San Diego Natural History Museum--Your Nature Connection[BRCC San Diego Natural History Museum: Paleontology]

Geologic History of San Diego County

The Peninsular Ranges: A Geologic Guide to San Diego Back Country by Dr. Michael J. Walawender Article by Dr. Michael J. Walawender, Professor Emeritus, Dept. of Geological Sciences, San Diego State University; Research Associate, Department of Mineralogy, San Diego Natural History Museum

The following article draws heavily from The Peninsular Ranges: A Geologic Guide to San Diego’s Back Country by Dr. Michael J. Walawender. Refer to that volume for additional details on the geologic processes discussed below.


Geologic Time ScaleIf we were to go back in time about 200 million years and view what is now all of southern California, a very different picture would emerge. Ocean waters would stretch eastward to Arizona and northern Mexico. There, ancient granites, as old as 1800 million years, and associated metamorphic rocks would form the western edge of the North American continent. A low rolling landscape with rivers that have long since disappeared would wash sediment weathered from those rocks into the shallow seas adjacent to the continent. These sediments would thin westward under the ocean waters. The landmass on which we all now live and work was yet to be created.

To understand the history of our area, we must first unravel the present geologic picture. Although earthquakes and active faults are now a very recognizable part of life in southern California, they are a very recent addition to a much longer and more complex pattern of geologic activity. Part of this picture is hidden by geologically young sedimentary rocks on which much of metropolitan San Diego is built. As we travel eastward, however, into the less populated mountain and desert areas, the exposed rocks record a long history of plate collisions, volcanic activity, and crustal uplift. From 250 million-year-old sedimentary rocks, through 100 million-year-old granites and 18 million-year-old volcanic eruptions, to the faults that shake us into today’s reality, the San Diego area has been a very active place indeed.

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