Somewhere in the Sierra Nevada, a granite terrace the size of a football field holds hundreds of mysterious stone basins representing what geologists believe is one of the earliest known American Indian "factories," created and used by ancient Mewuk Indians to make tons of salt to trade with tribes up and down California.
James G. Moore, a geologist with the U.S. Geological Survey in Menlo Park, learned of the strangely pitted terrace from detailed maps made more than a century ago and hiked the region east of Modesto in May to study what he saw were clearly hand-hewn objects.
He examined 369 of the circular artifacts — 325 of them carved in orderly ranks only a few yards from two streams of salt water fed by a nearby salt spring and a lake that was equally salty.
Moore and his colleague at the USGS, Michael F. Diggles, believe the circular basins were hand-made by the Mewuk people in an impressive display of early technology. They have published a detailed account of their findings in an official Geological Survey report, but because the area is now an "archaeologically sensitive" site and its location protected by law, Moore is permitted only to say the basins are in a canyon somewhere within the Stanislaus National Forest.
"This is quite likely to give us new insights into the lives of the Mewuk people in the Sierra," said Kent G. Lightfoot, a professor of anthropology at the University of California at Berkeley and a specialist in the history and culture of California's Indians.
Records show that early American Indians, including the Mewuk people, lived for thousands of years as hunter-gatherers in that area of the Sierra, Moore said, and it is filled with evidence of old settlements, with abundant middens, arrowheads and small stone tools. But how long ago the basins were carved awaits high-tech dating.
The basins average more than a yard in diameter and are more than two feet deep.
To create them, Moor and Diggles said Mewuk tribe members built fires on the granite surface that heated the stone until it fractured. They then crumbled and pounded the fractures with stone tools and removed the debris, inch by inch, until the basins were formed, the geologists said.
Diggles estimated it took Mewuk workers nearly a year to complete a single one. He calculated that each fire used to dig a single layer of rock deepened the granite by no more than a centimeter. The process, he said, must have been repeated 100 times to make a single basin.
Similar granite basins were discovered in 1891 by Henry W. Turner, a geologist exploring California's mountains in what is now the Sequoia National Park, Moore said in their Geological Survey report. Moore has examined those, too.
"I think of them as the Machu Pichu of North America," Moore said. Machu Pichu is the ancient city of stone in the Peruvian Andes, abandoned by the Incas nearly 1,500 years ago.
Salt springs are rare in the Sierra Nevada, but Moore said the salt in the nearby streams probably comes from a layer of ancient marine sediments formed many millions of years ago when the area was covered by an ocean.
He said he believes the Mewuk people carried water from the streams in watertight woven baskets, poured it into the basins and let it evaporate in the summer heat until the dry salt could be scooped out.
The salt content of the water and the rate of water flow indicate that the two streams probably yielded about 3 tons of salt each year, Moore said.
The people of the area, he said, "had a large and valuable surplus to trade with other tribes, an early example of commerce by hunter-gatherer people."
Chemical analysis of the water also shows high levels of arsenic — 170 times higher than the maximum allowed in drinking water by the Environmental Protection Agency, he said. It is unknown whether arsenic made its way into the salt.
"Salt was an important commodity for Native Americans," Lightfoot said. "It is certainly possible that salt harvested from these basins could have been traded to other native groups in California and the Great Basin (east of the Sierra). Further work will be needed to develop a solid chronology for the basins."