Franz Valley Obsidian
8 June
1997
Identification
of Franz Valley
obsidian in North Coast Ranges archaeological
assemblages has been limited to date. This glass group has been described in
primary and secondary geological contexts corresponding to the Glen Ellen
Formation (Fox et al. 1985) in areas north and northeast of Santa
Rosa including the uplands east and west of Franz
Valley, the southern rim of Alexander Valley,
and on the plains east of the Russian River between Mark West Creek and Windsor. Though geochemically
distinct, Franz Valley
glass is always found with greater amounts of Napa
Valley obsidian and never as a
discreet quarry flow (Jackson
1986). Recent thesis investigations by Psota revealed
greater Native American use of this material than previously supposed (1994).
While
experienced analysts achieve a 95% accuracy rate for visually sourcing the four
major obsidian glass groups in the North Coast Ranges of northern
California--i.e., Napa Valley, Annadel, Borax Lake,
and Konocti--(Wickstrom and
Fredrickson 1982), Franz Valley glass is marked by a higher degree of
macroscopic variability. Macroscopic attributes of this glass range from non-lustrous
grey or dark grey, frequently banded (Annadel-like)
to translucent, glossy, often with fine bands (Napa Valley-like) to minimally
glossy, non-translucent grey, often banded with phenocrysts;
the latter group being the most readily identifiable as Franz Valley
glass.
One hundred
forty-two Native American archaeological sites were included in Psota's research area; twenty-five sites with collections
containing 8 or more obsidian artifacts were examined for that study. Ten site
collections were comprised of samples containing 100 or more obsidian items;
within these, all formal and informal tools and approximately 50-100 flakes
were examined and visually sourced by Psota; all
materials in the other fifteen were visually sourced. This sample of visually sourced
obsidian amounted to 3,661 items; about 12% of these were also visually
characterized by Thomas Origer, Principal
Investigator of Origer & Associates, an experienced obsidian specialist. Items for which visual
source assignments were in dispute by these two researchers and many items
assigned by both researchers to Franz Valley obsidian were then geochemically characterized (n=106). Results of the
sourcing analyses confirmed the widespread use of Franz Valley
obsidian within the research area.
Hydration
values on 89 tested Franz
Valley glass artifacts
from the north, central, and southern study area localities show a range of 7.4
- 1.1 microns. Greater modal groups in this hydration span include 60 values
between 3.4 - 2.0 microns and fourteen in the 1.8 - 1.1-micron range. Although a hydration comparative constant has not
been established for this glass, and geochemical intrasource
variability has not been examined, it is provisionally suspected these
hydration periods correspond approximately with a span of time encompassing the
Middle Archaic Period through the Late Period based on artifact styles and
periods of site use.
The greatest
occurrence of this material in archaeological assemblages has been in sites near
and immediately south and west of the Franz Valley
source localities. Although artifacts made from this glass have been identified
in lesser frequencies in earlier assemblages in the region, it occurs most
often in sites containing later components. Additionally, in the southern
research locality the frequency of occurrence declines in proportion to Annadel glass and in southern Sonoma County, significantly diminishes in number. In Marin County,
for instance, only three artifacts--Late Period corner-notched points from
CA-MRN-307--had been identified (Jackson
1986).
Recent
excavations at CA-MRN-254 in San Rafael, Marin County, California
by Holman & Associates identified two items assigned to Franz Valley
glass. Like others in this glass group, they were marked by widely divergent
macroscopic attributes. Item 97-1-138, a small cobble core made on float
obsidian was glassy, clear to moderately translucent, moderately banded with
light grey striations, and contained small phenocrysts.
The dorsal exterior surface on this item had minimally eroded cortex
characteristic of geological tumbling. A biface,
97-1-997, was a matte grey, non-translucent glass with irregular black
striations, and also contained small phenocrysts; no
cortex remained. The two hydration values were 1.2 and 1.8 microns,
respectively.
Identification
of Franz Valley glass needs to be incorporated
into obsidian studies by archaeologists conducting research in this region.
Additionally, more work needs to be done with this glass group including
studies of intrasource geochemical variability and
development of comparative constants through induced hydration rates. Following
this, it should be possible to achieve greater advances in studies addressing
cultural technological trajectories of this material. Since Franz Valley glass
never occurs geologically without Napa Valley glass, it cannot be assumed
cultural procurement of the latter was focused on the Napa Valley glass flows
in the northern part of that valley; some, if not most, of the Napa Valley
glass identified in the same site assemblages might have originated from
displaced geological material or unidentified sources within Sonoma County.
· 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.
· Jackson, Thomas L.
1986 Late Prehistoric Obsidian Exchange in Central
California. Unpublished doctoral dissertation,
Department of Anthropology, Stanford
University.
· Psota, Sunshine
1994 Native American Use of Non-Quarry Obsidian in Northern Sonoma County:
A Preliminary Assessment. M.A. thesis. Rohnert Park: Department of Anthropology, Sonoma State
University.
· Wickstrom, Brian P. and David A. Fredrickson
1982 Archaeological Investigations at CA-SON-20, Santa
Rosa, Sonoma County,
California. Report on file at the
Anthropological Studies
Center, Sonoma
State University,
Rohnert Park,
California.
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