While volcanic activity has long since ceased in Southern California, it has left its mark. From cinder cones in the Mojave Desert, geothermal vents in the Imperial Valley, and extensive basalts in the Santa Monica Mountains, volcanoes have played a big role in the geology of Southern California.
The most recent volcanic activity was relatively recent in geologic time in the Mojave Desert. There, two large cinder cones were formed near Amboy on old US 66. Amboy Crater and Pisgah Crater were both active around 100,000 years ago. The basalts of the Santa Monica Mountains are much older. They date to the Miocene Epoch and are about 20 million years old. In the Agua Dulce area, the base of the Vasquez Formation has basalts dating back to the Oligocene Epoch (about 30 million years ago).
These pages will feature some of the more interesting aspects of these rocks and formations.