17 Undergraduate Research Posters (Posters)

Tuesday, 5 April 2016
8:30 AM - 5:30 PM
Ballroom C

SEARCH FOR THE SOURCE OF COGSTONES IN SOUTHERN CALIFORNIA

Paper number: 17-32
Program: Theme Sessions
Time: 8:30 AM – 5:30 PM

Authors

Abstract

Cog stones are 6000-3500 BC old Native American artifacts carved in a way that look like a cog gear. They have been only found in southern California and are dominantly made out of a variety of scoria basalt. The use and significance of these stones is unknown and no flawless theory has yet been agreed upon. The location of the basaltic source rock for the stones is also not known, however, multiple basalt locations crop out in southern California and it is possible the source rocks were derived from nearby. Thus finding the location of the source rocks may help narrow down the possibilities of what the stones were used for and reveal more about the lifestyle of Native Americans in Southern California.

We use petrography and geochemical analysis on four cog stone fragments collected at different sites in Orange County. We are comparing these to scoria basalt collected on the SE side of Catalina Island near Two Harbors (CL1B), a site known to have been inhabited by Native Americans, the Santa Monica Mountains near La Vina Gomez de Malibu along Mulholland Highway (SAM), at Van Winkle Mountains (SW Mojave National Preserve; VW1) and at Fossil Falls along Hwy 395 (FF1). Three of the four cog stones are vesicular basaltic scoria and one is volcaniclastic tuff. Petrographic analysis revealed the cog stones are plagioclase rich with various amounts of olivine, pyroxene, oxides and iddingsite and varying grain size. Samples SAM and VW1 were eliminated as source rocks for these particular stones based on differences in petrography. The texture, grain size and mineralogy of samples CL1B and FF1, however, could be both possible matches for cog stone CS3. XRF major oxide and trace element analysis shows similarities in chemistry between FF1 and CS3 with only minor differences in SiO2, CaO, Ba, Ni, Cu and Nb content. Samples CL1B and CS3 yield some overlap in major oxide and trace element concentrations, but enough variance to indicate a match is probably less likely. However, more geochemical analyses are underway to further test our working hypotheses. If the analyses yield positive match results with Fossil Falls (FF1) and Catalina Island (CL1B), then there is a possibility that Native Americans had a reason to travel far to collect cog stone source rocks, however, other potential source rock comparisons have indicated possible nearby locations as well.

Geological Society of America Abstracts with Programs. Vol. 48, No. 4

doi: 10.1130/abs/2016CD-274302

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