The story of the land beneath our feet is remarkable. It is the story of our Earth — this dynamic, majestic rock floating in space. Geology is the study of the solid earth and the processes by which it changes. Epochs pass, rock is created and mountains wash away. Tectonics rip continents apart and smash new ones together. From Earth’s geology comes wondrous beauty but also great devastation. It is a fundamental factor of all that takes place on our planet.
There are three distinct geologic regions of San Diego County: The Salton Trough in the desert to the east, Peninsular Ranges running down the center and Coastal Plains to the west. The Salton Trough contains the low-lying Colorado Desert (Anza-Borrego). Movements in the San Andreas Fault system perpetuate this feature as an extension of the Gulf of California. Interestingly, this depression is partially filled with sediments from the Colorado River’s carving of the Grand Canyon!
From almost anywhere on the coast, there are mountain peaks and ridgelines visible to the east. These are the Peninsular Ranges. These mountains run from Riverside County south through much of Baja. Mount San Jacinto, near Riverside is the tallest point at 10,835 feet. In San Diego, these mountains include Palomar, Hot Springs, Cuyamaca and Laguna. According to Jay Hill, geologist with the U.S. Geological Survey, the Peninsular Ranges formed when enormous pools of magma cooled and hardened while underground, called intrusive, igneous rocks (100 million years ago). These batholiths were then forced to the surface by plate tectonics. Because they are made of tougher rock, they weather differently than adjacent rocks, creating the recognizable peaks we see to the east. The Sierra Nevada Batholith to the north is essentially one giant granite rock creating the spine of California.
The San Diego Coastal Region consists of relatively young sedimentary rock. The large sea cliffs from Torrey Pines in the south to
Carlsbad in the north were created 3 million years ago when North America was locked in an Ice Age. The coastline was farther west then, as ice increased, sea levels decreased.
Ancient coastal wetlands laid the sediments for the Delmar Formation (50 million years ago), the greenish gray layer at the bottom of our sea cliffs. These wetlands were teeming with life; consequently this layer exposes abundant marine fossils. The Delmar Formation is also responsible for the reef systems that mark our coastline and provide the best shaped surfing waves in the county.
Above the Delmar, Torrey Sandstone (20 million years ago) is a large layer of pale, yellowish sedimentary rock. Once a giant sandbar, created by the flow of sediments from inland erosion and currents along the shore, the Torrey Sandstone is now eroded by Ocean waves to create new beach sand. The Bay Point Formation (1 million years ago) is the top layer of the sea cliffs. These are the youngest rocks in the county.
San Diego is a very geologically active area. A menacing network of fault lines and plate boundaries crisscross the county. Including the infamous San Andres Fault in the East. The Rose Canyon Fault runs from downtown, through La Jolla then along the coast of North County. These faults form the boundary between the North American Plate to the east and the Pacific Plate to the west. As the Pacific Plate moves north and the North American Plate grinds south, San Diego is caught in the middle: fracturing, converging, separating and uplifting.
Geologic forces are all around us. We cannot stop them, but we can enjoy the process of understanding how the Earth beneath our feet came to be. San Diego did not exist 500 million years ago and it will certainly look very different 500 million years from now.
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