S. The hills are here higher (200-400 is an accumulation of 12-24 inches of dark, humus feet elevation above the coastal plain). The upper bearing soil over a weathered zone of undetermined soil layer is 12-36 inches deep, often with a black "A" depth.] horizon 0-12 inches in depth over a brown "B" horizon 7. 4 - 7. 8 miles S. There are several road cuts 12-24 inches deep. The "A" horizon is particularly trough the crests of ridges. The latter are composed well developed on the lower slopes of the ridges. f sands and gravels similar to the deposits mentioned 21. 3 miles S. The crest of a ridge cut through by at the previous locality. There is a layer of mixed the highway. Soil horizon 12-24 inches, over weather- soil and rock fragments at the surface, 6-24 inches ing shale. Vegetation deciduous forest. eep. This overlies a weathered zone which in turn 21. 6 miles S. Ridge crest, or crest of a long spur. grades into unaltered gravel. Bedrock shale. Brown soil 12-36 inches deep. No 8. 4 miles S. The highway crosses-the Rio Piaxtla. black "A" horizon. No horizon differentiation. There are no ranches or villages. The river runs here 24. 5 miles S. Crest of a low ridge. Bedrock shale. rough a narrow, wooded canyon. The vegetation Soil brown, mixed with unweathered stones, 12-36 ontains some mezquite and other spiny plants, but inches thick, over deeply weathered parent material. eciduous trees predominate, together with herbaceous Other exposures in the vicinity show the soil very thin dergrowth. to almost absent on the summits of the knolls and 10. 1 - 12. 1 miles S. There are several exposures ridges. At the same time the lower slopes have a here the highway cuts through the crests of spurs or much deeper brown soil, 1-5 feet thick, with occa- idges. The parent material consists of gravels. The sionally traces of a black "A" horizon above the brown oils are variable, ranging from zero (weathered stratum. Vegetation here, in a definitely hilly region ravel exposed) to 18 inches in depth. Where visible is quite dense and consists of deciduous forest. e soil may consist of a thin layer of light brown 24. 7 miles S. Crest of a low ridge, or spur. Bed- aterial 0-12 inches in thickness, or may be overlain rock shale, deeply weathered. Soil 12-48 inches. y a black, humus-containing layer 0-6 inches deep. 25. 6 - 25. 8 miles S. Three small ridge crests, e latter condition appears to represent the mature close together. Bedrock shale. Two exposures show Foil type (i. e., an "A" horizon of dark soil, a "B" thin soil, 6-12 inches, the other a thicker layer, 12- Forizon light brown in color, and a "C" horizon con- 36 inches. | isting of gravel somewhat altered by weathering). 27. 3 miles S. The highway cuts across the crest of 12. 2 - 13. 8 miles S. Several ridge crests are cut a ridge. At the summit the soil is thin, and in a few l rough by the highway. The vegetation here shows a spots absent. In nearby valley bottoms the soil is 298 ANTHROPOLOGICAL RECORDS from 2 to 4 feet deep and is marked by a dark brown Disregarding any systematic change due to to black "A" horizon overlying a light brown "B" climatic shift, it is clear that the upper soil horizons horizon. The vegetation consists of deciduous forest. have been reduced or destroyed at two distinct points: 27. 5 miles S. Several ridge or spur crests are cut at the northern extremity of this transect, and in the by the highway. The soil varies widely from very thin basin of the Rio Elota. There is no perceptible change (6-12 inches) to quite deep (12-48 inches). The depth in the valley of the Rio Piaxtla. These findings con- and development depend upon very local conditions of form broadly to the pattern of human occupation. topography and in part are probably functions of For the moment the effect of livestock may be dis- moisture at the top and on the slopes of the ridges. regarded, because domestic animals have ranged freely 29. 1 - 29. 3 miles S. Several exposures where and uniformly throughout the region, and such surface small ridges are traversed by the highway. In one erosion as they may have engendered will be evenly place the soil is relatively deep (12-48 inches); in distributed and will not be localized. The soil picture others it is shallow (0-6 inches). then reflects actual settlement by sedentary popula- 29. 7 miles S. Highway crosses a ridge crest. Bed- tions in restricted localities, where agriculture and rock shale. Soil red brown, no horizon development, day-to-day living wear and dissipate the superficial 24-36 inches deep, overlying weathered bedrock. horizons. There are two such localities along Highway Vegetation deciduous forest. No. 15. The first is at the northern end of the transect [Southwest of 30 miles S. the same conditions pre- and beyond and is represented at the periphery of the vail for several miles. As the coastal plain toward complex by the villages of Abuya and Baila. The Mazatlan is approached the soils exposed at the second is the village of Elota, quite near the highway, summits of the road cuts continue to vary from one to on the north bank of the river. All these settlements several feet in depth.] have been foci of human activity for at least 500 years. From the descriptions given above and from the It therefore seems reasonable to conclude that the graph (fig. 10) two conclusions may be drawn. 1.h(i Proceeding from20 mosils north to 30a. mcorrespondence is established between intensity of 1. Proceeding from 20 miles north to 30 miles occupation and the degree of attrition of the topsoil. south of the Rio Elota there is a progressive deepening o tioniand the e o t n the topsoil . of the topsoil at the summits of small elevations and It is significant that all the topsoils examined the crests of ridges and lateral spurs of the hills. appear to have been at least partially depleted, those Simultaneously the soils in the bottoms, valleys, and near the inhabited localities more than elsewhere. if minor depressions are uniformly quite thick. This the villages concerned had flourished up to the late trend on the exposed summits seems to reflect the sixteenth century and had then gone out of existence climatic shift toward generally heavier rainfall to the we would expect to find evidence of soil recovery. OI0 southward, together with the transition from a semi- the contrary some degree of active erosion is still in arid monte to a relatively dense deciduous forest. In progress. Hence we must conclude that the areas nea any event the edaphic factors have operated independ- the villages have been subject to continuous degradati ently of human interference, for a long period, with no opportunity for reconstituti 2. Between 12 and 18 miles north of the Rio Elota of a mature profile. the observed soils are decidedly shallow (0-12 inches). The region as a whole, apart from the vicinity of Between 4 and 12 miles, in terrain very similar to that inhabited sites, demonstrates no fully mature, un- immediately northward, they are undoubtedly deeper disturbed topsoil profiles, but rather a moderately (6-24 inches). From 4 miles north to 4 miles south of severe, generalized reduction of the upper horizon. the river the soil at the gummits of knolls becomes This condition must be ascribed, as previously sug- extremely thin, and over wide areas has been totally gested, to the ranging of livestock. Since here, also, depleted (0-6 inches). From 4 to 18 miles south of there is no indication of reversal, or recovery, it the Rio Elota a much greater average depth is seen must be concluded that the mild damage to the soil (0-24 inches), and through the subsequent 12 miles to caused by livestock has been inflicted over four the southward, the soil, although varying widely with centuries and persists even at the present time. Th local circumstances frequently reaches a depth of 4 erosion caused by local habitation and agriculture is feet (range 0-48 inches). superposed upon that induced by unrestricted grazing V. PROFILE ANALYSIS OF HABITATION SITES ON THE COAST OF SINALOA AND NAYARIT This section concerns a series of habitation sites on the coastal plain of Sinaloa and northern Nayarit. SERIES 1 The first of these, which I examined, is a sand connection with some of the Tepic soils, lead to the mound, described, and designated by Kelly (1945) as supposition of recent, incipient profile development. site No. 60, lying just north of Eldorado, 500 meters The soil contains considerable acid-extractable carbon north of kilometer 59 on the paved road to Culiacan. dioxide, distributed in an erratic fashion from top to Only one profile was sampled, from an excavation bottom. Much of this calcium carbonate is undoubtedly close to the road. The matrix is filled with potsherds, derived from fragments of molluscan shell which are shell, charcoal, and scraps of bone. At this point the scattered at random through the site. The pH is deposit is at least 8 feet deep. The surface is covered slightly alkaline. -with mezquite and other thorny shrubs. The analytical The indications are, therefore, that this is a mound iresults are given below. constructed out of soil taken from the surrounding ,; The clay content is relatively constant to a depth of country, to which has been added certain residues of [80 inches. There is no evidence of recent clay forma- human occupancy, that is, shell, charcoal, etc. The on at the surface, nor is there any accumulation at construction was in the era of the Spanish conquest, in ower levels. One must therefore conclude that the the sixteenth century, or perhaps earlier-a view which lay is derived directly from the soil which went to is supported by the archaeological evidence (see Kelly, onstruct the mound, had been previously formed, and 1945). In four or five centuries the heaped-up soil has as incorporated in the mound matrix. The organic developed no visible horizons and has only begun to ontent is low, both in nitrogen and carbon. Neverthe- segregate organic matter and clay. This gives us some ess there is a slight increase (seen in the carbon) at notion of the time span necessary for the establishment e surface, over the more or less uniform values of a mature profile in the warm, damp, coastal climate, ound at lower levels. The C/N ratio diminishes with when the parent material is predominantly alluvial epth. These phenomena, as was pointed out in river sand. Carbon Sample Depth Clay Nitrogen Carbon dioxide pH C/N No. (in.) (per cent) (per cent) (per cent) (per cent) ratio 1 6 14.6 0.04 0.64 1.16 7.84 16.0 2 18 11.1 0.02 0.23 1.72 8.02 11.5 3 36 11.9 0.04 0.37 1.25 8.07 9.2 4 60 15.1 0.05 0.37 0.00 7.11 7.9 5 80 15.6 0.04 0.25 0.54 8.39 6.2 6 100 - 0.05 0.40 0.79 8.11 8.0 SERIES 2 The second series of analyses are of samples from by conclusions derived exclusively from these data. group of former habitation sites near Amapa, a The data from analyses follow. It will be noted that age four miles west of Santiago Ixcuintla, on the the organic content rests entirely upon analyses for oastal plain of northern Nayarit. The excavation was carbon, for we have not yet been able to carry out the de and the samples obtained by Professor Clement nitrogen analyses. However the carbon content is a eighan of the University of California at Los Angeles. very good criterion of total organic matter. At this location there is a closely aggregated group In Herman's Mound, except for a very slight begin- mounds. Soil sample series were secured from ning of clay formation in the upper 12 inches and a ree test pits in three separate mounds, the different very slight accumulation of organic matter, there is eries being designated respectively as "Herman's almost no variation from top to bottom. The implica- ound, " "Pit B, " and "Pit D. " Since we have a com- tion is that the mound was artificially constructed with rehensive statement from Professor Meighan (personal the use of surface soils found in the general vicinity. omunication) concerning the stratigraphy, archaeol- Such building materials would naturally contain and history of these sites, excerpts from which appreciable quantities of both clay and organic carbon tement may be quoted verbatim, it will be appro- which would appear with apparently random distribu- rate to present the data fromn the soil analyses and tion throughout the site. The slight excess of these en see to what degree they coincide with and confirm components found in the upper foot of the site may be e conclusions of the field investigators. The analyt- taken as the degree of new soil formation since the cal results therefore follow herewith and are amplified completion of the mound. [2991 300 ANTHROPOLOGICAL RECORDS Herman's Mound Carbon Sample Depth Clay Carbon dioxide pH No. (in.) (per cent) (per cent) (per cent) 1 4 11.84 0.84 0.00 7.0 2 12 12.65 0.42 0.00 7.3 3 20 8.66 0.24 0.00 7.6 4 28 8:35 0.25 0.00 7.6 5 35 8.70 0.27 0.00 7.7 6 43 9.11 0.30 0.00 7.9 7 51 9.11 0.26 0.00 7.9 8 59 8.18 0.26 0.00 8.0 9 67 8.42 0.30 0.00 8.0 10 75 8.98 0.33 0.00 8.0 11 83 7.32 0.32 0.00 7.6 12 91 10.26 0.27 0.00 7.4 13 99 8.38 0.31 0.00 7.0 14 106 10.52 0.32 0.00 7.2 15 114 13.48 0.30 0.00 7.3 16 122 9.80 0.31 0.00 7.6 17 130 8.31 0.34 0.00 7.9 18 138 7.77 0.32 0.00 7.9 19 146 7.07 0.34 0.00 7.9 20 153 10.13 0.32 0.00 8.0 21 161 8.90 0.36 0.00 8.0 22 169 8.49 0.35 0.00 8.0 23 177 11.45 0.34 0.23 9.2 Pit B Carbon Sample Depth Clay Carbon dioxide pH No. (in.) (per cent) (per cent) (per cent) 1 4 15.37 0.63 0.00 7.5 2 12 17.49 0.49 0.00 7.0 3 20 17.62 0.21 0.00 7.3 4 28 13.01 0.25 0.00 7.3 5 35 15.36 0.29 0.00 7.3 6 43 13.15 0.26 0.00 7.1 7 51 11.83 0.27 0.00 7.4 8 59 5.19 0.12 0.00 7.8 9 67 6.93 0.60 0.37 7.9 10 75 12.25 0.26 - 8.3 11 83 10.40 0.37 0.38 9.0 12 91 9.79 0.35 0.23 9.1 13 99 9.69 0.24 0.24 9.1 14 106 11.57 0.30 0.63 9.0 15 114 14.42 0.37 0.31 8.9 16 122 17.24 0.57 0.00 8.7 17 130 16.43 0.48 0.00 8.7 18 138 13.78 0.31 0.00 8.7 19 146 13.24 0.33 0.00 8.7 20 153 8.51 0.18 0.00 8.7 21 161 14.55 0.17 0.00 8.6 Samples 22 and 23 may indicate the original soil no formation of visible horizons. On the other hand surface. The clay content rises slightly, acid-extract- there is clear evidence of discontinuity in composition. able carbon dioxide makes its first appearance andthepH At samples 9, 10, and 11 (depth range approximately shifts markedly toward the alkaline side. The reaction from 65 to 85 inches) there is a sudden increase in pH, of the entire site is somewhat alkaline. At the same an increase in organic carbon, and the appearance of. time, above the mound base there is no carbonate. It acid-extractable carbon dioxide. With sample 16 (de may be concluded therefore that the alkaline reaction 122 inches) the carbonate disappears and the organic is associated with the presence of salinity, derived carbon rises. It is as if there had been three different perhaps from sea salts. and successive periods in the construction of the mou In Pit B there is also indication of recent weathering with an interval following each during which incipient on the surface, although the profile as a whole shows soil formation had proceeded at the surface. COOK: EROSION MORPHOLOGY AND OCCUPATION HISTORY 301 Pit D Carbon Sample Depth Clay Carbon dioxide pH No. (in.) (per cent) (per cent) (per cent) 1 4 7.58 0.89 0.00 7.2 2 12 3.49 0.60 0.00 7.5 3 20 4.28 0.65 0.00 7.7 4 28 6.38 0.25 0.00 7.9 5 35 4.98 0.26 0.00 8.1 6 43 3.21 0.27 0.00 8.1 7 51 2.67 0.27 0.00 8.0 8 59 2.51 0.27 0.00 8.0 9 67 2.31 0.79 0.00 8.2 10 75 4.52 0.33 0.00 8.4 11 83 6.00 0.28 0.00 8.2 12 91 7.11 0.29 0.00 8.4 13 99 9.81 0.33 0.00 8.4 14 106 7.66 0.43 0.00 8.8 15 114 2.09 0.11 0.00 9. 1 16 122 2.00 0.04 0.00 9.2 17 130 6.82 0.17 0.00 9.3 18 138 10.44 0.28 0.00 9.2 19 146 12.21 0.22 0.00 9. 1 20 153 11.57 0.24 0.00 9. 1 21 161 10.79 0.26 0.00 9.1 22 169 19.28 0.34 0.00 8.9 23 177 28.68 0.25 0.00 8.7 In Pit D the moderate rise in organic carbon suggests least 2 meters of refuse in it. Hence the present me surface modification, although very little new land surface is an artificial one.... y has been formed. There is some indication of "In this pit there is a fairly sharp cultural break scontinuous construction of the mound for there is a at 280-300 cm (about 120 inches). This may or ge in pH and a considerable increase in clay con- may not be accompanied by an abandonment of the t from the 110- to the 140-inch level. These shifts, site. The 280-300 cm. level dates from about ever, may have been due to variation in the mate- 600 A. D., according to our present typological 1 used for construction. It is possible that the cross-ties; hence everything lower is older and face of the original soil is reached with sample 23 everything higher is later in time. The topmost 177 inches depth, for the clay content suddenly rises level should go to about 1200 A. D." atly. However, in view of the wide fluctuation in The cultural break at 280-300 centimeters coincides rcentages of clay throughout the mound this sug- with the analytical break found at about 120 inches, and stion remains only a possibility. the two results reinforce each other. The relatively There are certain features common to all three of slight or moderate surface clay and organic matter se sample series which have definite bearing on the accumulation can now be stated as being not more et history of the area. First, the complete absence than 750 years old. If we bring Professor Meighan's horizon formation associated with weathering of the date of 1200 A. D. for completion of the site to its rent material and consequent redistribution of the most recent possible limit we get a figure of 450 years. athering products within the body of the mounds is Hence in this climate, and with this type of sandy or ost conclusive evidence that the eminences, or silty soil we get a range of from 450 to 750 years for uds, are of geologically extremely recent origin the formation of little clay and a little organic matter that they were built up rapidly by human agency. at the surface. ond, the erratic distribution of clay, organic carbon, The upper discontinuity, at approximately 70 inches, acid-extractable carbonate leads to the conclusion is not noted by Professor Meighan as having cultural t these mounds were deliberately constructed, rather significance. However it was no doubt of the same being the slow, casual accumulation of habitation character as the lower break, and occurred some time den or refuse. They cannot have existed in their between 600 and 1200 A. D. sent form more than a few centuries. Pit D, according to Meighan, is located in Now let us turn to the field observations. With re- an area which was built up rapidly during the later d to Pit B, Professor Meighan says: phases of the construction of the mound in which Pit B is located. This would put its completion as roughly "This pit was dug at the end of a constructed contemporary with the other site, a hypothesis which is nound. Over 3 meters of it are below the surround- borne out by the similar distribution of clay and organic ng soil surface, although the whole area. .. has at carbon. 302 ANTHROPOLOGICAL RECORDS The history of Herman's Mound is interesting in the SUMMARY light of our analytical results. Professor Meighan says concerning it: If we consider the site tested near Culia-can as well as those near Santiago Ixcuintla, it is clear that we "It is located about 200 meters north of Pit B-14E [Pit BI , in the end of one of the largest constructed have some idea of the rate of aging of these coastal pro.-' [Pit B]I, in the end of one of the largest constructed fie. A ne soi cosstn of rie-eostdsn mounds on the site. Thre is no visual profil files. A new soil consisting of river-deposited sand moucandes ont soite.n the Thereis columno,via proe tand silt begins to form clay and incorporate organic of~~~~~~~~~~~~~~ chne.nsi ntewhl oun xetta matter within a few hundred years such that a demon- the top foot or so is more friable and contains plant sterabl a ationofeithearo both tat t surfac strable accumulation of either or both at the surface roots, iguana holes, etc. The physical and cultural ( 1 evdec iniae'htti woemud.a (upper 12 inches) is evidence of plant growth and weathering for 500 to 1, 000 years. A deep, progres- built deliberately all at once, and in a very short s a period. The material used was river silt, appar- anslocation of orandeptevera feetw ently scrped up fom the ajacent aea out o translocation of clay to a depth of -several feet would ently scraped up from the adjacent area out of tu eur eea huadyas uhapro borrow pits which are ponds at the present time. wous far antedat theusanishcnues a any Ther arefew herd inthismateial,but hatwould so far antedate the Spanish conquest and any there are rew shterd in thhistomaterial, bute at known cultural occupation that the soil would have to be there are are late in the history of the site and this rgre santrl sopsdt ua om~ construction phase can be tentatively dated at about to,alwas alng at parent aterial israw 1000-1Z00 A.D." ~~~~~~~tion, always allowing that the parent material is raw 1000-1200 A. D." sand or silt and that the climate has not altered With respect to Meighan's conclusion that the site significantly during the past few thousand years. was built rapidly approximately 750 to 950 years ago, There is very little difference between Culiacan we observe the uniformly low-organic matter and and Amapa with respect either to new soil formation clay throughout the depth of the profile, and the random or to stratification of soil used as construction mate- fluctuations in these components. These findings con- rial. Culiacan No. 60 may have been finished near the firm his deductions from the archaeological evidence. period of the Spanish conquest, and the Amapa sites The slight increase in clay and organic carbon in the 200 or 300 years previously. For distinctions of this upper 12 inches, supports his implication of weathering character the archaeological evidence must be relied in this horizon and, furthermore, again gives us a upon. The soil features do not permit of such refined notion of the rate of horizon formation in this kind of differentiation. It is equally possible that Culiacan No. material and in this climate. Professor Meighan says 60 was completed decades before the conquest, or in that the mound does not go below the surrounding ground the same era as saw the construction of the Amapa level, but the sudden jump in pH and the appearance of mounds. In any case it is clear that this portion of the acid-extractable carbon dioxide in sample No. 23 at 177 Mexican coast was being actively inhabited from one to inches is strong indication that the original soil surface five centuries before the Spaniards arrived, and was had been reached. still occupied at the time of the conquest. VI. PROFILE ANALYSIS IN SOUTHERN HIDALGO A portion of the plateau north of Mexico City covers For this investigation two localities were selected, the north central portion of the state of Mexico and the and selected because they had been carefully surveyed south central part of the state of Hidalgo. The soils in with respect to erosion patterns more than ten years general give evidence of very severe use for many cen- ago. A report was made at that time and the localities uries, and the region prior to modern times was heav- were among those described in the monograph by Cook ily populated. In this respect Hidalgo differs from its (1949a). They are on the Laredo highway, between neighboring states to the north and west, such as San Pachuca and Actopan. On this visit particular attention Luis Potosi, Durango, and Chihuahua, which are equally was paid to the securing of samples in order to test arid but which until recently had never supported more weathering rates in this area. The two profiles con- than a very sparse population. cerned will be designated Nos. I and II. PROFILE I (Mexico City-Laredo highway, northwest of Pachuca, clay does not. Since the soil here consists of former at kilometer stone 88) top soil washed down from the slope higher up it may The location is about half way up an approximately be concluded that the clay was derived from this source five-degree slope, from 2 to 3 miles long. The soil and not produced in situ. The organic matter in the onsists of a gray, sandy wash derived by sheet erosion upper 12 inches probably was formed in place. The rom the slope above. There are 30 inches of this wash surface is slightly alkaline, becoming neutral below. esting upon what appears to be the smoothly eroded The calcium carbonate is below the measureable limit. surface of an ancient caliche. The exposure is by According to the history of the region, as previously eans of a roadside ditch. The analytical data are reconstructed (Cook, 1949a), the deposition of this ven below. slope wash occurred no later than the sixteenth century. This profile shows a moderately high clay content Within a minimum of 400 years, therefore, this soil d also a moderately high level of organic matter. has undergone only a very slight degree of weathering, he latter diminishes rapidly with depth whereas the if any. Carbon Sample Depth Clay Nitrogen Carbon dioxide pH No. (in.) (per cent) (per cent) (per cent) (per cent) 185 3 15.0 0.10 0.93 0.00 7.7 186 6 14.1 0.05 0.68 0.00 7.8 187 12 16.0 0.09 0.60 0.00 7.0 188 24 15.6 0.06 0.32 0.00 6.9 PROFILE II (Mexico City-Laredo highway, due west of the village alluvial deposit down to and into a heavily compacted f Arenal, at kilometer 111.5) caliche. This underlies the entire area, and perhaps A lateral dirt road runs at right angles to the high- represents the original, prealluvial land surface. The ay, and at 100 yards reaches an arroyo, an affluent of age of the primary gully (and of its subsidiary) may be large dry-wash which here for several miles holds a gauged from the fact that a pepper tree four feet in ourse parallel to the highway on the west. The terrain diameter grows in the flat bed of the gully at this local- s nearly level but with a slight downward gradient to the ity. Samples were taken from the bank of the lateral est. The surroundings are cultivated in part but are affluent close to its point of junction with the main redominantly covered with cacti, prickly pear, mez- stream and extended from the present surface of the uite, and very thin grass. ground to the top of the caliche, about ten feet below. The principal arroyo has cut a channel from 40 to At the spot where the samples were taken there is little 0 feet wide, with vertical banks from 10 to 20 feet if any vegetation, although the adjacent area has long igh through a deep, sandy alluvial deposit which has been cultivated to maguey and corn. There was no een washed down through a very long period from the visible formation of horizons in this entire profile. The ugged mountain range to the east. It has been shown analytical data are as given below. flsewhere (Cook, 1949a) that the deposition of this This soil, derived fromthe eastern mountain slopes, naterial was associated with a land use probably re- is calcareous, as indicated by the acid-extractable Lted to the civilization which centered at Tula. At the carbon dioxide. Yet the values at different depths show oint where this profile was examined the main arroyo only random variation, indicating no tendency toward a ,B joined by a lateral stream which has cut through the downward displacement of lime. The organic matter is [3031 304 ANTHROPOLOGICAL RECORDS not present in great quantity at the surface, and tends weathering and organization of a new profile on the to become even less at the lower levels. The clay alluvial deposit. The channelling of the washes, as content is low throughout the profile. The two high deduced from tree growth, artifact occurrence, and values at 24 and 36 inches may be ascribed to fortuitous documentary evidence, must have taken place no later variability in the material as it was deposited. Other- than the great population maximum in the Teotlalpan in wise there does seem to be a slight increase in clay at the early sixteenth century. The deposition of the 0-12 inches. detritus from the east undoubtedly antedated the arroyos by several centuries, perhaps by as much as 1, 000 As measured in terms of visible horizon develop- years. Weathering and horizon formation, therefore, ment and production of clay and organic matter at the are extremely slow in this area of moderate temper- surface, this soil shows only the bare initiation of ature and great aridity. Carbon Sample Depth Clay Nitrogen Carbon dioxide No. (in.) (per cent) (per cent) (per cent) (per cent) 196 3 7.8 0.08 0.80 0.91 197 6 8.8 0.07 0.71 0.91 198 12 7.9 0.08 0.64 0.96 199 24 14.7 0.09 0.36 0.74 200 36 15.2 0.06 0.38 0.75 201 48 5.2 0.06 0.23 0.74 202 60 6.3 0.06 0.43 0.94 203 72 4.1 0.03 0.19 0.76 204 120 4.7 0.11 0.73 0.83 VII. PROFILE ANALYSIS AND EROSION PATTERNS ON THE TEPIC PLATEAU On the more-or-less level area surrounding the city ing hillsides, such accumulation presumably being of Tepic, Nayarit, there are three types of soil, dis- caused by human agency. tinguishable on the basis of their history and their Here we present the data by means of a series of parent material. The first is that formed upon a white, samples. These were taken from fourteen profiles, coarse, friable pumice, or volcanic ash, which orig- selected as characteristic of the area. After setting inated from the volcanoes to the south and probably was forth the analytical results we submit the descriptions distributed within the past three millenia. The second taken in the field of the condition of the soil and of the is a red earth, or latosol, of very ancient origin, de- erosion pattern. The two types of evidence are then rived by deep weathering of the basaltic country rock synthesized so as to show the relation between them or, in places, ancient sediments. The third consists and to Feach general conclusions concerning the his- of coluvial deposits, or slope wash from the surround- tory of the area. RESULTS OF PROFILE ANALYSIS Profile I B (3. 6 miles north of the northern cor- which we may hope to obtain in the area. It has been ner of the park, on Highway No. 15) buried by the deposition of volcanic ash for centuries. There are former pastures or cultivation. At The clay content is high and increases in the lower present there is grass, small shrubs, and a few small horizons (as far as one can secure samples). The trees. This is a mature red earth, or latosol, buried organic matter is low throughout. However in this under a thick layer of pumice. buried soil the organic matter has been undergoing The analytical results are given below. slow oxidation since the deposition of the ash, and it Here is the nearest to a mature red earth, or latosol, is noteworthy that any is left at all. Sample Depth Clay Nitrogen Carbon C/N pH No. (in.) (per cent) (per cent) (per cent) ratio 15 147 28.4 0.01 0.14 14.0 6.09 16 156 46.7 0.02 0.27 13. 5 6.28 17 180 56.0 0.02 0.18 9.0 6.27 18 240 58.9 0.03 0.12 4.0 6.89 Profile VI (11.7 miles west of church at western high at the surface and diminishes only gradually with edge of Tepic, on highway to Compostela, at kilometer depth. The C/N ratio is constant throughout, showing stone 19, on the crest of a low ridge) long-term bacterial action. This soil is old and has This is a mature soil on white volcanic ash, or been undistrubed for a long time, very likely since the pumice, which is soft and highly permeable. volcanic ash was deposited. This soil, or something The analytical results are given below. similar, may be taken as the prototype of a mature The clay has been moved downward relatively rapidly soil formed upon this kind of substratum. to the level of 8-10 feet. The organic matter is quite Sample Depth Clay Nitrogen Carbon C/N pH No. (in.) (per cent) (per cent) (per cent) ratio 46 6 0.74 0.52 5.89 11.3 - 47 12 0.62 0.37 3.97 10.6 - 48 24 0.55 0.24 2.88 12.0 - 49 48 0.80 0.14 1.85 13.2 - 50 72 0. 68 0.05 0. 67 13.4 - 51 96 1.15 0.01 0. 14 14.0 - 52 110 15.60 0.00 0.10 - 6.30 53 124 10.70 0.00 0.08 - - Profile XII (5. 3 miles west of the plaza of Jalisco, road cuts, together with two large new arroyos in the on the road to Compostela) slope at right angles to the road, show 20-30 inches of On the slopes of the high range to the northwest, in black "A" horizon over 2-6 feet of lighter colored "B" open grass land, there is a new gravel side road. The horizon, the latter over white ash. This is a relatively [305] 306 ANTHROPOLOGICAL RECORDS mature soil formed on ash, similar to Profile VI. The pH is probably on the acid side of neutrality. The The analytical results are given below. C/N ratio is moderately high and stable to a depth of The analyses show quite high organic matter persist- three feet. This soil resembles Profile VI in that it is ing in quantity to a depth of approximately 6 feet. The an old, relatively undisturbed soil, although perhaps clay is substantially absent above a depth of 3 feet but not so mature. becomes fairly heavy in amount between 4 and 6 feet. Sample Depth Clay Nitrogen Carbon C/N pH No. (in.) (per cent) (per cent) (per cent) ratio 375 3 0.8 0.231 3.87 16.7 - 376 6 0.9 0.176 3.14 17.8 - 377 12 0.8 0.112 1.72 15.3 - 378 24 1.7 0.067 0.96 14.3 - 379 36 5.3 0.033 0.61 18.5 - 380 48 17.1 0.005 0.33 - - 381 72 21.6 0.062 0.35 - - 382 96 7.3 0.004 0.05 - - Profile XIII (10. 1 miles west of the plaza of inches. The underlying, unaltered ash is very deep, Jalisco; or 2. 0 miles west, or below the summit, of the and extends below the road cut. The analytical results Tepic-Compostela highway) are given below. The ash in large measure ceases to be found at the The analyses show relatively high organic matter summit and is replaced by soils developed upon older persisting to a depth of about 4 feet. The C/N ratio is substrates such as red earths and forest soils. How- stable and high to a depth of 4 feet. The clay is small ever, at this point there is a deeply matured soil over in amount to a depth of 3 feet and increases markedly a local area of volcanic ash, at the base of steep slopes. at 4-6 feet. In its general characteristics this soil The upper horizon (black) is 24-30 inches deep, and the resembles Profiles VI and XII. lighter colored intermediate, or "B, " horizon is 20-40 Sample Depth Clay Nitrogen Carbon C/N pH No. (in.) (per cent) (per cent) (per cent) ratio 383 6 0.0 0.106 2.94 27.7 5.6 384 12 1.0 0.120 2.55 21.2 5.8 385 24 1.5 0.081 1.90 23. 5 5.5 386 36 3.3 0.062 1.51 24.3 5.4 387 48 10.6 0.030 0.68 22.7 4.5 388 60 20.6 0.005 0.12 - 4.0 389 72 25.1 0.009 0.14 - 3.8 Profile I A (The same location as Profile I B) within one foot of the surface. It may be deduced that This is the overlying pumice layer, approximately the pumice here once had developed a mature profile, 12 feet deep. but was worn off or eroded very deeply. Subsequently The analytical results are given below. a new weathering cycle has begun and has developed a The pumice has been weathered to about 15 inches rejuvenated "A" horizon. This process must have with considerable formation of both clay and organic begun a fairly long time ago. The antecedent erosion matter. However, there has been no downward dis- may have occurred in the era of the Spanish conquest, placement of clay and the organic matter disappears that is, in the sixteenth century. Sample Depth Clay Nitrogen Carbon C/N pH No. (in.) (per cent) (per cent) (per cent) ratio 10 3 21.8 0.34 4.72 13.9 6.80 11 8 20.7 0.20 2.30 11.5 6. 98 12 15 7.3 0.01 0.27 - 6.56 13 120 3.1 0.00 0.00 - 7.00 14 144 2.1 0.01 0.00 - 7.09 COOK: EROSION MORPHOLOGY AND OCCUPATION HISTORY 307 Profile II (5. 4 miles north of the park on Highway a prehistoric mature profile this area has been denuded No. 15) and thoroughly eroded. There is some recent recovery This is a small road cut next to a level ploughed but it had proceeded only briefly when the road cut was field. The soil is formed on pumice. made. It will be noted also that the C/N ratio dimin- The analytical results are given below. ishes rapidly below the surface, indicating a relatively The clay formation is moderate in the upper hori- brief period during which the organic matter could be zons, the organic matter slight and shallow. Assuming reconstituted. Sample Depth Clay Nitrogen Carbon C/N pH No. (in.) (per cent) (per cent) (per cent) ratio 19 6 22.0 0.10 1.52 15.2 6.10 20 18 17.3 0.05 0.35 7.0 5.90 21 30 4.2 0.00 0.02 - 6.20 Profile III (2. 3 miles north of the park on High- The analytical results are given below. way No. 15) This profile is similar to Profile H. There is con- Here is a small borrow pit west of the road. The siderable clay formation and a moderate degree of parent material is pumice. The "A" horizon is thin, organic matter formation at the surface. Both diminish generally about 6 to 8 inches. The "B" horizon is rapidly with depth, as does also the C/N ratio. The variable, from 6 to 30 inches, forming pockets in de- soil has been profoundly eroded, up to recent times, pressions in the pumice. The surface shows recent and has but lately begun to recover with weathering at use for pasture or cultivation. The vegetation is grass the surface. It must be regarded as a new soil. and low shrubs. Sample Depth Clay Nitrogen Carbon C/N pH No. (in.) (per cent) (per cent) (per cent) ratio 22 6 24.9 0.20 2.47 12.3 5.70 23 12 6.6 0.10 1.03 10.3 5.40 24 24 10.3 0.06 0.74 1.2 5.50 25 36 3.5 0.00 0.25 - 5.10 26 48 1.7 - 0.08 - - 27 84 0.0 - - - - 28 180 - - - - - Profile IV (7. 4 miles north of the park on Highway depth, and a moderate organic content at the surface No. 15 on the crest of a long, low ridge in open pasture which diminishes markedly but persists to a depth of or formerly ploughed land) seven feet. The C/N ratio is relatively constant. The A deep ditch along the roadside to the west shows a profile may be regarded as a mature latosol, truncated light-colored upper horizon ("A") 1-2 feet deep, a at the surface, but with the main body undisturbed. second, darker brown horizon ("B") 2-3 feet thick, The surface erosion is probably due to relatively re- then the "C" horizon, red or brown earth, indefinitely cent overgrazing and cultivation. It is doubtful if this deep. area was ever extensively occupied or used in early The analytical results are given below. colonial or preconquest times. This profile shows an increasing clay content with Sample Depth Clay Nitrogen Carbon C/N pH No. (in.) (per cent) (per cent) (per cent) ratio 30 3 45.9 0.24 1.99 8.3 - 31 6 44.7 0.07 1.37 19.6 4.8 32 12 45.3 0.08 0.81 10.1 5.1 33 24 57.6 0.06 0.45 7.5 5.4 34 36 53.2 0.04 0.23 5.6 - 35 48 67.2 0.02 0.21 10.5 6.1 36 84 72.5 0.03 0.09 3.0 5.3 308 ANTHROPOLOGICAL RECORDS Profile IX (4. 8 miles east of the city limit of Tepic, The clay content here is high at the surface and on the gravel road to the sugar refinery at Puga) increases to very high values at depths of from 10 to This is just over the crest of the ridge separating 13 feet. This argues an old latosol, the upper layers Bellavista from Puga. It is on a north-facing slope. of which have been lost. The organic matter is only The parent material is the country rock, a heavily moderate in extent, even at the surface, but is present compacted sediment, probably derived from a basalt. in small amounts even to the lowest sample. The C/N The weathering is very deep. The upper 2-3 feet are ratio falls through the upper 2 feet but then remains dark reddish brown merging into a light brown horizon. constant to the 5-foot level--as low as analysis can The upper portion may be residual "B" horizon, for pick up nitrogen. The pH slowly rises throughout. even this stratum has disappeared in many places, These findings all show a badly truncated latosol of leaving the underlying light-colored soil ("C" horizon) ancient origin. The mature surface horizons have been exposed. The latter extends without visible alteration destroyed, probably by human occupation throughout a as deeply as it may be followed--10 or 12 feet. This considerable period extending up to the very recent condition is more or less typical of the generally past. This area may have been occupied at the time of denuded and eroded low hill slopes. Samples are from the conquest; it certainly has been occupied steadily the north side of the road cut. since then. The analytical results are given below. Sample Depth Clay Nitrogen Carbon C/N pH No. (in.) (per cent) (per cent) (per cent) ratio 65 3 38.1 0.06 1.16 19.3 4.80 66 8 31.5 0.06 0.97 16.2 4.90 67 15 38.7 0.05 0.76 15.4 4.90 68 24 46.7 0.05 0.58 11.6 5.30 69 36 49. 1 0. 03 0. 34 11. 3 5. 00 70 48 46.8 0.02 0.32 16.0 5.30 71 60 67.8 0.02 0.21 10. 5 5.20 72 84 60. 6 0.00 0. 18 - 5. 60 73 108 54.8 - 0. 15 - 5.90 74 120 77.1 - 0. 18 - 5.60 75 150 65.1 - 0.15 - 5.30 Profile VII (2. 9 miles east of the Tepic city limit The analytical results are given below. on the gravel road to Bellavista and Puga) The clay content is high but diminishes with depth. This is a north-facing slope fairly high on the hill- The organic matter is low but persistent to 72 inches. side overlooking Bellavista. The vegetation is grass The C/N ratio declines. This seems to be the badly and scrub oak. The rock is a deeply weathered shale. eroded residue of an old latosol cut down such that The upper horizon shows approximately one foot of the present surface is toward the lower levels of the brown soil with from 2 to 3 inches of organic matter original profile. at the surface, over a latosol. Sample Depth Clay Nitrogen Carbon C/N pH No. (in.) (per cent) (per cent) (per cent) ratio 54 6 49. 6 0.08 1.09 13. 6 - 55 12 42.3 0.05 0.63 12.6 5.30 56 24 46.1 0.02 0.37 18.5 5.30 57 48 39.3 0.02 0.24 12.0 5.20 58 72 31.0 0.02 0.15 7.5 5.30 Profile VIII (4. 4 miles from the Tepic city limit The C/N ratio is quite constant, indicating the stable on the road to Puga) lower horizons of an old soil, rather than a soil which This is at the crest of a ridge separating Bellavista is rejuvenating. from the low tableland of Puga. The vegetation is Profiles IV, IX, VII, and VIII may be regarded as grass and scrub oak and mesquite. The country rock, old latosols which have been surface eroded and des- a shale, is deeply weathered. There are no visibly troyed intensively and continuously within recent developed soil horizons toward the surface of this road centuries. Particularly the area east of Tepic, ex- cut. The soil generally is a red earth, or latosol. tending to Bellavista basin and Puga, seems to have The analytical results are given below, been subjected to incessant active occupation by Here again appears to be the residue of what was agriculture and cattle since the Spanish conquest. once a mature red earth. The clay content is that Prior to that time it is also probable that native utili- characteristic of the lower "C" horizon. The amount zation of the land was heavy. of organic matter is low despite existing vegetation. COOK: EROSION MORPHOLOGY AND OCCUPATION HISTORY 309 Sample Depth Clay Nitrogen Carbon C/N pH No. (in.) (per cent) (per cent) (per cent) ratio 59 6 17.5 0.06 0.73 12.2 5.50 60 12 19.2 0.06 0.64 10.7 5.40 61 24 15.3 0.03 0.42 14.0 5.40 62 48 14.9 0.03 0.35 10.7 5.50 63 72 19.9 0.04 0.47 11.7 6.00 64 96 9.5 0.00 0.05 - 5.60 Profile V (6. 1 miles west of the church at the washed in from former surface horizons on the upper western edge of Tepic, at the start of the road to slopes, and new formation in situ. Jalisco and Compostela) This deposit reaches back a long distance histori- The exposure is a recent deep gully caused by road cally, although it has no great age in terms of soil construction. It is compound, consisting of about 9 formation. The gully here is well developed, and has feet of slope wash deposited over volcanic ash, or been in existence for some time. The slope wash took white pumice. The uppermost 2-4 feet are discolored a good many years to accumulate. It would be safe to as by accumulation of organic matter and also contain estimate the original erosion and deposition of this numerous potsherds and obsidian chips. The locality soil as dating from the sixteenth century, or perhaps itself is not an archaeological site, hence these arti- earlier. facts must have been washed down from the slope When we examine the soil upon which the slope wash above. was laid we find a pumice soil in which the clay content The analytical results are given below. is moderate to considerable but dimishing greatly at 6 Considering the slope wash profile above 108 inches, feet below the surface. There is very little organic the clay content is variable and erratic. There is no matter, nitrogen, or carbon. The original organic obvious new formation at the surface and likewise no matter characteristic of a mature "A'- horizon may have significant accumulation in the deeper layers. To all been eroded off prior to deposit of the alluvial soil appearances the clay was washed down with the rest above the pumice, or it may have decomposed slowly of the loose surface material and was deposited in a in subsequent centuries. I think it probable that the random fashion above the old soil surface. Since this original condition was one of severe erosion with loss deposition there has been little if any sorting or re- of the top soil. Upon this erosion surface was deposited distribution. The organic matter was carried down the 9 feet of slope wash. If this interpretation is cor- from the slopes above in a similar manner. However rect then we have here an area of ancient occupation, there has been a tendency toward new soil formation certainly dating back to 1530, which denuded the original in the upper strata such that a layer 2-4 feet deep of soil and then caused the washing in of great quantities dark brown or black material has become manifest. of surface soil from the slopes above. That the sweep- The C/N ratio is equivocal since it is not constant ing down of this surface soil postdated the occupation throughout but, on the other hand, does not diminish of the land and the erosion of the original pumice soil sharply with depth as one might expect at the surface is proved by the presence of artifacts in the slope of a new exposure. We may conclude that two sources wash deposit. may contribute to the organic matter, the material Sample Depth Clay Nitrogen Carbon C/N pH No. (in.) (per cent) (per cent) (per cent) ratio 37 6 29.1 0.15 1.96 13.1 5.40 38 24 34.4 0.09 1.48 16.4 5.40 39 48 25.5 0.08 1.25 15.5 5.60 40 72 45.2 0.04 0.37 9.2 - 41 108 20.1 0.00 0.18 - 6.20 42 108 23.8 0.00 0.17 - 6.30 43 120 29.5 0.01 0.14 14.0 6.40 44 144 30.3 0.02 0.20 10.0 6.40 45 180 8.2 0.02 0.25 12.0 6.30 Profile X (9. 35 miles south of the intersection of inches deep, is dark brown to black. The "B" horizon, Highway No. 15 and Avenida Mexico, in Tepic, and on 0-15 inches thick is light brown. The "C" is profoundly Highway No. 15) weathered aggregate, or conglomerate. That this mixed This is a road cut through a low ridge, the latter formation is not of great antiquity in the geological itself being on a long, gentle, north-facing slope down sense is attested by the presence of potsherds and obsid- from the mountain to the southeast. The vegetation is ian flakes throughout the upper two feet of soil, as well grass, mezquite brush, and scattered corn fields. The as frequently on the surface for some distance around. bedrock is a solidly aggregated or consolidated deposit It appears as if the mixed detr~itus had been moved down of stones, rocks, and boulders -- almost a conglomerate, from the higher slopes of the mountain within relatively the soil is variable. In general the "A" horizon, 0-15 recent times and had since been compacted into an 310 ANTHROPOLOGICAL RECORDS incipient conglomerate at the same time that weathering as thoroughly matured as Profiles IB and VI. The was active from the surface downward. c,omposition of the parent material suggests a down- The analytical results are given below. ward movement of soils and rock from the overlying The clay content is quite high at the surface, but mountainside, perhaps caused by some natural cata- nevertheless increases perceptibly to a depth of 18 clysm. However the presence of potsherds in the inches, below which it may fall off somewhat. The or- upper 20-30 inches of this material is sure indication ganic carbon and nitrogen are moderately high at the of prior human occupancy of the region. If this is true surface and diminish to a depth of 48 inches, although then we may postulate some type of agricultural activ- organic matter is still present. The C/N ratio is ity as the agent inducing the deposition of the existing nearly constant. All these features imply a relatively pseudoconglomerate on this spot. If there was such new soil formation but yet one which has progressed human occupation then it must have been present in to a somewhat advanced stage, at least sufficiently for very early times in order for the newly formed deposit the downward translocation of clay to be evident and for to form a fairly well-developed profile. In terms of the C/N ratio to stabilize to a considerable depth. This historical events it must have long antedated the Span- is clearly a much more advanced condition than is seen ish conquest. in the pumice Profiles I and III, although by no means Sample Depth Clay Nitrogen Carbon C/N pH No. (in.) (per cent) (per cent) (per cent) ratio 76 3 23.8 0.15 2.04 13.6 5.20 77 6 30.2 0.13 1.81 14.0 5.00 78 12 35.8 0.09 1.14 12.7 5.20 79 18 38.0 0.08 0.74 9.3 5.20 80 24 37.0 0.07 0.77 11.0 5.20 81 36 29.9 0.06 0.58 9.7 5.40 82 48 34.0 0.02 0.28 14.0 5.40 Profile XI (11.0 miles south of Tepic on Highway time the clay increases to a maximum at 18 inches of No. 15, and 1.65 miles south of Profile X) depth, then diminishes; the nitrogen and carbon occur The vegetation is grass, mezquite, and other shrubs. in considerable quantity at the surface and diminish be-8, The upper 0-18 inches of this soil is brown, with a layer low. The nitrogen becomes too slight for analytical de- of loose stones on the surface, indicating recent wash- tection below 70 inches but the carbon persists. The ing out of the finer surface material. Underneath is a C/N ratio is quite stable to the same depth. very deeply weathered layer of old rock, probably a This formation appears to be the remains of a very sandstone or shale. Samples were taken from the bank old soil which was disturbed, with possible loss of the west of the highway. upper horizons, by erosion. Thereafter recovery oc- The analytical results are given below. curred and has reached approximately the same point This soil does not seem to be an accumulation of as is demonstrated in Profile X. This condition like- material washed down from higher elevations, but wise implies a very ancient, but intensive occupation b rather a native rock weathered in place. At the same man. Sample Depth Clay Nitrogen Carbon C/N pH No. (in.) (per cent) (per cent) (per cent) ratio 83 3 21.1 0.13 2.03 15.6 5.80 84 9 30.0 0.10 1.24 12.4 5.40 85 18 32.0 0.06 0.93 15.5 5.30 86 30 30.0 0.03 0.48 16.0 5.60 87 50 23.1 0.02 0.26 13.0 5.60 88 70 22.1 0.01 0.21 21.0 6.00 89 90 24.2 0.00 0.25 - 6.00 90 110 17.8 0.00 0.29 - 6.00 Summary of ProfileAnalyses. -From the preceding structed. Thus we can place preconquest populations descriptions it is evident that on the Tepic plateau a just west of the town of Jalisco, and some ten miles series of soil conditions may be recognized. Thus one southeast of Tepic. Continuous and intense occupation may distinguish mature, relatively unaltered profiles, probably dating back to the conquest can be ascribed either on the surface or buried under volcanic ash. to the Bellavista basin. Then there are soils which have been disturbed but It is of course tedious, expensive, and difficult to which show advanced recovery, soils which represent take series of samples from a large number of points; erosion within historic times but which have begun to along roadsides and riverbanks, or to put down deep recover, and finally soils which even recently have borings eve.ry few hundred feet, thereafter to carry been denuded. The past history at the points where the several analyses on each sample. Such a procedure,. profiles were examined can be to some extent recon- however, is probably, unnecessary. Once the chemici COOK: EROSION MORPHOLOGY AND OCCUPATION HISTORY 311 and physical properties of a certain soil type are es- horizons, the depth and extent of weathering of slope tablished these features can be associated with the washes and other alluvial deposits. The two approaches visible appearance of an exposure, and deductions of laboratory analysis and field observation should thus ,,made concerning the past experience of the soil. complement and reinforce each other, such that the use Therefore, in conjunction with the distribution of of either or both of them should lead to the same ulti- substances in the profiles, as just described, the ero- mate conclusions. An experiment in this type of sion patterns as directly observed should be considered. synthesis has been attemped in the Tepic area. The The erosion patterns should take into account the degree analytical results have been set forth above. We now and style of gully formation, sheet erosion as demon- present the field observations covering the same local strated by the depth of the topsoil and its constituent areas. RESULTS OF FIELD OBSERVATIONS In order to set forth the field data we follow the high- of alluvial slope wash from the hill to the west. This 'ways as radii emanating from Tepic as a center and wash contains pumice almost entirely, characterized by interpolate the data from profile analysis at the locations the presence of numerous large stones (any stones where they occur. This procedure differs somewhat in larger than very small rock fragments are rarely en- method from the previous mode of description but con- countered in untouched pumice deposits). The deposit solidates all information into a single account. indicates long-continued heavy erosion of the hill slopes Field data A. We start with the Compostela road, directly to the west. with the church (near the barracks) at the western edge 2. 0-2. 2 miles. Several exposures show the dark of Tepic as the zero point. In condensed form the field surface horizon reaching depths of 3-12 inches, but observations are as follows. in many spots being absent altogether. At such local- 0. 15 mile. There is a recent cut in a knoll or ridge ities the surface of the ground is formed by the white, which exposes a dark to light brown layer of soil 6-18 unmodified volcanic ash. This is the "C" horizon, or inches deep over unaltered white pumice, or ash, The the parent material, as one wishes to regard it. vegetation is mixed grass, mezquite, and weeds. Devastating land use is clearly evident. 0. 2 mile. This is aroad excavation. The parent 2. 3 miles. Here is another low ridge crest with material is pumice. There is a black, top horizon, approximately 3 inches of dark surface layer. 0-3 inches deep which may be designated "A, " and an 2. 4 miles. This is the foot of a steep slope facing underlying brown horizon, 2-6 inches deep which may east. There are about 5 feet of slope wash derived be considered "B. " Both this and the previous locality from the soil formed over pumice. The layer of indicate intensive land use continuously for a long black, organic matter, approximately 6 inches deep, period, long enough to dissipate the mature profile and includes stones from the wash, which are apparently reduce the organic matter to those few inches which can derived from the ancient soil underlying the volcanic be formed from contemporary vegetation, subject to ash deposit. There is no alteration due to weathering incessant grazing and agriculture. below a depth of 6 inches. Two pieces of obsidian and The horizon geometry is variable and produces what one potsherd were found in this slope wash. It was once may appear confusion in the descriptions. The super- an inhabited area; it was washed out; it has weathered ficial, humus-bearing layer may in some places be since to a depth of from 3 to 6 inches. All this indi- clearly differentiated into a definitely black upper hori- cates a relatively ancient occupation, followed by use zon below which the black gives way to dark and then of the land more or less continuously since then, light brown strata, or there may be a continuous down- presumably by the now resident population. ward bleaching of the black, or dark brown topsoil into 2. 6-miles. The crest of a low spur. The dark sur- the unaltered white ash deposit. In the latter case it face layer shows some differentiation such that the "A" is impossible to distinguish sharply between an "A" and horizon is 0-6 inches and the "B" horizon 0-6 inches a "B" horizon. These terms are used therefore only deep. in a purely descriptive sense, and only where an appar- 2. 9 miles. Here is an arroyo which could be clearly ent differentiation could be perceived by eye. observed in the summer of 1958. In the summer of 0.4 mile. Here is a healed gully. It is an arroyo 1960 it had been covered by new shrubs and brush such which was once cut to a now undiscoverable depth, but that the banks could not be seen. This is an indication hich has since been filled in and become overgrown of the rapidity with which new bank exposures become by grass and brush. The cutting took place some time overgrown with vegetation in this climate. ago for there has been opportunity to refill and become 3. 2 miles. An old arroyo, 5-10 feet deep and 100 covered by vegetation. Relatively recently the area has feet across overgrown with grass and brush, except been actively cultivated. for an active channel at the bottom perhaps 5 feet wide. 1. 5 miles. Another arroyo of the same character. 3. 5 miles. Here is a gully, 10 feet deep, primary, 1. 6 miles. A new bank cut in a small ridge shows a and of recent origin. It exposes an alluvial deposit of dark horizon, 6-12 inches deep, over white ash. silt and a little gravel. There were no artifacts obser- 1. 7 miles. New road excavations. Several expo- ved. The deposit is probably quite ancient. The zone sures showing 3-12 inches of dark brown soil over ash. of formation of organic matter is from 1 to 2 feet deep, * 1.8 miles. Relatively undisturbed pumice shows an but indicates a continuing loss of top soil. "A" horizon 2-12 inches deep and a "B" horizon 6-12 4.2 miles. Here is the plaza of the town of Jalisco. inches deep. Denudation of the surface, however, is The stretch from Tepic to Jalisco shows consistent sttested by the accumulation of stones and cobbles on severe denudation of the upper soil horizons together the surface and in the upper few inches of the soil. At with numerous arroyos, or gullies developed by the a point nearby a recently cut arroyo discloses 2-8 feet cutting of minor streams. Some of these arroyos 312 ANTHROPOLOGICAL RECORDS expose deep deposits of slope wash from the hills to the horizons found at the surface of ancient slope washes and northwest, at least one of which contains artifacts. other coluvial deposits which also contain relics of These conditions imply relatively dense settlement for human occupation. a long period. We know from documentary sources that 8. 6 miles. The soils here are weathered pumice, or this has been so since the time of the first entry of the an older brown earth, or a forest soil derived not from Spaniards in the early sixteenth century. pumice but from the underlying bedrock. The upper 4. 9 miles. Here is a fresh cut at the foot of a slope. horizons are variable but tend to be deeper than those The dark superficial layer is 6-12 inches deep. described hitherto, ranging from 6 inches to 3 feet for 5. 1 miles. This is a relatively flat area at the foot the combineci 'A" and "B" horizons. of hills to the west. Borrow pits to the west of the road 9. 5 miles. At this point there is a new gravel side and stream cuts to the east expose 10 feet of consoli- road. The soils exposed show consistently a dark "A" dated slope wash, composed of silt and gravel. There horizon 1-2 feet deep, and a brown "B" horizon 1-6 is an arroyo which, above the road, is 10 feet deep and feet deep. This is the location of sample Profile XII, 100 feet across. Its profile discloses two or more previously described. small terraces, indicating successive stages of forma- 10. 2 miles. Here are numerous exposures of the tion. No artifacts were found. The weathering zone at volcanic ash, or pumice, deposited over older red and the surface is several inches in depth. Here again is brown soils, some of them reaching great depth. ancient occupation, surface erosion, deposition, and 10. 8 miles. Here is a fresh exposure of a pumice incipient profile formation prior to the cutting of the soil on a ridge crest, with grass and brush vegetation. recent arroyo. The dark brown to black "A" horizon is 1 foot, the light 6. 1 miles. This is the exposure described in detail brown "B" horizon 3 feet in depth. under the heading Profile V. The indications were of 11. 1 miles. There is a wide arroyo to the south. It an accumulated wash, similar to that seen at 5. 1 miles is 15-20 feet deep, with vertical sides, and is recent, (see above), but containing artifacts and with a forma- showing a deep, alluvial soil, probably derived from tion of clay and organic matter reaching to a depth of the mountainside to the northwest. The black, upper more than 1-2 feet. The formation of the black organic horizon is from 3 to 5 feet thick. This accumulation layer was estimated as representing perhaps four must be relatively ancient, and must have been practi- centuries. Previously there had occurred severe cally undisturbed to have permitted the formation and erosion of the original land surface, with loss of clay retention of such a deep weathered layer. Land use and organic matter. The analytical data thus support within the past few centuries must have been light. the hypothesis that there was occupation and intensive 11.5 miles. Here are several deep arroyos, recenti land use long prior to the advent of the Spaniards. cut, and clearly associated with farming operations, the 6.6 miles. Here the exposures become quite variable. base for which is a group of nearby ranch houses. The Some show deep, dark horizons at the surface, some soil destruction which is now proceeding is serious. show the dark surface horizons to be very shallow. 11. 7 miles. Here was taken the sample series de- 6. 9-7. 0 miles. Here is the crest of a knoll, or ridge, signated Profile VI, previously described. It was the parent material being still pumice. At the crest the found upon analysis to be a mature profile formed on dark superficial layer runs 6-12 inches deep. At the pumice, with little sign of agricultural disturbance. foot of the lateral slopes "A" and "B" horizons are In this area there are numerous similar exposures all distinguishable, each ranging from 12 to 18 inches in indicating maturity and only superficial erosion, if any depth. at all. Evidently we have reached the limit of former, 7. 5 miles. Here the road is cut along the contour or ancient, habitation on the east side of the low range line of a gentle slope. In general the "A" horizon is separating the Tepic basin from the sloping plain which 12-18 inches deep and the "B" horizon may reach 4-5 extends from Compostela to the sea. feet in depth. The former horizon in spots may become To approximately nine miles southwest of Tepic, as deep as 24 inches, and is black soil. The latter is through Jalisco, the soils demonstrate intensive, unin- weathered, brown ash. The underlying parent material, terrupted erosion, and hence land use. The evidence o or "C" horizon, is white ash. this condition is seen in the almost complete removal 7. 8-8. 0 miles. There are several profile exposures of the original top soils and the deposition of silts and over pumice. At this point the darker layers on the gravels at the foot of the slopes. The latter have surface are again very shallow. Of particular interest weathered only superficially. It is likely therefore is a new (in 1960) exposure cut in a pre'dxisting cut. that this nine-mile strip has supported a heavy popu- There is a steep bank, some 30 feet high. This bank lation which occupied and used the land intensively sinc was excavated in approximately 1950 or 1951, through a period before the Spanish conquest. Beyond the south' raw, unweathered white ash. The new exposure shows western limit of this strip there probably was little and about one half inch of black organic matter formed perhaps no sedentary occupation. Along the strip it directly over clear white ash. This gives an idea of may be concluded that there was an unbroken line of weathering rate. There is no intermediate brown or villages and agricultural operations. yellow zone. Hence the black layer is not residual but Field data B l.--Here we start north on Highway No. has been produced de novo on the unaltered parent 15 from Tepic, beginning with the northeast corner of material, within a time span of certainly no more than the park, just west of Highway No. 15, in Tepic. ten years. The deepening of such an organic zone is not 0.0-0.5 miles. Approximately the first half mile a linear function of time, but its rate of formation dim- consists of built-up city. inishes progressively. Thus one half inch in ten years 0. 8 mile. Here is a bank exposure on flat ground, might indicate a full inch in from twenty to thirty years with 2-3 inches of dark surface layer over ash. and 2 inches in a century. Several inches would reason- 1. 0 mile. An arroyo has cut through flat ground.A ably imply several centuries of formation. This sug- There are 3-6 inches of dark surface horizon over ] gestion is in fair agreement with the depth of weathering white ash. COOK: EROSION MORPHOLOGY AND OCCUPATION HISTORY 313 2.0 miles. This is the junction of the road to Jalco- deep, the "B" horizon dark brown, 2-3 feet deep, the cotan and Mecatan. At this point there are two visible "C" horizon red, weathered rock. This is Profile IV, exposures. which was characterized as a truncated latosol, with One is a borrow pit to the east of the road, at the only the surface disturbed. This disturbance seems edge of a cultivated field. The parent material is ash, to have been recent, and is consistent with the persist- or pumice, over a weathered latosol. The weathered, ence of well-developed horizons. There is little evid- humus-containing surface layer is from 12 to 16 inches ence of ancient disturbance and more recent recovery. deep. The other is a bank exposure with 6-8 inches of 7.8 miles. Here is a deeply decomposed conglom- dark surface horizon. Both are in flat land. erate with the bedrock softened to a depth of 30 feet. The 2. 3 miles. There is a small pit, or excavation, in "A" and "B" horizons together vary locally from 2 to level ground, to the west of the highway. The "A" 5 feet in depth. There is some recent surface erosion horizon is 6-8 inches, the "B" horizon 6-12 inches, or but the soil underneath is not disturbed. deeper in depressions or pockets. Here is Profile III, Summary of field data A and Bl.--To summarize the described above. The analyses indicate a newly form- descriptions given above, it may be stated that along ing soil on an old erosion surface. The depth of the Highway No. 15, north of Tepic, surface erosion is organic zone implies a substantial recovery from severe to a point approximately 4 miles north of the earlier land use. city. Beyond this line the evidence indicates an early 2. 4-2. 6 miles. To the east of the highway there is occupation which produced serious loss of the mature a large new excavation used as a pit for getting road upper horizons but from which significant recovery has material. One series of exposures shows the dark taken place. These signs of early occupation are lost upper, "A" horizon to be 0-6 inches and the lower, at about 7 miles to the north of the city, beyond which brown, "B" horizon 3-6 inches. Another exposure appear deep latosols which may be superficially denuded, shows the dark surface layer to be 8-12 inches deep. but the lower levels of which seem to be relatively well 3. 2 miles. A fresh excavation on the side of a steep preserved. The superficial erosion may be referred to slope shows the dark surface layer to be 3-10 inches, agriculture and grazing within the past several decades. mostly 3-6 inches deep. In other words, at one time habitation, or at least 3. 3 miles. A bank exposure on a gentle south-facing agricultural use, of the land extended to the vicinity of slope. The "A" horizon is about 6 inches, the "B" seven miles north of the present city. It then contracted horizon 6-12 inches, over 2-3 feet of consolidated to an approximate four-mile limit, and within this limit sand, which in turn is over an ancient red earth. has persisted probably in varying degree until very 3. 6 miles. Here is pumice over red earth. This is recently. where Profiles I A and I B were taken. The combined Field data B2. - -At 2. 0 miles from Tepic a newly "A" and "B" horizons are about 15 inches deep. The paved road branches west from Highway No. 15, to run analyses indicate an early period of denudation followed to the low-lying banana-producing towns of Jalcocotan by a period of recovery. and Mecatan, near the coast. The distances are es- 4. 3-4. 4 miles. Here are exposures several years timated from the junction. old. The dark surface zone is 6-12 inches deep. 0.4 mile. A road cut; 0-12 inches of dark surface 4. 7 miles. Here may be obtained a view of the layer over white ash. considerably eroded hill slope to the west. There is 0. 5 mile. A road cut; 6-12 inches of dark surface generally ash over red earth and the ash exposures ap- layer over ash. pear to have about one foot of dark surface horizon. The 1.0 mile. A borrow pit; 3-6 inches dark layer. valley to the west descends from the road to run just 1. 7 miles. A bank at the foot of a slope. The dark north of the village of Puchon. The erosion in this surface horizon is 12-15 inches deep. valley is severe. There are many branched arroyos 1. 9 miles. A similar exposure at the foot of a slope; running toward the creek in the valley bottom. The the dark layer is 12-18 inches deep. plowing on the steep lateral slopes exposes white ash 2. 5 miles. At the foot of a slope there are 12-18 through the darker surface layer. inches of black organic horizon over 24-36 inches of 5.4 miles. Here is a road cut adjacent to a ploughed yellow brown weathering zone, or "B" horizon. This field. The soil is derived from pumice and the norizons is over white ash. are quite deep: "A" is approximately 18 inches and "B" 2. 7 miles. Here is a borrow pit dug in level ground. is nearly the same. Profile II was sampled here, a There is a dark horizon, 6-24 inches deep over ash. profile which shows a considerable accumulation of 3.0 miles. On a slope there is a dark upper horizon, clay and organic matter in the upper levels but no down- 12-18 inches deep over 12-24 inches of brown weathered ward translocation. Hence this appears to be a newly and accumulation horizon, the latter over white ash. reconstituting soil over an old erosion surface. 3. 9 miles. At the crest of a small ridge, near some 6. 9 miles. Here is a deep red earth with no over- farm houses, there is a 6-12 inch dark layer over ash. lying deposit of volcanic ash. The top soil is brown, 4. 2 miles. Generally on the hill slopes there is 1 roughly 3 feet in depth. foot of dark "A" horizon, and 1-2 feet of brown "B" 6. 6-7. 4 miles. . The red earths in ditch exposures horizon. uniformly show 1-2 feet of dark brown "A" horizon and 4. 2 miles to 5. 7 miles. On all sorts of terrain and 1-2 feet of pale brown or yellowish "B" horizon over a soil types there is characteristically a dark upper hori- dark red "C" horizon. There is little if any admixture zon 6-18 inches in depth and a variable underlying "B" of stones or rocks either at the surface, or above the horizon. zone of weathered bedrock. 6.0 miles. Here is a black surface layer 1 foot in 7. 4 miles. This is a very deeply weathered, ancient depth over the weathering or accumulation layer 3 feet so8il, a red earth, on the crest of a long, low ridge, deep, both over white ash. t with open pasture. There are unweathered rocks on 6. 3 miles. This is the summit of the divide between ithe surface. The "A" horizon is light brown, 1-2 feet the Tepic basin or plateau and the slopes which descend 314 ANTHROPOLOGICAL RECORDS to the coast. For the first two miles to the west there slope to the east. A red earth is weathered to a depth is oak and pine forest. The soils are still formed prin- of many feet but with no visibly differentiated horizon cipally over the typical white ash or pumice, but are at the surface. frequently much deeper than on the eastern side of the 4. 4 miles. The crest of a low ridge; grass, brush, divide. The dark brown or black top soil, or surface and scrub oak vegetation. Deeply weathered rock. No horizon is 1-2 feet deep and the light brown "B" hori- visible horizon developed at the surface. Here is zon may reach 4-10 feet in depth. At this point the Profile VIII. The clay content and the organic content ash ceases and is replaced principally by sedimentary are both low throughout and confer the appearance of rock. The moisture and temperature rise and the the mere residue of a former latosol. There is little weathering goes very deep. if any evidence of rejuvenation, despite the fairly pro- With regard to habitation history the first two miles lific plant growth. There has been continuous occupa- of this road displaythe same characteristics as are tion and denudation for a long period. seen in all areas close to Tepic, a verythintop soilover 4. 6 - 4. 7 miles. Similar bank exposures. unweathered parent material. Habitation here has been 4. 8 miles. A deep bank cut at the foot of a gentle long and intense. The next three miles show much less slope. Here is Profile IX. A residual "B" horizon soil wear although the horizon development is by no may be present, and there is a moderate accumulation means mature. It is suggested that there has been of organic matter at the surface. Otherwise the long-continued but perhaps intermittent and relatively conditions approximate those seen in the profiles des- light land use. There is no evidence of intensive cribed above. ancient occupation followed by an interval of abandon- 5. 3 miles. A shallow road cut. There is a thin ment. At and beyond the summit of the divide there is layer of brown soil on the surface at certain points, no indication of more than sporadic and casual defor- none at others. This is perhaps recent soil formation, estation and stock running. Furthermore, beyond the aLready removed where exposed to tillage and grazing. summit we enter an entirely different ecological pro- 5. 5 miles. This is another exposure similar to the vince where the criteria of soil erosion used near preceding one. Tepic can no longer be applied without modification. The area represented by mileage points 2. 5 to 5. 5 Field data C. --The area east of Tepic is traversed miles has been intensively populated for a long period. by a gravel road to Bellavista and Puga, along which The entire surface soil has been lost and recovery has are numerous profile exposures. Except near the town scarcely started. It is probable that this process of there is no pumice. The soils are all deeply weathered soil destruction has been going on since before the red earths. The zero point for distances is the city sixteenth century. The analytical data previously limit, at the eastern end of the bridge, at the eastern given indicate likewise that we have here a series of edge of the city. The road runs over a flat land and at soils which have been consistently exposed to erosion about 2. 5 miles crosses a low divide, or ridge. Close for a very long time, and from which "A" and "B" to the town are several exposures of pumice, all having horizons have been removed, with little opportunity very shallow surface horizons. for recovery. 2. 7 miles. Over the crest of the ridge; the vegeta- Field data D. --Starting from the junction of High- tion is brush. The surface is bare red earth. A bank way No. 15 and Avenida Mexico in Tepic, the former cut shows a much weathered shale at least ten feet deep, road may be followed southeast. but with no visibly differentiated horizons at the surface. 1. 3 miles. In the flat land south of the city an 2. 9 miles. This is also a north-facing slope, with excavation discloses several feet of black, alluvial grass and scrub oak. The road cuts show approximately deposit, containing numerous fragments of unaltered one foot of brown soil with 2-3 inches of dark organic pumice. Hence it is clear that this alluvium was matter at the surface, over the deep "C" horizon. formed after the pumice was deposited. Its source Here is Profile VII. The analyses show a high clay was the hill slopes to the west and northwest. content slowly diminishing with depth. The organic 2. 5 miles. At the railroad overpass there is a pure matter is appreciable at the top but drops off within pumice formation with a surface horizon of 6-30 inches 24 inches to a low value which persists to 72 inches. of dark soil. This soil is probably ancient, since it is These findings are interpreted to signify an old latosol overlain extensively by an alluvial deposit derived which has been swept clear of the original "A" and "B" apparently from the hills both to the east and the west. horizons. Little if any rejuvenation of the clay is to be 3. 1 miles. The hills directly east, at 2-3 miles, found, and the moderate increase in organic matter at appear to show an ancient arroyo pattern, now wholly the surface implies a brief, recent period of plant healed, but still marked by the lines of vegetation. On growth. The over-all picture is of a very heavy erosion the other hand, the more distant hills to the east and cycle several centuries ago with sufficient land occupa- southeast at 3-8 miles have perfectly smooth contours, tion in the period since then to prevent much recovery. especially on the lower slopes. The arroyo pattern This view is supported by the entire appearance of the indicates former land use and erosion in the area terrain for several miles northward, where much sur- indicated. face exposure and gullying is to be seen. 3. 0-7. 0 miles. The plain reaches to the beginning 3. 2 miles. A bank is cut on the same north-facing of a long rise at San Cayetano (6. 7 miles). There is a slope. There is one foot of loose soil over weathered black, deep alluvium covering the original land surface. shale. This soil contains many unweathered stones, a 7. 1 miles. Here is a cut made at the original con- good indication of the washing away of the finer soil struction of the highway. It shows what appears to be from the surface, leaving the heavier components be- the former weathered "C"' horizon of the bedrock, con- hind. solidated with slope wash and mixed with red soil at the 4. 0 miles. Several profiles show nearly complete surface. There are numerous unweathered stones mixed loss of the upper horizons. with the soil in the upper portion. Also there are a 4. 1 miles. The land is here quite flat, with a slight number of potsherds in the top one foot of the bank, as COOK: EROSION MORPHOLOCGY AND OCCUPATION HISTORY 315 well as many sherds on the surface of the ground in the washed in from the banks. vicinity. The surface sherds of course demonstrate a 11.0 miles. Here is a low ridge, or spur, with the former human domestic occupation, and their presence same vegetation. There is brown soil 0-18 inches in the upper horizon indicates that the occupation pre- deep, with a rock mantle, grading into a deeply weath- ceded the formation of the soil from the downward ered old rock, probably sandstone or shale. Here was washing of the slopes above and to the east. taken the series from Profile XI. The analytical data 7. 3 miles. To the west of the road, at the foot of a suggest an ancient occupation, with resulting denudation short, steep slope, and 100 feet from the highway, is of the then mature surface horizon. This would have an extensive new excavation. There are exposed about been followed by reconstitution of the exposed lower 5 feet of black slope wash, or alluvium, mixed with horizons with the formation of the present brown upper small stones and rocks. Among them are a few pot- layer. The latter appears to have been washed and sherds in the upper 1 foot of the profile. partially truncated in relatively recent times, pro- 8. 1 miles. The bank of a road cut shows 1-2 feet ducing the existing rock mantle. of weathered topsoil, brown and mixed with stones, There are no potsherds in this profile, since this over weathered volcanic rock. Potsherds are numer- soil has weathered in situ and there has been no de- ous to a depth of at least 1 and perhaps 2 feet. position of material from the slopes above, as there 8. 4 miles. The road has now climbed the first, was at the area of Profile X. lower slopes of the southeastern mountain. At this 11. 1 miles. Here are 1-2 feet of mixed rock and point there is a low ridge crest, or spur, covered with red soil over the weathered bedrock. There are pot- grass or low brush. The country rock is a hard sherds in the upper few inches of the bank exposure, basalt. There are 0-3 feet of brown, undifferentiated making it clear that the soil-rock mixture has been soil over relatively unweathered rock. This soil car- moved in from above. ries numerous inclusions of undecomposed rock frag- 11. 1 - 12. 7 miles. This stretch, which ends at the ments. Apparently there has been considerable recent summit of the divide between the Tepic basin and the weathering ot an old erosion surface on the basalt, ac- valley to the south, continues to show the same types celerated perhaps by some accumulation of wash from of soil exposure, mixed rocks and soil over the weath- the slopes above. In any case considerable time must ered and disintegrated bedrock. have elapsed since the erosional loss of the original profile. Summary of Field Observations. --As will be noted 9. 0 miles. Here is a small arroyo, shallow, filled from the foregoing descriptions, the long, lower slope with trees up to ten feet high. There is no recent of the high mountain to the southeast of Tepic seems gullying. The soil is red-brown and rocky, formed to have once been the site of intensive human settle- directly over slightly weathered basalt. ment. Erosion removed the mature upper horizons. 9. 1 miles. Here is a new road cut. There are 2-3 This period was followed by one of vacancy, during feet of mixed rock and red soil over the weathered which there was time for the redevelopment of a bedrock. Sherds are seen to the depth of 1 foot. weathering surface horizon, containing clay and or- 9. 35 miles. Iere is a road cut through a low spur ganic matter, to a depth of roughly 1-2 feet. This on the long, gentle, north-facing slope from the moun- newly weathered zone, as indicated by Profiles X and tain to the southeast. The parent material is a con- XI, was formed upon the old truncated soil surface, or glomerate, or consolidated aggregate, derived probably upon a mass of detritus moved down by water action in very ancient times from the southeastern volcano. from the higher ground to the east. If the new parent The "A" horizon is 0-15 inches, dark brown, the "B" material was such detritus, potsherds from the pre- horizon is 0-20 inches. There are potsherds and ceding occupation are still to be found buried in it. If obsidian flakes throughout the upper portion of the soil. the old erosion surface has been reweathered there Here is Profile X. The analysis of that profile indicated will be no sherds. a relatively new horizon formation but one in which the Conditions vary from one locality to another with weathering and associated movements. of soil compon- respect to the degree of preservation of the secondary ents had progressed appreciably. More than a few surface horizon, for the land has been used for agri- decades, or even centuries, would have been required, culture and stock raising at least to a moderate degree even in the warm, damp Tepic climate, to reach the in modern times. At a few spots, such as where Pro- present stage of development. The erosion cycle, files X and XI were taken, the new soil is clearly the recovery from which is displayed in this profile, evident as a light or dark brown superficial layer over- must have occurred at least a thousand years ago, and lying the invariably red soil which grades into frag- perhaps longer. mented or disintegrated bedrock. At most places, 10.0 miles. This is the crest of another spur from however, the existing surface is composed of the red the southeastern mountain. The vegetation is grass soil, mixed with sharp, unweathered rock fragments, and low brush. There is a red-brown surface soil ranging from the size of a pea to several inches in over deeply weathered rock, resembling the exposure diameter. Such a condition bespeaks an accumulation previously described. Potsherds were found in the from an old erosion surface, higher up the slope, which banks of the road cut. has been denuded a second time by modern human 10. 6 miles. Here is a broad hollow between spurs operations, or which has never been permitted to de- from the eastern mountain. It is overgrown with brush, velop a new soil on its surface. Of the two possibilities but the banks are very gently sloped and the bottom is the former is the more likely on historical grounds, relatively flat. There is a small existing arroyo in since there is no mention of villages to the southeast of the bottom which appears to have been cut during Tepic in the sixteenth-century documents, and there is recent decades but which shows at least partial recovery, no reason to suppose that this area was reoccupied On the bottom of the hollow is red-brown soil, mixed intensively until at least the end of the eighteenth cen- with stones. Both soil and stones probably have been tury. 316 ANTHROPOLOGICAL RECORDS CONCLUDING DISCUSSION The conclusions based upon examination of soil conditions conform in a general way to what is known The analysis of soil constituents combined with of the history of the region from documentary and examination of erosion patterns gives us an initial archaeological sources. Sites established by pottery- insight into the habitation history of the Tepic area producing cultures prior to the conquest have been which may eventually be supported by evidence derived described on the lower slopes of the mountains facing from cultural remains. Three principal occupation both sides of the valley. The earliest Spanish invaders cycles may be suggested. The first is ancient, pro- (as seen in the accounts of the expeditions by Francisco bably long antedating the Spanish conquest, and can be Cortez and Nuino de Guzman) found large villages or allocated to at least two areas; the lower slopes north- towns at Tepic and Jalisco. Secondarily Jalcocotan east and southwest of Jalisco and an ecologically sim- and Mecatan are mentioned, although Compostela was ilar region to the southeast of Tepic, beyond the a Spanish settlement. No villages appear to have village of San Cayetano. Both spots were inhabited by existed in the sixteenth century to the southeast of pottery-producing peoples. They no doubt had agri- Tepic, a fact which is in conformity with the absence culture, since they were responsible for profound of extreme, recent erosion in that vicinity. After the erosion of the native, mature soil profiles and for ex- conquest the population declined almost to the vanish- tensive alluvial deposits at the foot of the lower slopes. ing point but rose again in the seventeenth and eighteenth The second occupation, which to be sure may have centuries. At this period, or later, the Bellavista- been simply a phase of the first and continuous with it, Puga area was developed with sugar cultivation and a was in existence at the beginning of the sixteenth cen- cotton mill. The contemporary severe surface tury when the country was overrun by the Spaniards. erosion near these pueblos, with profound loss of top It covered, beyond much doubt, the lower slopes as soil, therefore dates probably from the past one or far as a few miles southwest of Jalisco, the escarp- two hundred years. It continues to the present day. ment country to perhaps five miles north of Tepic, and The region north of Tepic to and beyond the junction of the irregular hill and plateau area several miles to the road to Jalcocotan seems to have been less inten- the east and northeast. The bottom lands to the im- sively occupied in historical times, although it is now mediate south of Tepic should also be included, for suffering considerably. such a fertile stretch of land would not be neglected by I include a sketch map indicating the suggested pos- even a primitive culture. The presence of local re- sible areas of land use near Tepic (see fig. 11). covery of the surface soil horizon on the lower mountain slope to the southeast indicates a rather long period of N I Ist phase quiescence at this point and therefore relative absence t ---. r 2nd of use during the second phase of occupation. Where . 3rd .. it was intense this phase was accompanied by the gen- eral removal of all topsoils and the formation of numer- Jolcocotan 7 - . . ous and deep arroyos. The third stage extends from the mid-sixteenth *. Bellovista century to the present time, and includes most of the area mentioned in connection with the second stage. The effect of milpa agriculture and the livestock intro- ,, Topic - duced by the Spaniards has been to reduce the topsoil Puga about as fast as it is formed and to maintain very shal- low horizons of organic matter on both white ash, or iF pumice, and red earth. Locally the strain on the soil , Son A* has been variable. In a few relatively rare instances $ 4aIsco o Cayetano 9 the former erosion surface, or accumulation of the wash detritus from the slope above, has had an oppor- tunity to initiate the formation of a new soil. More commonly there has been serious truncation or com- plete elimination of the upper horizons, leaving unmodified parent material, or 'C" horizon, at the surface of the ground. Simultaneously many active primary arroyos are noted, some of the older of which may display the beginning of recovery. Figure 11. Areas of settlement near Tepic. VIII. PROFILE ANALYSIS AND EROSION MORPHOLOGY IN THE VICINITY OF GUADALAJARA The modern city of Guadalajara contains approxi- center of the city east-southeasterly so as to pass just mately 800, 000 inhabitants and is solidly built up over north of the suburban town of Tlaquepaque. Here is an expanse of roughly 40 square miles. The suburbs found the junction of Highway No. 80 with a side road extend from three to five miles beyond the formal city which proceeds south to enter Tlaquepaque. Along limits. The entire metropolitan complex lies in an the highway the country becomes open and subject to uneven plain, traversed originally by the small stream heavy cultivation. The land rises very gently for the known as the Rio Atoyac. The plain is bounded by the next two miles as far as the junction with a paved road Rio Santa Rita (an affluent of the Rio Grande de which passes north-northeast approximately 2-1/2 Santiago) on the east, northeast, and north, by the high miles to end at the town of Tonala. ranges north of Lake Chapala on the south, and on the To the east of this road and parallel to it is a low west by the broken, hilly country which merges into ridge culminating in a hill 200-300 feet above the level the valleys of Tala and of Cocula. The elevation is of the road and of the town of Tonala'. On the other close to 5, 000 feet, the climate relatively cool and side this ridge falls away southeastward to a level dry. plain, or bottomland, perhaps three miles in its long- Such a large area can not be subjected to a complete est dimension. Meanwhile if one continues to pursue and intensive soil-habitation study, particularly in view Highway No. 80 beyond the junction of the Tonala' road of the vast urban development in the center. Yet it is he descends another easy grade, dropping diagonally worth some attention as constituting a type of environ- down the southeastern slope of the ridge 1. 9 miles to ment intermediate between the warm, rather damp the very small village of Tateposco at the foot of the Pacific escarpment to the west and northwest, on the slope. East of Tateposco the highway crosses the one hand, and, on the other hand, the cool but arid bottomland, keeping a range of low hills one half mile interior plateau which in the east reaches from the to the north, for about three miles and then enters western Bajio to the Atlantic escarpment. It has been hilly country. Interest here is confined to the Tonala' necessary, therefore, to select certain accessible and ridge and immediately adjacent lowland. illustrative localities for detailed examination. The underlying country rock is predominantly a dense, gray basalt. This rock once underwent pro- found weathering to form red-colored soils such as are THE TONALA REGION now frequently seen throughout western Mexico. At a more recent date extensive portions of the territory The first test area may be designated the I'onala' were covered with a deposit of volcanic origin. This region (see fig. 12). One arrives there by way of material is at certain points a clear, pumicelike ash, National Highway No. 80, which emerges from the but more frequently is a relatively compact, usually stratified, mixture of ash with silt, sand, or even gravel, almost invariably containing finely fragmented 1030 obsidian. The ashy deposit may reach depths of five 30' 1020 or ten meters and weathers to form gray, brown, or 30' even black soils with highly developed, thick caliches, or hardpans. On slopes under extreme erosion the entire ash-sand deposit may be swept away, to expose the underlying red earth or basaltic derivative, or the latter may be brought to view by deep gullying or channel cutting. Wherever thus exposed, the surface of the red basaltic material shows an apparently extreme degree of erosion. There is never the semblance of a mature soil profile in place. The topmost layer of the red o Matotlan earth under the unconformity consists of partly or profoundly weathered fragments of rock, mixed with Guadalajara varying quantities of finely divided soil, perhaps clay, SoN\& 011 >