LOCATING THE VOLCANIC SOURCE ROCK OF PREHISTORIC COGGED STONES FROM SOUTHERN CALIFORNIA: WERE THEY CARVED FROM EL MODENA AND SANTA ROSA BASALTS?

Cog stones, hand-size Native American artifacts carved in the shape of cogs, have only been found in Orange County, CA and are dominantly made of basaltic scoria. The use of cog stones is unknown with ca. 40 different potential uses proposed to date. The purpose of this study is to identify the volcanic source location from which the cog stones were carved. This may help reveal the significance of the cog stones to 6000-3500 BC Native Americans.

To identify the source location of basaltic cog stones four cog stones fragments unearthed in Orange County were used for analysis. We focused on two potential source locations: the El Modena volcanics, which have been previously suggested as a likely source for the cog stones, and the nearby Santa Rosa volcanics in Riverside County. Petrographic observations and whole rock XRF geochemistry are used to compare the cog stones with the potential source samples. Given the variability in the composition of basaltic cog stones, it is unlikely that all cog stones are from the same volcanic source.

Preliminary results from petrographic analysis of thin sections of the cog stones show that they have porphyritic texture and are composed of mainly plagioclase laths with varying amounts of ortho- and clinopyroxene, olivine, opaque minerals, and iddingsite. Petrographic analysis of Santa Rosa basalt has the same mineralogical composition and texture as cog stone CS3. We are further testing this potential match through XRF analysis. Mineralogy and texture of the El Modena basalts do not resemble that of any of the cog stones analyzed, however XRF analyses of two El Modena samples suggest similar geochemical compositions to CS2. They contain ca. 55 wt.% SiO₂, 20-21 wt.% of Al₂O₃, 5-6.5 wt.% Fe₂O₃, 8-9 wt.% of CaO, and similar trace element concentrations, e.g. 200-230 ppm Zr, 320-400 ppm Ba, 16-20 ppm Nb, and 600-700 ppm Sr. El Modena basalt has smaller plagioclase exhibiting a sieve structure, lower vesicle and matrix abundance, and does not contain clinopyroxene and olivine. More analyses are underway to further examine these relationships. If we can confirm that both El Modena and Santa Rosa volcanics were sourced to carve cogged stones, it would suggest that Native Americans collected these rocks perhaps because they were soft to carve and found nearby; the collection site may have not been important.