LATE PREHISTORIC OBSIDIAN PRODUCTION AND EXCHANGE IN THE NORTH COAST RANGES, CALIFORNIA Thomas L. Jackson I NTRODUCHION OBSIDIAN SOURCES IN THE NORTH COAST RANGES This is a paper ofthree parts. The first part Obsidian in the North Coast Ranges originates in examines the geological distribution of obsidian in the either the Sonoma Volcanics or the Clear Lake Volcan- North Coast Ranges. A number of obsidian quarries ics (Figures 1 and 2). Obsidian fragments from the vol- and other potential sources not previously reported are canic fields are also found as clasts in the gravels of described and their trace-element geochemistry younger alluvial deposits along the fringes of the summarized. Secondly, I review the history and volcanics. In the Sonoma Valley and in areas west and concept of obsidian artifact "sourcing" by archaeolo- north of Santa Rosa, obsidian pebbles "apparently gists and geologists. Although the discussion relates derived through erosion of obsidian in the upper specifically to North Coast Ranges obsidian, the member of the Sonoma Volcanics" are definitive of the example is appropriate to a much broader audience Glen Ellen Formation (Fox 1983:11). Erosion of the concerned with determining the original source of raw Sonoma Volcanics and the Glen Ellen Formation has, material for any lithic artifact. I offer a "cautionary in turn, resulted in the incorporation ofobsidian pebbles tale" about how we come to "know" where obsidian into localized Quaternary alluvial and fluvial deposits. was obtained by prehistoric people. A number of Fox (1983:10) has divided the Sonoma Volcanics semantic conundrums arise and I make a recommenda- into two units, according to their age: "The lower tion for greater care in the use of terminology. Finally I member occupies most of the southern part of the discuss the selective exploitation of obsidian in the volcanic field as it is exposed today. The member North Coast Ranges by late prehistoric populations. consists chiefly of silicic basalt, andesite, and dacite Although there are numerous obsidian sources in the flows, with subordinate interlayered ash flows and region, not all were exploited for the manufacture of all rhyolite flows, and thus contrasts with the predomi- types of chipped stone artifacts. For example, only nately tuffaceous rock of the younger part ofthe field to certain obsidian types were used for projectile (arrow) the north" (Figure 2). Rocks of the lower member of points, while a broader inventory of obsidians was used the Sonoma Volcanics date from 5.5 million years for other artifact types. Available data strongly suggest (m.y.) to >7.1 m.y., and the youngest dated material in that the production and distribution of certain obsidian the Sonoma Volcanics is assigned an age of ca. 2.9 m.y. artifacts was closely controlled by social elites. 80 Contibudons ofthe Archaeological Research Facility Number 48, December 1989 FIGURE 1 DISTRIBUTION OF SONOMA VOLCANICS AND OBSIDIAN LOCALITIES IN THE SOUTHERN NORTH COAST RANGES. 4 WA (Geologc base mm after Fox 1963:Pato 1) [ Glen Ellen Formation Upper member Lower member Sonoma Volcanic* * city * geologic sample locality: quarry: A-Annadel SV-1 = Oakmont BC-Blossom Creek SV-2= Trinity CS-Crystal Summit seob 0 5 10 1s 20 25km MC-Meg's Crown NGM-Napa Glass Mtn. Obsidian Produedon andExchange in the North Coast Ranges, California 81 FIGURE 2 OBSIDIAN FLOWS IN THE CLEAR LAKE VOLCANICS Borax Lake obsidian flow o 5 lOmi 0o 5 i0k. Cache Formation Mt. Konoct i obsidian flow (Fox 1983:11; Mankinen 1972:2065). Obsidian occurs location of obsidian "quarries" in the North Coast in association with rhyolitic rocks in both members of Ranges. Heizer and Treganza (1944) examined the the Sonoma Volcanics. ethnographic and archaeological literature to determine Obsidians of the Clear Lake basin were produced the sources of various rocks and minerals (including during eruptions of the Clear Lake volcanics, and are obsidian) used by native Californians. Their inventory largely confined to the basin itself. The Clear Lake ofobsidian quarries contains numerous errors, stem- Volcanics are much younger than the Sonoma Volcan- ming in part from their identification of certain chert ics, generally dated as less than ca. 2 m.y. Obsidian quarries as obsidian sources. Ball (1941) reports a flows of the Clear Lake Volcanics have been radiomet- number of obsidian sources in the study area but rically dated, and range from ca. 1.01 m.y. to .088 m.y., locational data are minimal. Efforts at correlating more the younger date being for the Borax Lake obsidian recently discovered quarries in the Napa Valley region flow (Donnelly 1977:3-4; Donnelly-Nolan et al. 1981). with Ball's inventory are stymied for lack ofprecise Obsidian occurs as massive flows, domes and brecci- map data. Elsewhere (Jackson 1973; 1974)1 have ated tuffs on the south side of Mt. Konocti (Brice 1953; discussed the problems related to sorting out the lists of McNitt 1968)). Borax Lake, on the east side of Clear archaeologically reported obsidian sources in the Lake, was created when the small basin in which the region. lake is situated was sealed by an olivine dacite flow, Other terminological and comparative problems which is capped by a rhyolite obsidian flow (Anderson arise. For example, Ericson's (1977:101) obsidian 1936). Obsidian pebbles are a minor constituent of samples reportedly from "Napa Glass Mnt." were, in Cache Formation deposits southeast ofClear Lake fact, collected not only from Glass Mountain itself, but (Brice 1953:33; Figure 2). also from "E. Dago Valley", "W. Dago Valley", and There is some confusion regarding the number and "Hill 450+". Obsidian collected by Ericson from Dago 82 Coneribudons ofthe Archaeological Research Facility Number 48, December 1989 Valley (ca. 1A kilometers north of Glass Mountain) manufacture, and has not been detected in archaeologi- would almost certainly have been material eroded from cal collections. The obsidian is chemically unique the source I call "Crystal Summit" (discussed below). (Figure 3), and its distinctive physical appearance The location of his "Hill 450+" is uncertain, as there are would make it conspicuous in any collection of several hills near Glass Mountain with summit altitudes debitage or artifacts.- in excess of450 feet. The point is not to denigrate Ericson's important efforts, but with increased resolu- OBSIDIAN IN THE LOWER MEMBER OF THE tion in our knowledge of the complexity of the geology SONOMA VOLCANICS (apart from the semantic problems discussed earlier), there is a need for explicit geographical mapping of Obsidian occurs in both the upper and lower sources with a concern for distinguishing among inusitu members of the Sonoma Volcanics. The best known versus redeposited obsidian. obsidian in the lower member occurs in the vicinity of Obsidian occurs throughout much of the Sonoma Annadel State Park in the upper Sonoma Valley. In Volcanics field, for the most part, as very small pebbles addition to this archaeologically well known obsidian at ("Apache tears") unsuited for tool manufacture. I make least two other chemically discrete obsidian types are no attempt to discuss these occurrences, except those now recognized: Los Guilicos and Trinity. for which I am aware that obsidian pebbles are of In the upper member of the Sonoma Volcanics we sufficient dimensions to possibly have been used in tool find the famous 'Napa Glass Mountain" quarry and manufacture. I concentrate on obsidian in those areas several other newly documented quarries which I call where there is evidence for prehistoric quarrying. Blossom Creek, Crystal Summit, and Meg's Crown. Likewise, I do not attempt to describe the geographical These quarries all yield obsidian which is chemically distribution of obsidian which occurs as float in stream and physically very similar, although slight chemical channels. For example, obsidian is a constituent of variability exists among them. In addition to these gravels in the Napa River and in the streams which quarries, a chemically distinctive obsidian is found in drain the area of the Mt Konocti obsidian flow. tuffs ofthe upper member of the Sonoma Volcanics near Franz Valley. Other quarries are likely to exist. Burdell Mountain Annadel A highly weathered obsidian crops out on the southwest slope of Burdell Mountain in Marin County The Annadel obsidian quarry is located in what is (Figure 1). This obsidian consists of a black glassy now Annadel State Park in Sonoma County. The matrix containing a nearly equal volume of crystalline aboriginal quarry area is still covered with with vast inclusions, primarily feldspar. The obsidian is exposed amounts of obsidian flakes, partially completed tools, over an area ofapproximately 20 square meters, with and rejected raw material. Heizer and Treganza no evidence of quarrying. A recent excavation for a (1944:304) cite a personal communication with L. L. water tank adjacent the outcrop reveals isolated, nearly Loud, pioneering California anthropologist, to the effect disintegrated obsidian masses to a depth of more than 3 that quarry depressions excavated by the Indians were meters, suggesting that the remains are the last vestiges still visible ca. 1940. More recently Parkman (1983) of a small obsidian extrusion which has been nearly indicates that some of these features remain. It is obliterated by weathering. certainly true that distinct lithic reduction activity areas The obsidian and associated rhyolitic tuffs have not are still discernible at the site. Although the Annadel been recognized in published geologic mapping. The quarry has been explored by many archaeologists over area of the obsidian and tuffs has been mapped as part the years, to my knowledge no comprehensive mapping of serpentine and marine sedimentary units of the of the quarry exists. A map of the main quarry area Franciscan Complex (cf. Fox 1983; Koenig 1963; Sims made by archaeology classes from Santa Rosa Junior et al.