Presidio National Park
San Francisco
An Unofficial Look at Place Known Since 1776 by Handle “Presidio” Gathered by Our Research Department
1. San Francisco Bay is relatively young: only about 8,000 years of ocean water flooding into what was once a large valley. Melting ice from the last glaciation raised ocean levels and thus the bay began to form.
2. Interesting rocky base in these parts: The Franciscan Complex. (Sometimes called the “World Famous” Franciscan Complex.)
3. Rocks Beneath Our Feet: Graywacke sandstone and argillite also lessor amounts of greenstone (altered submarine basalt), radiolarian ribbon chert, limestone, serpitinite (altered mantle material), and high-grade metamorphic rocks such as blueschist (high-pressure), amphibolite, and eclogite – these words typically fractured and mixed together to form a “melange.”
4. Range in age from 200 to 80 million years old
5. Franciscan Complex composed of semi-coherent blocks, called tectostratigraphic terranes, which were episodically scraped from the subducting oceanic plate, thrust eastward and shingled against the western margin of North America.
6. The Earth is a coolish design concept -- a dynamic place of moving pieces. Some of the best geologic detective work centers on the understanding of tectonic plates -- and how the lithosphere is broken up into seven or eight major plates and many, many minor plates.
7. A piece of the Presidio is an interesting rock called Chert. Chert epitomizes how movement and change define life on Earth.
(We spend so much time with rocks here because of recent the rocks call the shots – they determine what grows where, what the terrain is like, how much any species, even humans, can survive given their chemical composition.)
Note: This is not to suggest the rocks care, mind you. From our careful observations, we can say they are entirely absorbed with their own (losing) battle with wind and water, as well as with tectonic shifts. Thus their understandable (and forgiveable) wild indifference.
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