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 "factories" created and used by ancient Miwok 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 in May to study what he determined were clearly hand-hewn objects. He examined 369 of the circular artifacts only a few yards from two streams of saltwater fed by a nearby spring and a lake that was equally salty. Moore and his colleague at the USGS, Michael F. Diggles, believe the circular basins were handmade by the Miwok 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 that the basins are in a canyon somewhere within the Stanislaus National Forest. [...]