The San Francisco Bay forms an exceptional natural harbor that spurred the Spanish to establish a presidio at this site and was an ideal setting for Native Americans to live for millennia before Europeans arrived. The formation of the bay and the San Francisco Peninsula is the result of forces developed along the Pacific Plate and North American Plate margin, where the Pacific Plate now slowly creeps northward past the North American Plate at rate of about one inch a year on the San Andreas fault. However, the rocks that compose the geologic foundations of the Presidio were formed on the edge of a subduction zone from 200 million to 100 million years ago, long before the San Andreas fault came into existence about 28 million years ago. During this subduction process, mantle rock was altered to serpentinite, which now forms part of the Franciscan Complex on the coastal bluffs at the south end of the Golden Gate Bridge (seen above). This zone of highly fractured rock (melange) tells of the immense forces developed in a subduction zone. Directly overlying these rocks are poorly consolidated sediments of the Colma Formation, dating back a mere 100,000 years and recording a time of high sea level developed during a warm interglacial period. These rocks are locally covered by recent sand dunes formed since the sea level rise following the last glacial period that ended 12,000 years ago.

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Select the following links to learn more about Presidio geology

Plate Tectonics Build the Presidio - Life at the Plates' Edge

Geologic Foundations of the Presidio - Plate Subduction and the Franciscan Complex

The Colma Formation - Deposition During an Interglacial

Recent Sand Dunes Cover the Presidio

Geologic Map and Cross Section of the Presidio and Surrounding Area