Most of the surrounding hills are composed of the late Miocene Castaic Formation. This rock is marine in origin. It overlies the Mint Canyon Formation in the Saugus Area. The hills to the south are composed of the Plio-Pliestocene Saugus Formation which are terrestrial in origin. The alluvium covering the valley floor is not very deep, maybe only a few hundred feet, and is about 1.5 million years old at the latest. One rather interesting rock formation, known as the Violin Breccia, is visible in hills just west of Interstate 5 near Palomas Canyon. The rock has a broken and loose appearance, and is reddish brown in color. Over the Liebre Mountains, Interstate 5 lies mostly atop the Ridge Basin Group, a rock formation that has been measured to almost 30,000 feet thick!
Rivers and Drainage
Most of this area is drained by Castaic Creek. Castaic Creek is a major tributary to the Santa Clara River. Most of the drainage has been cut off by the construction of Castaic Dam. Piru Creek runs about 10 miles to the west of here and drains the mountain north of Violin Summit. It drains into the Santa Clara River through Piru in Piru Canyon. Pyramid Lake sits in Piru Canyon as well. This may give an example of how deep and long Piru Canyon actually is. Terraces edge the main valley as shown here in this photo. This photo below was taken about one mile south of Castaic facing east.
The San Gabriel Fault runs right through here. It roughly follows Interstate 5 to the west most of the way over the Liebre Mountains. The low pass between the rugged Liebre and Topatopa Mountains that Interstate 5 traverses is caused in part by the fault-weakened rocks. The San Andreas Fault also runs nearby. It is about 30 miles north of here in Leona Valley and Cuddy Valley. Leona Valley, along with Pine Canyon, Cuddy Valley, and Anaverde Valley are a part of the San Andreas Rift Zone.