Sibley Volcanic Regional Preserve, in the Oakland hills. View north. The two peaks in the distance are Grizzly Peak (l) and Vollmer Peak (r). The foreground has equipment pads related to the Kaiser lava-rock quarry that was here for about 75 years. The basalt beds hold up the ridge on the right and are extensively quarried, revealing deep cross sections of a small volcano that existed here about 10 million years ago. Since that time the volcano and the sedimentary rocks beneath it have been folded and tilted eastward. You can see the layering of the sedimentary rocks in the nearmost ridge. Conglomerate rock has less soil and dries out brown, while the shales retain water better and support thicker growth. You can see the rocks are now tilted almost vertically. State route 24 runs behind the ridge in the middle distance, entering the Caldecott Tunnel under the ridge just off to the left.