Over ten thousand years ago, California's Sonoma Coast lay at the end of a broad coastal terrace. Today, a cluster of mushroomlike outcrops—some nearly sixty feet high—have odd polished areas from ground level to fourteen feet high. These rocks lie near what may have been a terminus for now-extinct megaherbivores (herbivorous mammals exceeding one thousand kilograms) migrating seasonally from valley to coast. The now-submerged coastal savanna attracted mammoths and mastodons from interior pastures, and they may have bathed near the outcrops in what archaeologists suspect is a prehistoric wallow, and then rubbed themselves clean on the outcrops.

Domestic livestock have grazed in the area for over a century, polishing fence posts and rock outcrops to an oily sheen by frequently rubbing against them. This suggests that animals other than megaherbivores could be responsible for the polished areas. However, cows cannot account for rubbings fourteen feet high. Rain and wind weathering has been considered as an explanation, but such weathering would polish the rocks indiscriminately, not just where the rubbing patterns occur.

Recently, researchers analyzed samples of the rubbed rock using microscopes, confirming that the polishing was not caused by weathering. Instead, the scratches worn into the stone closely resemble those on wooden rubbing posts used by zoo elephants to remove grit from their fur after mud baths.