[1] Volcanic clastic rock cobbles of rhyolite, in a sandstone matrix in this area are named Poway clasts.
[2] The ancient Ballena River brought rhyolite-gravel, or "Poway" clasts, from a region in present-day Sonora, Mexico to the Pacific Ocean.
Its sediments deposited into an alluvial fan–submarine canyon–submarine fan complex extending for miles offshore.
Remnants of submarine fan facies outcrops are found as far west as the northern Channel Islands.
Inland Ballena River deposits outcrop discontinuously over 16 miles (26 km) in a west-southwest trend from Whale Mountain to San Vicente Reservoir; here, the river was up to 2 miles (3.2 km) in width through the Peninsular Ranges.