Here in Southern California, San Diego County was under water off the coast of Mexico. Volcanic islands had formed offshore (the shoreline was far inland from here so the volcanoes were east!) as the Farallon Plate was dragged eastward under the westward moving North American Plate. Some volcanoes made lava flows of a reddish-purple rock called rhyolite. Pieces of the rhyolite began to be carried downhill by fast streams, and tumbled into rounded shapes called cobbles. These cobbles can be seen on our beaches today. The lava flows they came from are not in California now, but remained in Mexico when Baja and Southern California were split away from the Mexican mainland by movement along the San Andreas Fault; but this is many millions of years ahead in our story.