Reexamining the Potential to Visually Source
Western Great Basin Obsidians

updated 8 June 1997

The following paper presents the results of a study conducted by Anthropological Studies Center personnel several years ago to reassess the potential for visually segregating obsidian glass groups commonly recovered in western Great Basin and Central Sierran archaeological sites. The study was conducted as an adjunct to an archaeological investigation of several prehistoric sites situated along Highway 395 near Bridgeport in Mono County, California. Previous studies of the efficacy of segregating western Great Basin glasses employed only archaeological specimens. While their accuracy rate was fair, the study samples were overweighted in favor of the most easily recognized glass, Casa Diablo. Errors occurred most frequently in the smaller samples of Bodie Hills items in their study.

In the ASC study, we decided the first step was to determine whether macroscopic attributes of non-archaeological samples from each glass group were distinctive enough to correctly identify the particular glass a large percentage of the time. The results of this first level of analysis were promising: a couple people sorted most of the groups with a 90+% success rate which seemed to be based primarily on the researcher's familiarity with recognizing certain attributes. Not surprisingly the next step, dealing with archaeological materials, had a lower success rate.

Reexamining the Potential to Visually Source
Western Great Basin Obsidians

Sunshine Psota
19 August 1990

Paper presented at Annual Meeting of Society for California Archaeology, Foster City.

Sunshine Psota
Anthropological Studies Center
Sonoma State University
Rohnert Park, CA 94928


Previous studies suggest that certain western Great Basin obsidian sources are more reliably identified than other sources. Given the intra-source macroscopic variability and inter-source similarities of these obsidians, tests were designed to access accuracy rates. Three tests were conducted using a reference collection of five obsidian sources, then selected researchers using archaeologically recovered obsidian from the Mono area, further accessed reliability. Conclusions will focus on results of reference collection tests and XRF assignments to archaeologically recovered obsidians.


In 1989 the Cultural Resources Facility at Sonoma State University investigated five sites near Bridgeport, California for proposed California Department of Transportation highway improvements. Situated in the eastern Sierra Nevada, north of Mono Lake, these sites included both sparse and extensive lithic scatters, comprised overwhelmingly of obsidian. Given the size of the archaeological collection and the expected predominance of various local sources, it was decided to re-evaluate the potential for visually sourcing western Great Basin obsidians as an economical method for characterizing prehistoric use of this resource. A multi-phase test, consisting of both macroscopic characterization of reference materials and comparison with x-ray fluorescence (XRF) assignment of archaeologically recovered materials, was established.

Obsidian sourcing has been an important tool for developing inferences about exchange patterns and subsistence strategies. There are a variety of techniques available for determining the geologic origin of obsidian such as neutron activation analysis and XRF. Although these techniques have been used with success, labs employing these methods are either not readily available or the costs for processing large numbers of specimens are prohibitive. Researchers using magnetic sourcing (McDougall, et al. 1983), and microprobe analysis (Merrick and Brown 1984) have developed faster and more cost efficient approaches, but the equipment required is expensive and for various reasons not always an acceptable alternative. Perhaps the most expedient and economical technique for identifying large amounts of obsidian is visual sourcing. This method of sourcing has had variable results. Wickstrom and Fredrickson (1982) have demonstrated a 90% or better success rate in identifying four obsidian sources in the North Coast Ranges of California. In Chiapas, Mexico (Clark and Lee 1984) and southern Italy (Ammerman 1979) researchers have had similar success with visual sourcing. It was hoped visual sourcing of the major western Great Basin sources situated in the Bridgeport and Long Valley regions could be equally reliable.

In visual sourcing tests employing macro and microscopic techniques using archaeologically recovered obsidians from the western Great Basin, Bettinger, et al. (1984) produced results of 83 to 89% success. Their third test which consisted of 60 specimens predominantly from the area north of Mono Lake was most similar to our sample. It consisted of a sample in which 37% were Mt. Hicks, 25 % were Queen, and 17% were Bodie Hills; three Casa Diablo and 10 specimens from a northern source were also included. They interpreted their decreased accuracy rate of 83% to have been affected by the inclusion of Bodie Hills. Criteria for accurately sorting Bodie Hills obsidian from others in this region was not accomplished, and they suggest that it may not be possible.

In contrast, Hull (1988) restricted her experiment solely to the degree of translucency exhibited by obsidian specimens. She found that equating Casa Diablo with the most opaque samples, resulted in a high degree of reliability in collections from Yosemite National Park. Both of these studies also noted difficulty in distinquishing some Mt. Hicks obsidian from Bodie Hills and found better success with some sources than others.

Reference Collection

In the Mono area four major sources occur in relative close proximity: Casa Diablo, Bodie Hills, Mount Hicks, and Queen/Truman Meadows. A non-cultural reference collection was developed by obtaining un-modified cobbles from prehistoric quarry areas from three locations at Casa Diablo and Bodie Hills, and from smaller areas at Mount Hicks and Queen/Truman Meadows. Less economically significant sources such as Mono Glass Mountain and Pine Grove, which have been sparsely represented in archaeological collections (Jackson 1989, Turner 1989, and Moore 1989), were not examined due to time and cost limitations. To compensate for some of the biases of these tests, a non-local obsidian, Fish Springs, was included in the test samples. Since Fish Springs can contain similar visual attributes as some Bodie Hills and Mount Hicks material, it was included to determine whether it could be successfully separated out from the other sources. These sources then represent the same sources used by Bettinger, et al. (1984). It is recognized that the materials collected do not represent all prehistorically used areas within these sources, the full range of visual variability found in archaeological collections, or the only sources prehistorically used.

Test Preparation and Procedures

At the Bridgeport sites, some investigation areas characterized by secondary biface reduction were identified. Using the reference collection cobbles, similar biface reduction activities were duplicated, with each cobble's materials bagged separately. Three tests were prepared, each containing 90 specimens. These tests were designed to: (1) reflect the different locations collected from each source; (2) exhibit the greatest visual variability within the sample; (3) include varying amounts of cortex on 20% of each source's sample; and (4) include predominately thinner and smaller flakes which reflected the archaeological materials. Test specimens ranged in size from 6 to 20 mm, and in thickness from 0.35 to 27.7 mm.

Prior to taking the tests, participants familiarized themselves with Bettinger, et al.'s (1984) descriptions, the reference materials, and descriptions characterizing the reference collection compiled by Thomas Origer and Kim Tremaine, both from Sonoma State University. Seven people, with varying amounts of related obsidian experience, took these tests. Each specimen was checked for attributes such as color, flow structure, inclusions, texture, luster, and other characteristics such as cortex. In total 1800 identifications were made. For the first test, Test A, participants made identifications utilizing a number of approaches, such as direct sunlight, incandescent, and fluorescent lighting. More standardized procedures were established for Tests B and C which included using a light table, an incandescent lamp, review of previous test results, use of Origer and Tremaine's descriptions while taking the tests, and grouping like attributes with like attributes.

Results of Reference Collection Tests

Results of the first reference collection test suggested these western Great Basin obsidians could be visually sourced. From the results of Tests B & C it was concluded: 1) participants tended to improve with each test, with some participants consistently more adept in sorting visual attributes than others; 2) the amount of exposure to these sources through other avenues, such as laboratory work, usually yielded better test results; and 3) smaller debitage tended to be more difficult to identify, as it tended to contain less diagnostic characteristics. Most participants achieved a 85% to 100% success rate for identifying specimens of Casa Diablo and/or Queen/Truman Meadows. Bodie Hills and Mount Hicks were confused, at times, by all participants. In the third test, scores reflected a range of success with two participants achieving a poor accuracy rate, while four of the most successful participants accomplished an overall success rate of 87%.

Visually Sourcing Archaeologically Recovered Obsidian

With wide-eyed optimism, two of the more successful participants were selected to sort the archaeologically recovered obsidian sample from the Bridgeport sites. The researchers began by examining two bags of obsidian debitage, containing approximately 100 flakes. In contrasting the reference collection tests with the archaeologically recovered obsidian, it was found that patination can obscure some visual attributes. Also, most of the debitage was characterized by greater variability than the reference collection and exhibited some different attributes. The obsidian was sorted into groups characterized by like attributes and not by presumed source. By sorting like attributes with like attributes, 10 groups were developed with one defined as miscellaneous. It was expected that the debitage sample would be characterized by fewer sources, unlike formal tools which were expected to comprise greater variability of sources.

Using these attribute groups, the researchers then attempted to sort 60 diagnostic projectile points from the sites. Additional groupings were identified upon discovering that a wider range of visual attributes existed for these tools. This may in part be related to their greater size. At least 16 groupings were eventually delineated on the basis of differences in visual attributes. After independently sorting an additional 273 formal tool specimens according to these groupings, the researchers agreed approximately 42% of the time.

From these groupings, 18 pieces of archaeologically recovered debitage, representing nine categories, were subjected to XRF analysis. This small sample is biased towards primarily small, thin flakes, which may be the least identifiable but which constituted the majority of the archaeological materials. Comparing the XRF results with these assignments was discouraging, the best overall accuracy rate was 44%. XRF analysis assigned all but one flake to the Bodie Hills source with the remainder assigned to Mount Hicks. These results indicate that the established categories may not represent sources, but only variation within sources.

Next, the 60 projectile points were visually sourced into the groupings, assigned to a presumed source, and submitted for XRF analysis. In comparing the XRF assignments with these identifications, one researcher assigned approximately 48% correctly, while the other assigned 85% correctly. Additionally, the latter assigned specimens correctly to the Bodie Hills source better than 97%. Remaining sources assigned by XRF analysis comprise very small samples which will not be further elaborated.


In summary, an attempt was made to reevaluate the reliability of visually sourcing archaeologically recovered obsidian from the western Great Basin. The reference collection tests were designed to familiarize researchers with these obsidians, the range of variation, and to provide preliminary assessments of how reliably sources could be identified in archaeological collections. Test results indicate a moderate success rate in sorting the reference collection obsidians, but did not achieve a 90% or better success as some researchers in other regions have yielded (Ammerman 1979, Wickstrom and Fredrickson 1982, Clark and Lee 1984). Archaeologically recovered obsidian, however, was not as accurately attributed to sources. Although the accuracy rate for sourcing the XRF assigned debitage was poor, both participants improved significantly when sourcing the XRF assigned projectile points. This may be attributable to experience gained from previous tests and to the larger size of the materials.

In addition the following conclusions have been reached: 1) most participants exhibited an ability to improve significantly with each additional reference collection test which gave participants important experience for sorting archaeologically recovered obsidian; 2) projectile points, bifaces, and large to medium size flakes were more successfully visually sourced, whereas source identification of flakes smaller than 15 mm tended to be less reliable; 3) individuals need to evaluate their accuracy rates as some were better able to visually source obsidian than others, regardless of related obsidian experience; 4) accuracy rates might improve if microscopic examination was combined with macroscopic examination (Bettinger et al, 1984, Roper 1989, and Hale 1989); and 5) as previous studies have noted, inclusion of Bodie Hills and Mount Hicks adversely affects the accuracy rate within a sample.

Comparisons of our success rates to other regional researchers may not be entirely appropriate given the variable proportions of different obsidians within a geographic region. Thus, assemblages derived from areas in close proximity to the Casa Diablo source may be expected to contain high proportions of this obsidian and visual sourcing results should be more reliable than those for archaeological sites within the Bodie Hills/Mt. Hicks region. Also, although one participant's accuracy rate was virtually identical to Bettinger et al.'s (1984), this comparison does not reflect similar amounts of problem sources as only a small amount of Bodie Hills was represented in Bettinger et al. collections and the Bridgeport archaeologically recovered sample contained a high percentage of Bodie Hills obsidian.

In conclusion, visual sourcing is viewed as a preliminary means to discriminate between obsidians and of practical importance when examining large amounts of material. Accuracy rates for visually sourcing western Great Basin obsidians will vary geographically due to subsistence strategies and exchange patterns. Although the best overall accuracy rates for the study region are not as reliable as rates obtained for other areas, the sample size of materials tested by XRF analysis is much lower than necessary for an accurate assessment. More detailed experiments need to be implemented under controlled conditions with specific objectives in mind. In this manner, visual sourcing of obsidian materials from specific regions, in combination with selective XRF analyses, may prove to be a viable cost-effective method for determining the composition of archaeological assemblages. For this region of the western Great Basin by these researchers, however, visual sourcing has not had a high enough accuracy rate to be reliable for large scale use.

References Cited

·  Ammerman, Albert J.
1979 A Study of Obsidian Exchange Networks in Calabria. World Archaeology 11(1):95-110.

·  Bettinger, R.L., M.G. Delacorte, and R.J. Jackson
1984 Visual Sourcing of Central Eastern California Obsidians. In Obsidian Studies in the Great Basin, edited by Richard E. Hughes, pp.63-78. Contributions of the University of California Archaeological research Facility No. 45. Berkeley.

·  Clark, John E. and Thomas A. Lee, Jr.
1984 Formative Obsidian Exchange and the Emergence of Public Economies in Chiapas, Mexico. In Trade and Exchange in Early Meso-america, edited by K.G. Hirth, pp.235-275. University of New Mexico Press, Albuquerque.

·  Hale, Mark
1989 Personal Communication.

·  Hull, Kathleen L.
1988 Obsidian Studies in Yosemite National Park: Preliminary Observations. In The Proceedings of the Society for California Archaeology, edited by Susan M. Hector, Lynne E. Christenson, G. Timothy Gross, and Martin D. Rosen, pp. 169-188. Volume 1. Society for California Archaeology, San Diego.

·  Jackson, Robert J.
1989 Personal Communication.

·  McDougall, J.M., D.H. Tarling, and S.E. Warren
1983 The Magnetic Sourcing of Obsidian Samples from Mediterranean and Near Eastern Sources. Journal of Archaeological Science 10:441-452.

·  Merrick, H.V. and F.H. Brown
1984 Rapid Chemical Characterization of Obsidian Artifacts by Electron Microprobe Analysis. Archaeometry 26(2):230-236.

·  Moore, Joe
1989 Personal Communication.

·  Roper, C.Kristina
1989 Personal Communication.

·  Skinner, Craig E.
1983 Obsidian Studies in Oregon: An Introduction to Obsidian and an Investigation of Selected Methods of Obsidian Characterization. M.S. Thesis, University of Oregon, Eugene.

·  Turner, Arne
1989 Personal Communication.

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