SACRAMENTO, Dec. 29 (UPI) -- Geologists say they've found more than 350 ancient granite basins used by California's Miwok tribe to produce salt for trade.
The Miwok were a hunting and gathering tribe who lived in the northern Sierra Nevada mountains for nearly 5,000 years before contact with European Americans in 1769.
They were one of the first groups America Indian tribes to begin producing salt for trade, said Jim Moore, a researcher with the U.S. Geological Survey.
The basins Moore's team found were carved in granite in an area the size of a football field, each basin several feet in diameter. The basins were carved near a salt spring and the water carried to the basins to dry and leave a salt residue.
"To deepen the basins just 1 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," said geologist Mike Diggles. That process had to be repeated nearly 100 times to carve a basin 3-feet deep, Diggles said, estimating it would have taken several workers nearly a year to create one basin.
