All the lava flows at Fossil Falls are basaltic. The Coso Volcanic Field brought flows from the north east and later Red Hill, which can be viewed from Fossil Falls, released the younger lava. The flows occurred between 400,000 years ago and 10,000 years ago. During this period, the glacial flows would run through Fossil Falls and smooth the vesicular basalt. The erosion found at Fossil Falls was formed by the youngest glacial runoff, called the Tioga, from the Sierra Nevada Mountain Range about 20,000 to 10,000 years ago.