Hand-hewn granite basins at Native American saltworks, Sierra Nevada, California
“…the relatively uniform size, lack of overlap, and smooth hemispherical shape indicate that the (369) basins are not of natural origin, as reported in previous work.
Making these basins was challenging and required concerted effort by this group of Native Americans, though the exact techniques used to excavate so many basins in this glaciated bedrock are not known.
“Fire was probably used to heat the rock reducing its strength and making it easier to grind,” said Mike Diggles, USGS geologist and co-author of the report. “To deepen the basins just one centimeter, they had to build and maintain a hot fire on the rock, let it burn out, and then pound the bedrock with stone tools.”
The Miwok had to repeat this process about 100 times to carve a basin three feet deep into the stone. It would have taken several workers nearly a year to make just one basin.
The full report (http://pubs.usgs.gov/sir/2009/5225/), titled Hand-hewn granite basins at Native American saltworks, Sierra Nevada, California, was published in the beginning of November (2009).
Details of photos from relics of the ancients: