Obsidian Studies at CA-SOL-356

8 June 1997

An Obsidian Study by Bieling & Psota

Obsidian Studies at CA-SOL-356

These analyses showed that obsidian use at this site occurred during the Upper Archaic and Emergent Periods in Central California prehistory (ca. 1500 B.C. - A.D. 1800). Hydration studies showed the predominant temporal range of obsidian use occurred during the Emergent Period (ca. A.D. 1200-1800). With the exception of several lanceolates and bipoints, the majority of typological projectile points recovered were small serrated arrow tips. Most of the latter were characterized by expanding bases and a large barb on the proximal blade. Additional obsidian artifacts included dozens of flakes blanks, several preforms of the expanding base serrates, many simple flake tools, a few cores, and a number of sheared elements, mostly classifiable as core forms.

Virtually all obsidian was visually identified as Napa Valley glass. Considerable variability within this glass was recognized in attributes such as opacity, color, constituents, and structure. Only a few items were macroscopically and geochemically assigned to the Annadel group.

Results of these studies indicate three dominant technological strategies for tool production during the Emergent Period. The primary mode of reduction centered on use of locally obtained cobbles but supplies might have been supplemented by similar raw materials acquired through either direct quarry access in or near the Napa Valley, or exchange with intermediaries. This is supported by high proportions of debitage and tools retaining cortex in the collection. A number of larger pieces of debitage and some cores comprised significant sized fragments of small cobbles. Second, tool recycling was evident in biface fragments re-used as cores. Thirdly, multiple hydration bands identified on ventral face flakes, tools, and some debitage marked by differential patination suggest scavenging from this or other sites in the area.

As noted above, virtually all obsidian items examined were assignable to the Napa Valley glass group. At least two items were visually ascribed to Annadel. Ten items submitted for XRF analysis showed these two plus one other were from that glass group; the remainder were consistent with Napa Valley glass. Their hydration values have been adjusted to the Napa Valley rate using Tremaine's comparative constant where appropriate (Tremaine 1989).

Previous studies of cultural obsidian recovered in the Green Valley locality have addressed macroscopic variability in the assemblages. While the present study made no attempt to quantify the various glass macroscopic groups, examination of material recovered during the investigations confirmed the range of diversity recognized in previous studies. Glasses recovered in Green Valley archaeological sites includes (1) a homogeneous opaque black, glossy-to-vitreous glass often associated with a coarse cortex; (2) a slightly opaque to translucent glossy glass (translucense frequently dependant on thickness) often characterized by a range of internal variability including fine dust specks to subtle banding; and (3) a slightly opaque to translucent glossy glass marked by a root beer amber cast that often included somewhat parallel bands. Version 2 probably accounted for the greatest range of macroscopic variability in the Green Valley collections. As noted in an earlier study, "some opaque black material contained small inclusions or vesicles, and the translucent and glossy materials had variable shades of banding, smudging, or dustiness" (Bieling 1992:219). To reiterate, these groups are not exclusive--some attributes suggest intergradation of materials.

With respect to addressing the research questions, several data sets were primary among those selected for hydration testing.

(1) Small serrated projectile points with expanding bases.

(2) Small serrated projectile points with straight stems and basal fragments from these types.

(3) Non-serrated projectile points.

(4) Ventral face flakes (with or without dorsal patination) - among the first flakes detached from the ventral face of a larger flake blank/core having a remnant of that unmodified surface on the dorsal face.

(5) Small triangular or teardrop shape bifaces and edge-modified flakes (Type 4), some with preliminary notches at a break, presumed to represent either point blanks or preforms for the small serrated forms recovered.

(6) Interior flakes with and without dorsal patination.

(7) Other bifaces and tool forms.

Applying the current aspect of this study to the research questions, many of the items selected for hydration were marked to indicate the placement of the cut. The test cut locations included combinations of unmodified ventral surfaces, seemingly unmodified dorsal surfaces, dorsal areas with differential patination, and or fracture faces (see below). Some unmarked items were submitted to the hydration laboratory at Fresno State University with the understanding the test cut location should be situated to intersect surfaces capable of yielding multiple bands.

One hundred-three items were submitted for hydration testing. Ninety-one items returned hydration band values; fourteen yielded multiple bands. The remainder had either diffuse or weathered rims or lacked visible bands. In several instances, second bands of variable width or diffuse fronts were recognized. The hydration technician also commented on the nature of several extensively weathered surfaces that could be attributable to exposure to intense heat.

Most of the hydration values classed as Band 1 identified obsidian use during the Emergent Period whether adjusted for temperature or not. Adjusted values exhibited three modal peaks during the hydration span attributed to the Emergent Period--at about 1.1 microns, 1.8 microns, and 2.1 microns. The non-adjusted values also show three hydration band groups but the clusters are more diffuse. Several other adjusted hydration bands were associated with Middle and Upper Archaic Period use. One primary band and two secondary bands could represent much earlier cultural use of the area while at least two other second bands were probably obtained from non-cultural surfaces on the artifact.

A number of items were submitted for hydration testing to evaluate aspects of technological history through the potential identification of multiple bands. One of these data sets included twelve pieces of debitage consisting of four ventral face flakes (three which were also EMFs) and eight un-modified items, all with dorsal patination. Of these, three of the pieces of un-modified debitage and two of the EMFs yielded second bands. Four of the bands were between 4.1 microns and 6.5 microns while one was approximately 14.8 microns; although it was very small, the latter appeared to have been a non-cultural surface area. One other piece of debitage without a distinct patinated dorsal surface yielded a second band off its platform.

In addition, as many as 51 artifacts (mostly small serrated projectile points) characterized by remnants of original ventral and/or dorsal surfaces were submitted. Of these, 10 yielded second bands. Most second bands from these items were between 1.8 microns and 8.9 microns, adjusted. Two others that yielded 10.8 microns and 13.0 microns (adjusted) were suspected of having tested non-cultural surfaces. All items yielding multiple bands were visually assigned to the Napa Valley glass group.

Seven Napa Valley items from Unit 1 submitted for hydration testing yielded adjusted bands between 1.8 microns and 2.4 microns. These had a mean value of 2.1 microns with a low standard deviation (0.20). These figures suggest a possible single-component area dating to the latter part of the Lower Emergent Period. Given the location of this unit nearly 30m west of the core area, this could represent occupation expansion during a period of increasing population.

Hydration analysis pertaining to typological studies revealed most forms were not temporally well-separated. This could be attributed to cultural or environmental factors affecting hydration band formation or resolution limits of optical microscopy used by technicians (Stevenson, et al. 1989). Some patterns recognized in other studies, however, were supported.

Non-serrated corner-notched forms tended to be later in time than most serrated types. Specifically, types 5A (n=2) and a fragment assumed to be a type 5 had smaller hydration bands than most type 2s and 3s. Five Type 3s had a mean hydration value larger than nine Type 2As, six possible 2As, and five Type 2Vs. Although five 2Bs showed a larger mean, four of the items had 2.0 microns or smaller bands. Two Type 3V were characterized by mean values that might support their association with Type 1 items.

All obsidian debitage recovered from units 1, 2, 3, and 8 were included in a technological study. Selection of these units for the study was predicated on location, depth range, recovery rate, and recovery strategy. Units 1 and 3 were sampled in the field by 3mm screen while the other two units were subject to 6mm recovery. Recovery rates for these units were in the higher ranges for units excavated below 70cm. Units 2, 3, and 8 were situated within the central area of the site while Unit 1 was more than 30m northwest of that area.

Given the research objectives and certain expectations about Late Period technological organization derived from previous investigations, classification focused on attributes deemed most useful for assessing the importance of early reduction and manufacture stages. Diagnostic flake types identified for this study included ventral face flakes (VFF), flakes with bifacial platforms (BIE), flakes with bifacial distal edges (i.e., overshots; BIO), notching flakes (NOT), and impact flakes (IMP). All these flake types were also counted and weighed separately when they were identified in the debitage samples of other units during the cataloging phase; this supporting information will be addressed below. All other flakes and fragments were grouped as "general"; these included a variety of pressure flakes, thinning flakes, alternate flakes, miscellaneous fragments, and other diagnostic and non-diagnostic elements. Each category was counted, weighed, and the presence of cortex or dorsal patination was recorded.

In all, the sample was comprised of 2,136 pieces. Cortex was present on 522 items (24%) defined by a mean weight of 0.59 grams. Items with dorsal patination accounted for less than 4% (n=85) and had a mean weight of 0.40 grams. Non-cortical non-patinated pieces accounted for almost 72% (n=1529) with a mean weight of 0.24 grams. Flakes directly attributable to biface manufacture and repair (BIE, BIO, and NOT) accounted for less than 2% of the sample (n=32). Flakes with bifacial edges comprised 1% of the sample (n=30). If thinning flakes and late-stage pressure flakes were included in an assessment of the relative importance of biface reduction, it is still unlikely the proportion would be considered significant.

The percentage of items retaining cortex was most consistent in units 2, 3, and 8, where the proportion ranged from 20% to 29%. Likewise, mean weights for cortical debitage in these units ranged from 0.61 g to 0.64 grams. On the other hand, the sample from Unit 1 contained 13% cortical material marked by a mean weight of 0.32 grams. The greatest differences in mean weight were evident in the 3mm recovery units, 1 and 3.

In contrast, non-cortical non-patinated materials from units 1 and 3 were smaller on average than those from the 6mm units, 2 and 8, as would be expected. In the former units, the mean weights for these categories were 0.13 g and 0.18 g, whereas in the latter 6mm units the values were 0.29 g and 0.31 grams. Ventral face flakes comprised a greater proportion of the debitage assemblage in units 2 (7%), 3 (5%), and 8 (8%) than they did in Unit 1 (3%) but hardly to a significant degree. Given the proportions of cortical materials and VFFs, it is concluded activities at the Unit 1 location included less primary reduction than the core site area.

Debitage from units other than those used in the technological analysis was also examined for ventral face flakes. On average, these flakes accounted for 3-11% of the obsidian debitage recovered from those units. The frequency of ventral face flakes in either the units selected for technological analyses or the non-sampled units does not differ significantly.

Twenty-six additional BIE flakes, three BIO, one NOT, three IMP, and a possible uniface rejuvenation flake were identified in the non-sampled units (units 6, 7, 7E, 7EN, 7ES, 7W and 8). Two of the BIOs were classed as early-stage and the other was defined as middle-stage. BIEs had a mean weight of 0.47 g, virtually the same as defined for the sampled BIEs.

Although not quantified, cortex on items varied. On some pieces cortex was coarse but weathered; some pieces had coarse, grey, unweathered cortex; while other pieces were marked by weathered surfaces lacking evidence of conchoidal fracture. The first of these groups was assumed to derive mostly from surrounding hills and drainages. The group with coarse grey cortex resembled material at the Glass Mtn. source in St. Helena. The third group had surfaces reminiscent of cobbles occurring within an ash flow, such as that at Glass Mtn., in which pieces separated as they cooled but remained physically articulated.

Several recently investigated prehistoric sites were situated within about three miles of SOL-356. Two of these, SOL-315 and SOL-355/H, were also the subject of obsidian studies by Holman & Associates. SOL-315 and SOL-355/H were Archaic Period sites situated within one mile of SOL-356. SOL-315 was used throughout the Archaic Period, whereas SOL-355/H was an occupation site used almost exclusively during the Upper Archaic. Obsidian assemblages recovered from these two sites exemplify diverse strategies of technological organization and in some respects contrast sharply with those identified at SOL-356.

Recovery rates of obsidian materials were highest at SOL-315 while debitage mean weights were lowest, whereas SOL-355/H yielded the lowest number per cubic meter but higher mean weights. High recovery rates and small sizes of debitage and tools at SOL-315 were attributed to (1) potentially intensive use throughout an extended time period within a spatially restricted area; and (2) curation of volcanic glass materials (Bieling 1992). Conversely, higher mean weights of debitage at SOL-355/H were attributed to a reduction strategy characterized in part by more middle-stage tool production and less curation (Bieling 1993). A briefer occupation span at the latter could account for lower recovery rates.

Although higher recovery rates at SOL-356 could be partially attributed to occupation span, a technological strategy emphasizing production of small points and somewhat larger biface forms from cobbles was probably responsible for contributing greater amounts of material in a concentrated area. Debitage mean weights were more than 50% greater than those at SOL-315.

Other factors contributing to these differences are quantities and sizes of flake tools and their implied functions. As noted above, tools at SOL-315 were generally small and it was unlikely many of the modified flakes were part of a biface manufacturing trajectory. The reverse appears to be the case at SOL-356. Modified flakes (those not reclassified as blanks) were still characterized by higher average weights and a greater size range than those at SOL-315 or SOL-355/H (the sole exception was a single formally shaped scraper at SOL-355/H). Ninety-seven percent of the EMFs and 87% of the RTFs weighed less than 4.0 g; 80% of the EMFs were less than 2.0 grams. Although many had been re-classified as items comprising part of a manufacturing trajectory geared towards small points, other less diagnostic items within the EMF and RTF categories could represent fragments of broken blanks, and could thus be contributing to higher mean weights reflected in larger standard deviations.


Obsidian source and technological studies and hydration results on selected items provide information about material procurement. Multiple hydration bands on particular items, a relatively high proportion of debitage retaining cortex coupled with the additional items marked by dorsal patination support a strategy characterized by three methods of procurement: (1) naturally occurring cobbles were obtained from nearby stream beds and hillslopes where the materials eroded out of the Sonoma Volcanics geologic unit (Fox 1983; Fox, et al. 1985); (2) pieces of raw material was obtained either directly from quarry workshops in the Napa Valley region or through exchange with people to the west; and (3) previously worked material--possibly from nearby sites--was acquired directly or indirectly, or both. Of these strategies, the foregoing analyses indicate procurement of local un-modified materials probably represented the greatest contribution to the site deposit and might have been the dominant mode during the Emergent Period.

The amount of items retaining cortex plus the high number of ventral face flakes and early-stage examples of small point forms indicate a technological pattern characterized strongly by local procurement of raw material and on-site tool production. Tool manufacture was overwhelming represented by production of small serrated projectile points, from the flake blank stage to final form. A number of larger, early- and middle-stage biface forms could represent either the production of formal tools or preparation of bifacial core forms. Another dominant element of the flaked-stone toolkit were used flakes. Although many could be recognized as potential flake blanks, many others were too small, had irregular edges, or wear patterns inconsistent with blank shaping.


Studies of the collection from SOL-356 have shown Emergent Period obsidian use at this site was characterized in large part by the production of small serrated projectile points and simple flake tools from locally obtained cobbles. Various biface forms and cores were also produced. Temporal ranges for selected artifact types were presented above and appear consistent with findings by other researchers in spite of expectations; factors affecting hydration bands development and alteration, however, can not be ruled out. Specifically, hydration data derived from small serrated projectile points compared favorably with results from ALA-555 and the NCR studies by Fredrickson and Origer (1995). Finer resolution of the temporal relationships of the types defined will have to await further investigations with larger samples.

Compared to cumulative hydration data from the Napa Region, the Green Valley data shows less intensive use during the Upper Archaic Period and earlier times. Overall, this difference could be attributed to (1) archaeological hydration sampling strategies and changing research issues; or (2) a condition of development related processes as they pertain to site impacts; or (3) a combination of these and other factors. In Green Valley, SOL-71 exhibited hydration results indicating coeval use with SOL-356 during the Upper Emergent Period. Earlier use of the region was shown by data from SOL-69, SOL-71, SOL-315 and SOL-355/H.

Initial use of SOL-356 occurred during the Archaic Period. Given the absence of a larger number of temporally diagnostic artifacts, this use might have been of a task specific nature, such as hunting or fishing, by peoples occupying other sites within Green Valley. The obsidian assemblage attributed to this earlier use might consist of various bifaces and fragments plus debitage characteristic of tool maintenance.

Site use during the Emergent Period was marked by increased obsidian working and might represent a growing population, as well. Greater amounts of obsidian were recovered in upper excavation levels and late Phase 1/early Phase 2 point types were the most represented temporally diagnostic forms. It must be noted, however, that certain manufacturing trajectories can be responsible for the production of large amounts of debris and byproducts and should not be directly correlated with increases in populations.

A shift in reduction strategies from somewhat thicker, fully bifacial, parallel sided-stemmed serrated points to thinner serrated forms with expanding bases more often made on flakes was identified. Earlier reduction stages for the parallel-sided stemmed forms was not identified during these studies, however, some triangular bifacial blanks could be attributed to this trajectory on the basis of hydration results. A more robust manufacturing assemblage was defined for the later serrated point forms.

Certain reduction objectives of the people at SOL-356 were virtually identical to those of people at ALA-555--an archaeological site near PLeasanton--during the latter part of Phase 1 of the Emergent Period. Both populations were engaged in the production of small serrated projectile points made of Napa Valley glass. Manufacturing strategies focused on production of flat, simple flakes of suitable size from larger cores. Flakes were often detached from flat faces of these larger pieces. Factors defining the technological organization of the two groups, however, were quite different. As noted above, occupants of SOL-356 obtained the raw materials necessary for the production of small points primarily from the surrounding hills, splitting cobbles to generate surfaces appropriate to produce the desired flake blanks, whereas the individuals at ALA-555 obtained quantities of large flakes in trade from people in the Napa Valley. These large flakes, produced by various peoples during the preceding millenia and left at the quarry as workshop residue, served as cores and blanks for the manufacture of the small serrated points. Like the split cobbles used by people at SOL-356, the flatter faces of these flake cores were frequently selected for producing small flake blanks.

The presence of second hydration bands on several items has been interpreted as the product of recycling of scavenged cultural materials and use of non-local raw materials. Similar explanations have been posited to explain certain elements of assemblages with second bands from the Sierra Nevada and elsewhere (Skinner 1988). Recycling and reworking of scavenged materials often constitutes a technological strategy employed by people in areas of scarce resources providing an alternative to regularized exchange relationships. Given the results of the present study, it appears this strategy might have operated even in locations with abundant materials simply as one option to fulfilling resource requirements.

Social distance between the people occupying SOL-356 and those of the surrounding region can be addressed through the quantification of obsidian types and the forms in which they were found. The present studies showed that Napa Valley glass dominated the assemblage at all time periods represented. Much of this glass was probably obtained within a short distance of the site or the nearby hills. Some, however, derived from source areas further west and could be attributed to either exchange with other groups or the result of direct access through territories controlled by other peoples. The few formal tools made on Annadel glass recovered point to some interaction with peoples from the west during the Upper Archaic and late Phase 2 but probably on an ad hoc basis.

The preceding analysis has examined a number of aspects of cultural systems in the Green Valley locality during the past 3000 years and presented information useful for developing models of technological organization relative to obsidian use. Certain conclusions have been reached based on the available data but it is recognized that refinement of these models must await investigations of other single component deposits in the region. The materials recovered from SOL-356, however, have contributed significantly to emerging models of extinct cultures in the region.

References Cited

·  Bieling, D. G.
1992 Analysis of Obsidian Materials Recovered from CA-SOL-69 and CA-SOL-315. In, Archaeological Data Recovery at Sites CA-SOL-69 and CA-SOL-315, Green Valley, Solano County, California. Randy S. Wiberg, Holman & Assoc., San Francisco.

1993 Analysis of Obsidian from CA-SOL-355/H, Green Valley, Fairfield, Solano County, California. In, Final Report: Archaeological Data Recovery at Prehistoric Site CA-SOL-355/H, Green Valley, Solano County, California. Randy S. Wiberg, Holman & Assoc., San Francisco.

·  Fox, K. F.
1983 Tectonic Setting of Late Miocene, Pliocene, and Pleistocene Rocks in Part of the Coast ranges North of San Francisco, California. United States Geological Survey Professional Paper 1239.

·  Fox, K. F., Jr., R. J. Fleck, G. H. Curtis, and C. E. Meyer
1985 Potassium-Argon and Fission-Track Ages of the Sonoma Volcanics in an Area North of San Pablo Bay, California. United States Geological Survey Miscellaneous Field Studies Map 1753.

·  Fredrickson, D. A. and T. Origer
1995 Temporally Sensitive Attributes of Late Period Serrated Arrow Points in Sonoma County, California. Paper presented at 29th Annual Meeting, Society for California Archaeology, Eureka.

·  Skinner, E.
1988 Scavenging and Reuse: An Alternative to Models of Late Prehistoric Trans-Sierran Exchange in Central California. Paper presented at the 22nd Annual Meeting of the Society for California Archaeology, Redding, California.

·  Stevenson, C. M., D. Dinsmore, and B. E. Scheetz
1989 An Inter-Laboratory Comparison of Hydration Rind Measurements. International Association for Obsidian Studies Newsletter 1:7-13.

·  Tremaine, K. J.
1989 Obsidian as a Time Keeper: An Investigation in Absolute and Relative Dating. Master's Thesis, Department of Anthropology, Sonoma State University. Rohnert Park.

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