Geology of Castaic and Adjacent Areas

General Facts

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!

Violin Canyon and the Violin Breccia. The San Gabriel Fault passes through here as well.
Violin Canyon and the Violin Breccia. The San Gabriel Fault passes through here as well.
Steeply dipping Ridge Basin Group in Piru Gorge.
Steeply dipping Ridge Basin Group in Piru Gorge.

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.

Osito Canyon, a major tributary to Piru Canyon. Interstate 5 runs along the eastern edge of the canyon.
Osito Canyon, a major tributary to Piru Canyon. Interstate 5 runs along the eastern edge of the canyon.
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Stream terraces southeast of Castaic.

Faults and Folds

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.

Small landslide in Piru Gorge within the Ridge Basin Group. Slides are very common in this formation.
Small landslide in Piru Gorge within the Ridge Basin Group. Slides are very common in this formation.

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