The underlying rock of the Puente Hills from the Bedford and Southern California Batholith basement to the Miocene and Pliocene sandstone, siltstones, and shale tells us a story about the natural history of the Puente Hills, about how they were made and how they came to be Hills. This was not a short process. Time passed slowly but surely for millions of years, tectonic plates floated on molten rock, bumping and grinding against each other. Where two pieces rubbed edges, the earth twisted, slipped, folded, and buckled over and up. The land was, for the most part, covered by an ocean. Water creatures lived in forests of seaweed, and when the creatures and the forests died, their bodies settled to the bottom. When mixed with sand and mud, and heated and pressurized, they became the thick layers of rock and oil underlying the Hills. This happened for 245 million years, and some hundreds of thousands of years ago, the Puente Hills emerged from the ocean, as it followed one of its many ebbs and flows, slowly becoming dry land [ref]Kim Stanley Robinson, The Gold Coast, (New York: Tom Doherty Associates Book, Inc., 1988), 43 – 45[/ref].
As the Hills emerged from their oceanic existence, wind and water immediately began to shape them. The soft sedimentary rock underlying the Hills would ultimately combine with the climatic effects of wind and rain and the surface streams and creeks, resulting in the deep erosion evident in the Hills today. Ultimately, the rock and water would combine, creating the landscape natural history of the Puente Hills. Wind and rain and streams carry their own stories of the natural history of the Puente Hills.
Excerpted from Richard H. Ross. “From Rock, Wind, and Water: A Natural History of the Puente Hills.” Claremont Graduate University, 2006.