10 SAND TEMPER IN PREHISTORIC POTSHERDS FROM THE TO'AGA SITE W. R. D cKINSON T WENTY-NI RETATIVE sherds selected by T. L. Hunt from the collection of artifacts excavated at the To'aga site near the south coast of Ofu Island in the Manu'a Group of American Samoa were examined pergraically in thin section. As there is no current rason to suspect that any of the To'aga ceramic ware was made elsewhere, the purpose of the study was to provide baseline infor- mation about Manu'a temper sands. All the sherds examined contain volcanic sand as temper, aithough this basaltic detritus is mixed with calcareous grains derived from reef sources in some of the sherds. As would be predicted for Manu'a and other parts of Samoa, the volcanic sand is typical of the oceanic basalt tempers common to intra-oceanic Pacific archipelagoes (Dickinson and Shutler 1968, 1971, 1979). Several variants of temper sand are present in different sets of sherds, and available information is inadequate to pinpoint their ive sources. All could p ly have been collected on Ofu or nearby Olosega Island, but derivation of some from Ta'u in Manu'a or even elsewhere in Samoa is not precluded by the petrgric data. Their petrologic compat- ibility with Samoan lavas, however, and their overall resemblance to the spectrum of basaltic tempera studied to date from Samoa, makes importation from outside Samoa quite unlikely. As none of the temper variants are identical to tempers known from Tutuila or Upolu, all are regarded provisionally as indig- enous To'aga temper, with the proviso that petro- graphic evidence alone cannot indicate how far afield ancient potters may have gone in their seach for suitable clay and temper within Manu'a. TO'AGA TEMPER VARIANTS The following variants of basaltic temper sand are all pr in varying numbers of To'aga sherds, and each is described in detail in subsequent pas- sages: (a) Profise Basaltic Temper: Seven sherds (1-5, 7, 14) contain ferromagnesian basaltic sand so abundant that it forms 50-60 percent of the sherd bodies. The proportions of grain ypes in six (1-5, 7) of the sherds are statistically indiuis e, but the seventh (14) contains a related volcanic sand of slightly different composition. (b) Sparse Basaltic Temper: Twelve sherds (6, 10, 13, 16, 19-23, 27-29) contain sparse feremnagne- sian basaltic sand of somewhat different composition and texture. The mineral and lithic grains form only 5-15 percent of the sherd bodies, but about a third (25-40 percent) of the temper grains used may have been fragments of broken pottery. (c) Feldpathic Basaltic Temper: Three sherds (15, 17, 18) contain feldspathic basaltic sand, which 152 The To'aga Site fonns a normal prport (30-40 percen) of the sherd bodies and is both mineralogically and textur- ally disinc t from the two ferroma an variants of Toaga tper. Another sherd (9) contins a similar but sparser temper sa i (A15 percent of body) that appears to be a hybrid sand with a significant e of detritus from the kinds of bedrock sources that yielded the feromagesia basaltic sands. (d) Mixed Temper Sand: Seven sherds (8, 11, 12, 24-26, 30) contain mixed teper sands com- posed of both basaltic and reef detitus in varying poportions. Fcrwmagnesian mineral grai and basaltic litc fragmens do the volcanic sand components of the mixed tempers, but their propor- tions are highly variable and plagioclase feldspar grains are al pesent in some sherds. TEMPER GRAIN TYPES The non-calcareous components of all the To'aga temper types are without exception composed of mineral grains and lithic fragments derived entirely from basaltic bedrock sources, eiter lavas or pyroclastic deposits, together mi some cases with fragments of broken pottery. The mineral grains, originally phenocrysts or micrphenocrysts in basalt, include clinopyroxene, olivine, opaque iron oxides (magnetite anid/or ilmenite), and plagioclase feldspar. The lithic fr s, represing aphanitic ground- mass of basaltic lava or tephra, display a spectrum of internal textures reflecting an inherent range of constituent grain sizes. Microphanocrysts in lithic fragments include all the mineral species that were also p t in the temper sands as separate mineral gams Routine distinction between pyroxene and olivine in thin section was based upon key diagnostic features visible for each grin, and their identifica- tions were checked by observations of optic axial angle and birefringence on suitably oriented grains. Pyroxene grains generally display faint green tints, and many show either cleavage or prismatic shapes. Untinted olivine grains are brighter in plane light, and many are altered along edges and fIracues to bright reddish iddingsite. Most basalt lithic frag- ments have an intergranuar internal texture, al- though the finest gained (here tenmed "tachylitic") ae intersertal with plagioclase microlites set in black basaltic glass (tachylite). The coarsest grained (here temmed "slabwor') display blocky to prismatic subhedral pyroxene crystals iergrown with aggre- gated and multiply twined plagioclase crystals of slabmike aspect. Lithic fragments of intermediate grain size (here termed "latiwotk) display disor- ented mosaics of twinned plagioclase laths with equant and largely anhedral pyroxene grains within their interstices. As all gradations are seemingly present between "slabworkW and 'lathwodc" and 'tachylitic" grains, the distinction made among hem is useful in a qualitatve sense only. Groundmass iron oxides in lithic fragmes range from equant or granularto skelet or elongate in form without apparent regard to other aspes of internal texture. PROFUSE BASALTIC TEMPER The fenomagnesian volcanic sands in sherds with profuse basaltic temper are moderately sorted assemblages of subangular to subounded grains with a texture suggestive of alluvial orgin. Unmis- table rounding of the edges of many gains indicates n rally occuming sand, rather than artificially cnmshed aggregate, and local ravine streams may have provided the sources of the temper. As would be expected for such a setting, lithic fragmens are generally but not uniformly larger than mineral grains. Abundance of subangular silty basalt detritus within the clayey paste in which the temper sand is imbedded suggests that potters collected naturally tempered sandy clay. This circumstance may account for the superabun- dance of temper sand in proportions higher than typically encountered in Pacific Island sherds. Proportions of grain types in most sherds containing this alluvial temper are quite consistent (table I0.1): half pyroxene, a quarter lithic fragments, a fifth olivine, and a trace of opaque iron oxides. All micrhenocrysts in lithic fragments are pyroxene and olivine. As the inteal texture of lithic frag- ments is somewhat variable, being ree-quarters 'lathwok" in two sherds (1, 7) and two-thirds "slabwok" in four others (2, 5), collecting sites were evidently closely related but not identical. Neverthe- less, the average temper composition (table 10.1) for the six sherds in which proportions of grain types are essentially the same is taken here to be the best estimate, petrlogically speaking, of proportions of consituents for characteristic To'aga temper. Sand Temper in Prehistoric Potsherds 153 Table 10.1 Frequency Percentages of Ferromagnesian Mineral Grains' and Basaltic Volcanic Lithic Fragments (VRF) in Sherds Containing "Profuse Basaltic Temper" Sherd n Py 01 Fe VRF Py(Py+01) 1 130 51 21 2 26 0.71 2 160 50 23 2 25 0.68 3 210 53 20 1 26 0.73 4 105 51 17 4 28 0.75 5 185 50 24 4 22 0.78 7 260 54 23 1 22 0.71 Ave 52 21 2 25 0.71 14 36 14 3 47 0.72 'Py, clinopyroxene; 01, olivine; Fe, opaque iron oxides Note: n=number of grains counted in each sherd and average (Ave) composition is calculated for sherds 1-5 plus 7 but not 14. A seventh sherd (14) contains distinctly more lithic volcanic sand (table 10.1), although its pyrox- ene/olivine ratio is very close similar to that of the other sherds. Lithic fragments, mostly "lathwork," are also more irregular in shape and some are microvesicular. Curved re-entrants on some lithic fragments and the presence of a few grains of microvesicular brownish basaltic glass suggest a pyroclastic component lacking in the more charac- teristic six sherds whose tempers were probably derived entirely from bedrock lava sources. SPARSE BASALTIC TEMPER The ferromagnesian volcanic sands in sherds with sparse basaltic temper are well sorted aggre- gates of subrounded grains with a texture suggestive of beach origin. The lack of finer grained grit within the clayey paste suggests that potters added artificial temper to clay bodies. Dark angular blotches within the clayey paste are probably ghosts of broken pottery fragments also added as pain of the tempering process. Although recognition of this grog constitu- ent is equivocal in some sherds owing to indistinct outlines of the pottery fragments, its presence may account for the low overall proportion of volcanic sand, which amounts alone to much sparser temper than typically encountered in Pacific Island sherds. Frequency percentages of grain types are highly variable for different sherds (table 10.2), but so few grains are present in each sherd (average of only 20 per sherd in 10 of the sherds) that the statistical significance of individual counts is questionable. Consequently, all grains (250 total) were summed from all sherds counted to yield net frequency percentages, but net and average temper composi- tions are almost identical (table 10.2). The fact that both measures of bulk composition are similar to values for the single sherd (22) containing the most grains (60) gives confidence that either measure is a valid estimate of the overall temper composition. The sparse basaltic temper, probably beach sand, is less pyroxenic and more lithic than the average composition of the dominant alluvial variant of the profuse basaltic temper, but grain proportions closely resemble those in the more lithic variant of alluvial sand. In general, differences are not great enough to suggest wholly different provenance except for the contrast between stream and beach collecting sites. Proportons of lithic grain types are quite variable from sherd to sherd, but all three types are present in subequal amounts within the suite of twelve sherds as a whole. The net pyroxene/olivine ratio (0.63) is only slightly lower than in the alluvial sands (0.72), and may have been reduced marginally by preferen- tial cleaving of pyroxene grains and winnowing of resulting cleavage fragments during prolonged reworking in a beach environment. 154 The To'aga Site Table 10.2 Frequency Percentages of Ferromagnesian Mineral Grains and Basaltic Volcanic Lithic Fragments (VRF) in Sherds Containing "Sparse Basaltic Temper" Sherd n Py 01 Fe VRF 6 20 5 10 5 80 10 10 40 20 10 20 16 15 47 33 7 13 19 30 33 13 4 50 20 10 30 20 10 40 21 10 40 20 10 30 22 60 42 20 2 36 23 25 32 20 8 40 27 30 40 40 3 17 28 20 25 5 5 55 29 20 25 15 5 55 Net 250' 34 20 4 42 Ave 33 20 6 41 'Summation of n for 11 sherds listed. Note: n=number of grains counted in each sherd (note that net and average compositions are essentially the same). Sherd 13 too weatherd to allow accurate count. FELDSPATHIC BASALTIC TEMPER The volcanic sand in sherds (15, 17, 18) contain- ing feldspathic basaltic temper essentially lacks fenrmagnesian mineral grains (table 10.3), and nearly all microphenocrysts in lithic fragments are plagioclase rather than pyroxene or olivine. One olivine grain is present, however, in one sherd (18), and one olivine microphenocryst is present in another (15). Lithic fragments are consistently larger than separate plagioclase mineral grains, although the two are present in about the same frequency (table 10.3), and the sand overall is only moderately sorted. Most lithic fragments are "tachylitic," many have smoothly curved re-entrants typical of tephra clasts, and some are microvesicular. The textural features of the sand jointly suggest that scoiaceous basaltic as, possibly reworked locally, was added as artificial temper to the clay body by potters having some selective aim in using such a tempering material The feldspathic basaltic temper shows no compositional overlap with the fenomag- nesian basaltic tempers, but the geograhic separa- tion of their respective sources need not have been great. As if to underscore that point, one sherd (9) contains well-sorted and subrounded temper, probably a beach sand, that apparently represents a mixture of feldspathic and ferromagnesian volcanic sands. This anomalous sherd has an apparently sparse temper (- .15% of body) but also includes a few fragments of broken pottery as part of its overall temper component. MIXED TEMPER SAND The volcanic sands in sherds containing an admixture of reef-derived calcareous grains (15-75 percent) are highly variable in mineralogical compo- sition (table 10.4). Although all are dominantly ferromagnesian volcanic sands, nearly half contain feldspathic components as well. Coupled with the presence of the calcareous grains, the good sorting and rounding of the sands is diagnostic of coastal origin on beaches where mixing of detritus from multiple sources is to be expected. Proportions of temper sand vary from 10-25 percent (8, 11, 12, 24) Sand Temper in Prehistoric Potsherds 155 Table 10.3 Frequency Percentages of Plagioclase Feldspar (P1) and Opaque Iron Oxide (Fe) Mineral Grains and Basaltic Volcanic Lithics Fragments (VRF) in Sherds Containing "Feldspathic Basaltic Temper" Sherd n P1 Fe VRF Py 01 15 60 43 8 49 - - 17 35 50 6 44 - - 18 125 56 4 40 - - Ave 50 6 44 - - 9 60 18 7 59 13 3 Note: n=number of grains counted in each sherd and average (Ave) composition is calculated for sherds 15 plus 17-18 but not 9 (Py and 01 are clinopyroxene and olivine mineral grains in sherd 9). Table 10.4 Frequency Percentages of Calcareous Grains (calc), Silicate-Oxide Mineral Grains, and Lithic Fragments for Sherds Containing "Mixed Temper Sand" Sherd Calc n Py 01 Fe P1 VRF 8 13 65 32 32 3 - 33 11 22 35 14 7 7 - 72 12 20 12 33 17 25 - 25 24 15 30 27 23 - - 50 25 44 70 21 17 1 1 60 26 18 160 22 18 1 5 54 30 77 60 39 14 2 20 25 Note: n=number of non-calcareous grains counted in each sherd (percentages reported sum to 100 exclusive of calcareous grains) to 40-60 percent (25, 26, 30), the "tachylitic" variety of lithic fragments form about half the lithic popula- mon, and the overall pyroxene/olivine ratio ('0.60) is similar to that in the other sherds thought to contain beach sand temper. Computation of an average or net composition for the sherds containing mixed sand temper would be meaningless, given their inherent compositional variability, but their volcanic sands fit broadly within the spectrum of temper types present in other sherds. Sherds containing only sparse mixed temper sand also contain fragments of broken pottery in uncertain amounts. DISCUSSION: TEMPER COMPARISONS Although each of the To'aga temper types has clear distinguishing characteristics, compositional links argue that they form a related temper suite that is presumably indigenous to Manu'a. The two ferromagnesian basaltic tempers have contrasting textures tat reflect different sedimentological origins such as stream and beach sands, but the same grain types are present in both in slightly different proportons. The admixture of similar fenomagne- sian constituents in one of the sherds containing 156 The To'aga Site feldspathic basaltic temper suggests hat bedrock sources for the fenomagnesian and feldspathic volcanic sands exist not far apart This inference is strengtned by the observation that mixed beach sands containing calcareous grains contain a varied spectrum of fenomagnesian and feldspathic constitu- ents. Prehistoric sherds examined previously from Upolu (Dickinson 1969,1974,1976) contain generally similar basaltic temper sands composed of the same basic grain types, but none of the 'Upolu tempers is identical in texture or composition to the To'aga tempers. Fenromagnesian basaltic tempers from 'Upolu commonly contain a higher proportion of lithic fragments, typically have a higher ratio of olivine to pyroxene, and generally contain a subordi- nate proportion of brownish basaltic glass particles not present in To'aga tempers. Moreover, Upolu sherds with a ferromagnesian tempers contain well-sorted coastal sands texturally unlike the stream sands evident in well tempered To'aga sherds. Feldspathic basaltic tempers from 'Upolu are broadly similar to their To'aga analogs, but internal textures of lithic fragments differ in being coarser gained in the lJpolu sherds studied to date. Felds- pathic trachytic tempers present in all available Upolu collections apparently have no counterparts at To'aga. The generic resemblance of all Samoan temper types examined to date pennits the strong inference that the tempers in To'aga sherds are indigenous to Samoa. On balance, there is no reason to suppose on petrographic evidence that any of the To'aga temper types were derived from sites elsewhere in Samoa. The fragments of broken pottery present in about half the To'aga sherds are not common consti- tuents of Pacific Island wares, but do occur in sherds from the Ryukyu Islands, Palau, and the Nan Madol site on Ponape (Dickinson and Shutler 1979; Dickin- son et al. 1990). Their presence at To'aga presum- ably reflects a common paucity of suitable local temper sand, rather than any close cultural relation- ship between the Caroline Islands region and Samoa. REFERENCES CITED Dickinson, W. R 1969. Temper sands in prehistoric potsherds from Vailele and Falefa. IN R. C. Green and J. M. Davidson, eds., Archaeology in Western Samo, Vol. I: pp. 271-73, Auckland Institute Museum Bulletin 6. . 1974. Temper sands in shends from Mulifanua and comparison with similar tempers at Vailele and Sasoa'a (Falefa). IN R. C. Green and J. M. Davidson, eds., Archaeology in Western Samoa, Vol. II: pp. 179-80, Auckland Institute Museum Bulletin 7. 1976. Mineralogy and petrology of sand tempers in sherds from the Feriy Beth site, Paradise site, and Jane's Camp. IN J. D. Jennings, R N. Holmer, J. C. Janetski, and H. L. Smith, eds., Excavatios on 'Upolu, Western Samo, pp. 99-103. Pacific Anthrpological Records 25. Honolulu: Bernice P. Bishop Museum. Dickinson, W. R, and R. Shutler, Jr. 1968. Insular sand tempers of prehistoric pottery from the southwest Pacific. IN I. Yawata and Y. H. Sinoto, eds., Prehistoric Culture in Oceania, pp. 29-37. Honolulu: Bishop Museum Press. 1971. Temper sands in prehistoric pottery of the Pacific islands. Archaeology and Physical Anthropology in Oceania 6:191-203. -.1979. Petrography of sand tempers in Pacific islands potsherds. Geological Society of America Bulletin 90 (Part I, summary):993-95; and Part II (microfiche), No. 1 1, Card 1, pp. 1644-1701. Dickinson, W. R. J. Takayamna, E. A. Snow, and R. Shuder, Jr. 1990. Sand temper of probable Fijian origin in prehistoric potsherds from Tuvalu. Antiquity 64:307-312.