The Poway Conglomerate is an Eocene formation, covers approximately 125 square miles in western San Diego County, California, ranges in thickness up to about 1000 feet, is nearly horizontal, and in most places lies unconformably on the Cretaceous Peninsular Range batholith and accompanying metamorphic rocks.
Most of the formation consists of pebble and cobble conglomerate, although boulder conglomerate and sand lenses occur. More than 70 per cent of the clasts are soda rhyolite porphyry and tuff. Aplites, granophyres, and metamorphic rocks are other important constitutents.
The volcanic rocks and granophyres have not been found in place, and the writers suggest that they were shallow intrusives into and effusives onto the roof of the batholith and have been removed by erosion. The resistant rhyolitic rocks withstood weathering and abrasive action and were deposited during the Eocene as the Poway Conglomerate. A textural, mineralogical, and chemical gradation from batholithic rocks to rhyolitic rocks of the clasts is taken as evidence supporting this theory. Deposition was largely continental but also marine in part.