Obsidian Studies at CA-ALA-42

8 June 1997

An Obsidian Study by Bieling & Psota

Obsidian Studies at CA-ALA-42

These analyses show that obsidian use reflects a span of site occupation during the Lower Emergent period in Central California prehistory (A.D 793 - A.D. 1254). Typological projectile points recovered were thick, semi-serrated lanceolate forms made on Napa Valley glass. Other biface forms included a few early-stage items and fragments that appear consistent with the few diagnostic artifacts.

Macroscopic visual and XRF source assignments show that Napa Valley glass dominated the assemblage. Debitage was typically very small and derived primarily from retouch of formal tools and tool shatter. A few thicker second hydration bands point to a procurement strategy based on exchange with people situated closer to the Napa Valley region. Some of the items might derive from older flakes scavenged close to the glass source area and traded southward in an unknown form.

Seventy-nine obsidian items were recovered as a result of these investigations. These included seventeen bifaces and fragments and sixty-two pieces of debitage. Two biface items were classifiable as semi-serrated lanceolate forms; one was marked by extensive impact damage. Several of the other biface forms included tips, medial sections, and bases that were not inconsistent with the two diagnostic items. One small piece was identified as a biface margin. Two of the biface fragments were minimally modified forms retaining much of the original flake blank surfaces. Cortex was present on two of the biface forms, a finished point and a blank.

Recovered debitage was marked by a mean weight of 0.13 grams. Even though one item had residual cortex, virtually all could be attributed to late-stage manufacture or repair activities. Some appeared to derive from impact sustained by formal tools. These had attributes such as complex dorsal surfaces consistent with patterned pressure-flaking, sheared ventral faces or sides consistent with compressive force, absent platforms, or bifacial edges.

All items in the collection were examined for macroscopic attributes and assigned to probable sources. Virtually all possessed characteristics attributable to Napa Valley glasses. Two exceptions were one piece of debitage that resembled a darker variant of Bodie Hills glass and a biface fragment with sililarities to Annadel material.

Twenty-eight items were submitted from Ala-42 for hydration testing (35%). Twenty-three specimens yielded measurable bands; two of those items produced second bands, as well. Four had diffuse hydration fronts and one had no visible band. Following hydration testing, ten were submitted for XRF analysis. Eight possessed geochemical signatures of Napa Valley obsidians and two others were too small to generate reliable results and were not tested.

A few items selected for hydration were cut with the expectation of identifying multiple bands to address aspects of tool procurement, manufacture, and re-use as defined in the research questions. Test cut locations included combinations of unmodified ventral surfaces, seemingly unmodified dorsal surfaces, and/or fracture faces. The two items yielding multiple bands were both biface fragments. The smaller bands on these measured 2.5 microns and 2.6 microns, whereas the older surfaces yielded 7.0 and 4.2-micron fronts, respectively. Although other items submitted were cut across surfaces capable of representing older scars, none yielded second bands.

Excluding the two second band values, the twenty-three hydration rims had a mean value of 2.6 microns (range=2.2-3.4 microns; sd=0.27). With the exception of the two larger values (3.3 microns and 3.4 microns), the results strongly indicated a single component deposit. Typologically, however, the two items with hydration values greater than 3.0 microns also conformed to the rest of the assemblage.

Obsidian materials recovered during the ALA-42 archaeological investigations represented a single-component assemblage marked by very little diversity in either tool forms or debitage data sets. If it is presumed morphological diversity equates with functional diversity, the limited number of tool and debitage classes can be interpreted in part as a gauge defining a minimum range of activities requiring application of this material. Similarly, low recovery rates evinced by the sample also reflects a limited amount of tool manufacture, maintenance, and discard during the time period represented.

The second hydration bands on some items were interpreted as representing a mode of acquisition similar to that operating at ALA-555 several hundred years later. Although exchange systems were undergoing increased regularization, people in the north do not appear to have produced finished items for export so much as scavenged suitably large pieces from previously used workshops; it is also possible these were shaped into biface blanks for exchange. Unfortunately, the assemblage was not of sufficient size to test this hypothesis. In this respect, the ALA-42 assemblage contrasts sharply with that recovered from ALA-555.

A shift in both site settlement and obsidian use identified by earlier studies of assemblages from the Pleasanton sites has been supported by the present analysis. To reiterate, ALA-483 was marked by the earliest period of obsidian use. Obsidian at that site was dominated by NV material but ES/WGB sources were also represented as well as some Annadel. NV and ES/WGB glasses were equally represented during the earlier time periods of site use at ALA-483 while Annadel specimens corresponded to the most recent period of use. Two Annadel specimens from ALA-555 indicated use of this glass at the Pleasanton Locality may have began almost 1400 years ago.

Obsidian use was minimal during earlier periods of the Laguna Oaks Subdivision site occupations. Assemblages from that time appear to represent maintenance of tools acquired as finished or nearly-finished exchange products derived from nothern and eastern locations on an ad-hoc basis. Hydration data indicated some use of NV material during the latter half of the Middle Period but a notable increase relative to other glasses during the early phases of the Late Period. Results from the present study indicated NV dominated the obsidian assemblage at ALA-42. During these periods, obsidian use at ALA-483 declined; four of the rim values from the latest period of obsidian use at this site were from Annadel items. Four specimens at ALA-483EXT also reflected this time period.

Hydration values for NV glass at ALA-555 during the most recent period of site use appeared to represent a dramatic increase in flaked obsidian related activities corresponding with a decrease at the other sites. Technological organization at that site 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 obtained in the Sacramento region during the Late Period (cf. Jackson and Schultz 1975).

The present studies yielded a series of data sets applicable to a model of Late Period obsidian use in the East Bay Region. It is recommended subsequent investigations in the region examine obsidian assemblages with these analytic strategies and research topics in mind. More rigorous examination of contemporaneous assemblages from nearby late Middle Period and Late Period sites and existing collections would also be a productive avenue for examining the propositions outlined above. Additional substantive studies may provide important specific information about Late Period social interactions and exchange networks in the East Bay.

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Go to ALA-555
Go to SOL-356
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

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