The Santa Clarita Valley has a very diverse geologic history and makeup. Fossils abound here, as well as gold, howlite, and oil. During the mid to late 1800’s, there was gold fever here. Acton, Soledad City (later Ravenna), and Placerita Canyon were the hubs of the mining activities. Only Acton remains as a gold producing area today. There was an actual gold rush in Placerita Canyon starting in 1842 – six years before the discovery at Sutter’s Mill. Oil was also discovered here during the same time period. During the 1870’s and 1880’s, oil was king here. Mentryville, a town about 3 miles west of Newhall, was an oil boom town. It was founded in the mid 1870’s. Oil is still being pumped from the ground around here, only now the main operations are in Placerita Canyon and near Castaic.
Most of the valley is composed of sedimentary rocks ranging from 30 million years old to about 1.8 million years old. The valley floor is composed of alluvium from rivers and streams. Some of the oldest rocks in Southern California are located just five miles from here. They are a part of the San Gabriel Basement Complex and have been dated to about 1.7 billion years old. However, the last five million years here are the ones I will concentrate on, as this is when this valley began to take its present form.
During the Pliocene Epoch, much of this area was covered by the Pacific Ocean. It was shallow and was teeming with life. The life that once was here has left numerous fossils. These fossils include gastropods, clams, oysters, plants, fish, whales, and even dolphins. The waters that were here were warm but not warm enough to support more tropical forms of life such as coral. This whole area underwent drastic changes starting at around 1.5-2 million years ago. All the way up the coast, mountains were forming. These mountains exist today as the Coast Ranges. The sea receded and the land rose from below sea level to over 5000 feet in some places. This valley also took on a very different appearance. During the early Pleistocene, the Santa Clarita Valley was a much broader and shallower valley. The uplift that help to create the Coast Ranges also caused the dissection of the older valley floor creating the many stream terraces visible today along the sides of the canyons. Not all the mountains were formed around this time, however. The San Gabriel Mountains started forming about 12 million years earlier during the Miocene Epoch. Most of their uplift has occurred only in the last three million years.
One of southern California’s major fault lines can be traced right through here. Its name is the San Gabriel Fault and is an inactive branch of the San Andreas Fault system. Tracing its path is very easy. You can follow it from Pacoima Canyon, Bear Divide, Placerita Canyon and just west of Violin Summit on I-5. The photo below shows the fault as it runs through Pacoima Canyon.
In the Santa Clarita Valley, it runs through Placerita Canyon, through the summit on Sierra Highway and SR-14 near Golden Valley Road, along Magic Mountain Parkway from near Bouquet Junction, through the industrial center, and then running sub parallel to I-5 from Castaic north. The San Andreas Fault is another major fault that runs in the area. It, however, is about 15 miles north of here. Even so, it can still have a devastating impact if a major earthquake was to occur on it. The fault, in this area, runs through San Bernardino, through Cajon Pass, Wrightwood, Devils Punchbowl, Palmdale, Leona Valley, under Frazier Park, and along the western edge of the Great Central Valley. The last time that segment broke was in 1857. It ruptured from Parkfield all the way to San Bernardino – over 200 miles! Fault slippage is estimated to be at about 30 feet.
The following sections will give more detailed descriptions of each areas geology. Please feel free to ask any questions if you cannot find what you are looking for.