After erosion exposed the laccolith, volcanic activity continued and the youngest flows are 1-1.6 million years old. Many volcanic cinder cones can still be seen in the foothills of the Pine Valley Mountains and these have been dated at around 20,000 years old. As a result of these lava flows, the valley for which the mountain range is named and in which the town of Pine Valley is situated, was formed when lava dammed off the Santa Clara River and formed a lake. Sediments eventually filled the lake until they reached the height of the lava dam. These sediments form the floor of the present day Pine Valley, Grass Valley, and Grassy Flat.