Over the centuries, the small amount of acid in "unpolluted" rainfall still manages to carve caverns and fantastic shapes in our sandstone: "Tafoni". Lichens colonize the exterior stone, but that inside the cavity is more smooth. Above this ancient cavity remains evidence of Native American grinding -- acorns and perhaps other plants. Jasper Ridge Biological Preserve, Trail a. 7/17/2022, 2:17 pm.