San Francisco Bay Region Geology

The San Francisco Bay region is underlain by some of the most complex and active geology in the world. The earth here is made up of everything from iron- and magnesium-rich rocks formed deep in the interior of our planet to sand and gravel recently deposited by our streams and rivers. All of these rocks are constantly being bent and tilted by the forces that also drive our San Andreas, Hayward, and other faults. The same forces that cause earthquakes have shaped the landscape that makes our region unique: the hills, the valleys, and the Bay itself. That landscape is still being formed, the Earth's forces driving hills and mountains up, rain and gravity dragging them back down, sometimes in massive landslides or dangerous flows of mud and debris.

This website provides a gateway to all kinds of information about the geology of the region, including geologic maps and paleontology (fossils), and links to other sites with even more information about marine geology, earthquakes, landslides, and more.

Above: Geologists examine the contact between the dark brown lava-rocks of the Moraga Volcanics (upper right) and the green-gray sandstone and conglomerate (pebbles and cobbles in sandstone) of the Orinda Formation at the east end of the Caldecott Tunnel between Oakland and Orinda. This contact formed about 10 million years ago when lava from a nearby volcano flowed into an ancient river valley and onto the sand and gravel of the riverbed.