NPS photo

Shale beds displaying faulting

The topographical relief of the park ranges from sea level to 2,300 feet above mean sea level near the top of Mt. Tamalpais. Hillslopes range from almost flat marine terraces and alluvial deposits to steep canyons along some creeks, and near vertical bluffs above some beaches. Most watersheds are less than one square mile in area, and flow through narrow V‑shaped stream beds cut through bedrock. Stream channel gradients range from 3 percent, in Elk Creek, to 35 percent, in steep tributaries on Bolinas Ridge.

Golden Gate is located in a seismically active area along a fault line. The Pacific Plate has been slowly creeping northward past the North American Plate at a rate of about one inch a year along the San Andreas Fault for the last 28 million years. The San Andreas Fault extends northwest from near Fort Funston, and runs through Bolinas Lagoon and Tomales Bay. The San Andreas is the major fault in the area, but many smaller faults also exist. This movement is expressed as a violent earthquake occurring about once a century. Many earthquakes of lesser magnitude occur along the length of the fault.

Golden Gate also sits above a subduction zone where oceanic crust is moving under continental crust and being recycled. The rocks that compose the geologic foundations, or bedrock, of the Bay Area were formed along this subduction zone over 100 million years ago. Sandstone, Shale, Basalt, and Chert are among the bedrock types present. These rocks belong to the Franciscan Assemblage and were originally deposited on the ocean floor 80 to 140 million years ago. The rocks were greatly deformed and partly metamorphosed as the ocean floor was thrust under the western edge of the North American Plate, resulting in a landscape of easily eroded, sheared and crushed sandstone and shale, with occasional blocks of more resistant rock forming prominent outcrops.

NPS photo

Serpentine rock exposed at Inspiration Point in the Presidio

There are also some interesting rocks in our parks that have limited distributions or unique origins. The Presidio showcases our state rock Serpentine, which is found only in areas where oceanic crust is subducted and then pushed up again along fault zones. Worldwide, Serpentine is sporadic in distribution and high in heavy metals, creating rare plant communities on its soils. The Marin Headlands contains a rock of biological origin, Radiolarian chert. Composed of innumerable skeletons of these phytoplankton slowly raining down to the ocean floor over eons, these fossilized radiolaria underly about half of the Headlands. Because of its resistance to weathering, it makes up nearly all of the ridge tops and summits. The contorted layers in this chert express the plate tectonic actions in this area and are frequently visited by geology classes.

Our younger rocks are linked to ocean level rise and fall over the history of the San Francisco Bay Area. The Colma Formation, dating back a mere 100,000 years, records 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.  The sea cliffs at Fort Funston were formed from the oldest of these fossil‑rich beds of sand and clay, the Merced Formation. Many abandoned quarries are found within GGNRA. Dogtown Copper Mine, located just off Bolinas Ridge, is the only known mineral development in the park. It was developed in 1863 and re‑worked around the turn of the century. Its two shafts are now abandoned.