Geo Jocks: Mountains

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Author: Donovan Dennis

North America’s mountain ranges constitute a rather homogeneous crowd. Outside of Alaska and a sprinkling of mountains in Utah and the Desert Southwest, most of the ranges (think: Coast Mountains, Appalachians, Rockies, Sierra Nevadas, Adirondacks) stretch from north to south. They reflect main tectonic forces that once acted on the continent. Tension and compression from the east and west cause extension and uplift from north to south. The mountains of Southern California are a notable contradiction to this trend, though they still are a product of the active California tectonic regime.

As most Southern California outdoor enthusiasts and Griffith Park dog walkers know, mountains in the greater Los Angeles area trend in almost every direction. They are a hodge-podge of peaks seemingly thrown together in mismatched directions, both offering beautiful views of LA and trapping the smoggy haze that occasionally caresses the cityscape. The cause of the disorganization, like many hazards of California life, can be attributed to the San Andreas Fault.

Thirty million years ago, the petering remains of the Farallon Plate — an ancient oceanic plate now divided into several others — subducted below the North American Plate. In its absence, the Pacific Plate took up the action of the tectonic movement and changed the orientation of forces acting on western North America. Instead of pushing from the west, the subduction of the new Juan de Fuca Plate near Seattle pulled the Pacific Plate to the northwest, thereby initiating the formation of the ancient San Andreas Fault. Over the past 30 million years, the movement of the fault realigned the Transverse Mountains of Southern California, so named for their orientation transverse to the common north-south trend.

Around 20 million years before present, the Transverse Ranges were found nestled close to the San Jacinto Mountains, near the current location of the Santa Ana Mountains. Over the course of the following several million years, northwest movement on San Andreas Fault rotated the mountains nearly 90 degrees to their current locations opening up a void for the Los Angeles Basin. Sediments have since continued to fill the space, creating the relatively flat-lying plain on which LA is built today, and the mountains around it offer panoramic views of the region. Though they may be oriented quite differently, certain components of almost every mountain range from Santa Barbara County to San Diego County are identical, and can be dated to over 100 million years ago.

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