Roadside Geology of Northern California

For the last 200 million years the old western margin of North America has ground over the Pacific Ocean floor moving slowly eastward beneath it.

Sand and mud bulldozed from the ocean bottom and jammed against the continental margin have become California, one of the newest parts of the continent. Volcanoes erupting from the heated interior sent floods of fiery black basalt over thousands of square miles in the northeast and covered the Sierras in a deep blanket of ash.

Earthquakes continue as the Pacific Ocean bottom drags the western edge of California northward along the San Andreas fault.