This rock is displayed in
Treasures” collection on the roofed patio of the visitor
center in the Robert Sibley Volcanic Regional
Preserve. The above caption is the same that is given on-site
(June 11, 2012).
Sandstone is a sedimentary rock composed of quartz and feldspar grains. These sand-sized mineral grains are typically held within a silt or clay matrix.
The Siesta Formation in the Berkeley Hills consists of fine-grained, light gray sediments deposited from lakes that formed when lava flow, caused by volcanic eruption at Sibley's Round Top, dammed rivers. Kaitin Maguire describes the interesting geology and paleontology of the Miocene deposits of the Berkeley Hills, including the Siesta Formation: Bay Area Field Guide: Tilden Park