An estimate is that the speed of the landslides must have at times exceeded 100 miles an hour to create such a large boulder breccia. The rocks and minerals include asbestos, serpentine, actinolite, fuchsite, epidote, chlorite, glaucophane, pyrite, magnetite, and quartzite. In the Middle Miocene Epoch a great sea inundation, associated with the development of deep basins, resulted in massive accumulations of diatomite, an almost pure deposit of one-celled, glassy, plant shells which are commercially mined for use as swimming pool filters. The area again changed as the deep basins filled and the sea began to retreat. The Puente Formation was deposited in the Late Miocene and is composed of four members.
Sandstone provides a good reservoir rock for oil which has been trapped by faulting and folding.
Sandstone of the Puente Formation composes most of the Puente Hills. The Burruel Hills are also cloaked by the Puente Formation which has a maximum thickness of 5,400 feet. The marine Capistrano Formation is found in the southern part of the county. It ranges from Late Miocene to Early Pliocene in age and consists of poorly consolidated, fossiliferous, sandy-siltstone and mudstone. Sediment failures have caused extensive landsliding in San Juan Capistrano and San Clemente areas.