The Poway Formation is an Eocene Formation which includes a conglomerate member and a sandstone member. It is exposed in western San Diego County, California, and has a maximum thickness of 450 feet. The conglomerate member consists of boulder, pebble and cobble conglomerate with some sandstone lenses. More than 70 percent of the clasts are volcanic porphyries and tuffs of dacite-rhyolite composition. Lead-alpha age determinations on zircons from dacite and rhyodacite clasts from the conglomerate member of the Poway Formation have yielded ages of about 260, 250, and 190 million years. Quartzite clasts from the same formation yielded ages of 1450 and 2200 million years. Poway type clasts are primarily found in Eocene or younger conglomerates throughout southern California. Based on the dates, lithologic similarity, availability of source rocks, and occurrence of the clasts in Eocene and post-Eocene conglomerates, the writer suggests that the probable source area was in the region now occupied by the Mojave Desert.
