8 June
1997
Obsidian Studies at CA-ALA-42
These analyses
show that obsidian use reflects a span of site occupation during the Lower
Emergent period in
Macroscopic
visual and XRF source assignments show that
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
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
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
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
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
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Obsidian Sources in California & Western Nevada
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hydration results from Pleasanton studies
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