SANDSTONE FROM THE SIERRA Mighty rivers washed granitic rock from the Sierra Nevada westward into deep basins. You can see the evidence in the caves and castles at Mount Diablo's Rock City.

The TRAIL THROUGH TIME series features naturalist Ken Lavin and is sponsored by the Mount Diablo Interpretive Association in partnership with Save Mount Diablo. Videography by Macha Rose. Animations by Tanya Atwater, UC Santa Barbara. Music by Phil Heywood. Produced by Joan Hamilton.