The monument in the visitor center describes the use of the mountaintop as the initial point for all of survey lines for Central California and Nevada. Any property lines using the township and range system of measurements in the region are based on measurements from the summit that started in the 1850s.
Despite the mountain's distinctive shape, it is not volcano. Even though some of the rock making up the mountain is volcanic, it is basalt and related rocks that were once part of the ocean crust, and not something erupted from a terrestrial cone. The origin of the mountain can best be described as ocean floor crust and Franciscan trench deposits breaching the Earth's surface like (oh, geologist friends forgive me) some gigantic Godzilla character in a movie. Described more properly, the rocks from deep in the crust have been thrust upwards along active compressional faults.
Actually though, the oak-covered ridges in the foreground reveal something of interest. The Great Valley is a deep trough with upwards of 25,000 feet of sedimentary layers dating back to the Mesozoic Era, the age of the dinosaurs. We would know little about them except that along the eastern Coast Ranges, the sediments have been curled upwards and exposed by erosion. The rocks surrounding the core of Mount Diablo are in fact called the Great Valley Group, and they contain a rich fossil record, including plesiosaurs, ichthyosaurs, mosasaurs, and even an occasional dinosaur bone.
lots of interesting flower species. The views included the Livermore Valley and the South Bay. A short distance later, I reached the end of the trail and we headed down the mountain. We made one more stop, at the very cool sounding Rock City.
Black Diamond Mines Regional Preserve.
Mt. Diablo State Park is an absolutely fascinating and beautiful island of wilderness in the midst of the Bay Area. It is a great place to visit, and a marvelous geological wonderland. If you are ever in the region, check it out!