An Investigation by Holman & Assoc., San Francisco

Archaeological Excavations and Burial Removal at ALA-483, -483Extension, and -555, Pleasanton, Alameda County, California: Obsidian Studies, by David G. Bieling.

8 June 1997

Archaeological investigations by Holman & Associates of San Francisco at Ala-555, Ala-483 and Ala-483EXT--the Laguna Oaks Subdivision--near Pleasanton, Alameda County, California revealed several archaeological components representing multiple occupations spanning about 3000 years (Wiberg 1995). Two hundred fifty-one graves and cremations were recovered from both cemetary and dispersed contexts and seventeen radiocarbon dates were obtained. One thousand twenty-seven obsidian items were recovered, nearly all (97%; n=996) from Ala-555. One hundred thirty-one obsidian hydration bands obtained from this and a previous study revealed a complex history of volcanic glass use at these three sites illuminating our knowledge about past technological and exchange systems.

If you wish to download any of the following images, place the mouse's cursor on the image, click the right mouse button, and select "Save Image As".

Click here to go to a map of obsidian sources.

Click here to go to charts of obsidian hydration values for the sites and sources.

Building on previous studies by Basin Research, the analyses showed that obsidian use reflected periods of site occupation during the Early, Middle and Late periods in Central California prehistory (ca. 1400 BC - AD 1800). Early and Middle Period use of obsidian was represented primarily at Ala-483. Obsidian associated with these time periods was from the Napa Valley (about 70 miles north), and Casa Diablo, Bodie Hills, and Mt. Hicks (Eastern Sierra/Western Great Basin glasses). Napa Valley glass dominated the recovered obsidians during those times (60%). Assemblages from early times appeared to represent maintenance of tools acquired as finished or nearly-finished exchange products derived from northern and eastern locations on an ad-hoc basis.

Ala-555 was initially occupied during the latter part of the Middle Period. Increased obsidian use at this site appears to correspond with a period of decreased use at Ala-483 and -483Ext. The obsidian assemblage from this site was dominated by Napa Valley glass (99%; n=991). The intensity of tool manufacture and use at Ala-555 can be correlated with an influx of Napa Valley glass flake blanks over 400 years ago. Hydration and technological studies indicated these blanks were produced during the preceding millenia. Primary and secondary reduction flakes were apparently scavenged--probably by people occupying the Napa Valley region--and traded southward to people in the Pleasanton locality.

With the exception of several lanceolate and stemmed forms, the majority of typological projectile points recovered during the present investigations were small serrated arrow tips, mostly from Ala-555. These had a mean hydration value of 1.8 microns (or about A.D. 1500 b.p.). Additional obsidian artifacts included some flake blanks, point preforms, many simple flake tools, a few cores, and large edge-trimmed flake blanks. Technological organization at ALA-555 was defined by acquisition of large NV flakes which were treated as cores to produce small points, preforms, and miscellaneous simple flake tools. A similar strategy appeared to have operated in the Sacramento region during the Late Period (cf. Jackson and Schultz 1975).

Obsidian findings from two of the graves were particularly interesting. Burial 25 contained 16 small serrated points and point fragments all made of Napa Valley glass. Six of the points were hydration tested yielding band values ranging from 1.4 -1.6 microns (mean=1.5 microns; ca. A.D. 1690).

Burial 107 was a cremation accompanied by a unique mortuary offering comprised of many large obsidian flakes, a small serrated point, a uniface fragment (the distal end of an overshot thinning failure), and a thick flake core. After refitting many of the broken pieces, a total of 67 whole flakes and 102 additional flake fragments were tabulated. Many of the pieces had been heat-affected and exhibited crazing, crumbling, and crenated fractures. This might have affected hydration rims. Other items were presumably farther from the heat and appeared unaffected.

Seven specimens were selected from the Burial 107 assemblage for hydration testing. The sample included five large flakes (45.0-78.0 mm wide) devoid of any obvious effects from the cremation heat, the serrated point, and the uniface (burned on the proximal end). The latter artifact yielded a rim value of 3.5 microns from an unburned location. The serrated point had a 3.0 micron band. A single large non-cortical flake returned a value of 1.7 microns. Two of the large cortical flakes returned single values of 1.1 and 1.3 microns. Another cortical flake had two bands: 1.9 and 2.4 microns. The other cortical flake did not have a visible band. The obsidian hydration laboratory did not report any unusual characteristics of the hydration bands that might indicate alterations resulting from the heat of the cremation. Given these results, it seems likely the cremation took place approximately 550-185 years ago (ca. A.D. 1440 to A.D. 1800), as determined by the smaller cluster of hydration band values.

The technological organization of site inhabitants of Ala-555 may have developed, in part, in response to factors such as complexity of social organization, exchange relationships, and distance from obsidian source. The development of more regularized exchange systems during the Emergent Period has been well-documented (Fredrickson 1974). Other factors, such as inter-group relationships with neighboring peoples, might also have played a significant role in the obsidian requirements of the site's inhabitants. The obsidian studies for the collections from these sites yielded a series of data sets useful for constructing a model of Late Period obsidian use in the East Bay Region.

·  Wiberg, Randy S.
1995 Archaeological Excavations and Burial Removal at ALA-483, -483Extension, and -555, Pleasanton, Alameda County, California. Holman & Associates. San Francisco, CA.

·  Fredrickson, David A.
1974 Social Change in Prehistory: A Central California Example. In, ?ANTAP: California Indian Political and Economic Organization. L.J. Bean and T.F. King, eds. Ballena Press Anthropological Papers No. 2. Ramona, California.

·  Jackson, T. L. and J. E. Ericson
1994 Prehistoric Exchange Systems in California. In, Prehistoric Exchange Systems in North America (pp. 385-415). Edited by Timothy G. Baugh and Jonathon E. Ericson. Plenum Press, New York.

·  Jackson, T. L., and P. D. Schultz
1975 Typology, Trade, and Trace Analysis: A Test of Local Manufacture of Sacramento Valley Obsidian Tools. Journal of New World Archaeology 1(2):1-8.

Return to the top of this document.

Go to ALA-42
Go to
Go to
Franz Valley Obsidian
Go to
Visual Sourcing of Eastern Sierra/Western Great Basin Obsidians
Go to
Map of Major Obsidian Sources in California & Western Nevada
Go to
Charts of obsidian hydration results from Pleasanton studies

Return to Archaeological Studies Page


Return to Home Page