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Geologic Resources National Park Service U.S. Department of the Interior Golden Gate National Recreation Area The Golden Gate National Recreation Area is truly a park living on the edge, located on the far western end of the North American tectonic plate and flanking the Pacific plate. Whether formed from the interaction of ocean crust and mantle rocks under heat and extreme pressure like serpentine, or built by millions of microscopic sea creatures under pressure and over time like radiolarian chert, our park has anything a rock lover could want. Geologic Thrusts from the Past The rocks that compose the geologic basement, or bedrock, of the Bay Area were formed over 80 million years ago. They are part of the Franciscan Complex—ocean floor material erupted and deposited during a 100 million year journey from a mid-ocean ridge to the edge of the North American continent and a subduction zone. These rocks were greatly deformed and partly metamorphosed as the ocean floor was thrust under the western edge of North American. Some material was scraped off the subducting plate to form a mélange or mixture of blocks of serpentine, oceanic crust (basalt and greenstone), open ocean sediments (chert and limestone), and submarine landslide deposits (graywacke sandstone) shed from North America. Today’s landscape around the Golden Gate is the result of the more easily eroded sandstone and shale forming valleys, and the blocks of more resistant rocks forming prominent outcrops on the ridge lines. Slip Sliding Away Today, the Golden Gate is located in a seismically active area along a transform fault plate boundary. About 28 million years ago the Pacific plate started slowly creeping northward past the North American plate instead of moving under it. The San Andreas is the major transform fault in the area, but many smaller faults also exist. In the park, the San Andreas Fault extends northwest from Phleger Estate to near Fort Funston, where it goes offshore and then runs through Bolinas Lagoon and Tomales Bay. On Average it moves about one inch a year. This movement is expressed as a violent earthquake occurring here about once a century. Many earthquakes of lesser magnitude occur continually along the length of the fault. Movements on the fault created a “pull-apart basin” in which the Merced Formation underlying Fort Funston was deposited. The fault also may have resulted in a 130° clockwise rotation of the block of rock forming the Marin Headlands, creating the gaping passageway that is now known as the Golden Gate Strait. A right bend in the San Andreas offshore of Fort Funston forms a hole in which the Merced Formation was deposited. Golden Gate includes six terranes of the Franciscan Complex including the rotated Marin Headlands terrane block. Brand New to Really Old Talking geologic time is hard for most people because brand new is a few thousand years old in geology! Golden Gate has a wide variety of rock ages; from the Franciscan Complex, representing events that took place over a hundred million of years ago; to the Colma Formation, which tells of a time just a hundred thousand years ago when San Francisco was an island; to sand dunes only a few thousand years old containing sand grains that migrated from the Sierra Nevada Mountains down the Sacramento River to the coast. In fact, Golden Gate’s diverse geology has played an important role in the history of geology; from Berkeley professor Andrew Lawson’s delineating the San Andreas fault in the late 1800s, priming the world for plate tectonics; to our Franciscan rocks helping provide the first real understanding of subduction tectonics in the 1980s. Ranger Will Elder assists students from the teacher credential program at San Francisco State University. Park staff in the Interpretive Division have done a great job of sharing our park’s exciting geologic resources with the public. Whether visitors are downloading rock field guides, or Bay Area middle school students are making a field trip to the Marin Headlands, the story in the stone is being told. Many employees may be surprised to know that there are some amazing fossils in the park that are just now being inventoried. Since rocks are the foundation for all of the vegetation and wildlife that live upon them, it benefits all park staff to have a basic understanding of the earth beneath our feet! Whether studying serpentine sites for restoration potential or researching fossil clams at Fort Funston, staff are learning more about the park’s geology. Franciscan Field Guide Our Park has a downloadable field guide to the geology of the Golden Gate Headlands on both sides of the bridge. Rock hounds love this park because of the truly bizarre geology found here. The Presidio showcases our green and scaly state rock serpentinite, which is found only in areas where oceanic crust is subducted and then pushed up again along fault zones. Serpentinite is high in heavy metals and naturally occurring asbestos. Also mind-boggling is a rock of biological origin in the Marin Headlands, radiolarian chert. This chert is composed of innumerable skeletons of zooplankton that Rocks on the Move and PARK Teachers Rocks on the Move provides an opportunity for Bay Area middle school classes to hike through underwater volcanoes that erupted millions of years ago. Students study the global geologic forces that formed today’s dramatic Bay Area landscape, and then come to the Marin Headlands to conduct a geologic investigation to unveil how these global forces are recorded in local rocks. Activities challenge the students to use observational and critical-thinking skills to speculate about possible future geologic developments. slowly rained down to the ocean floor over eons. Basalts of the Franciscan erupted from volcanoes at mid-ocean ridges and were altered to greenstone, and the underwater landslides that deposited graywacke sandstone carried sand, silt, rock fragments, and plant and animal debris from the continental margin into deep ocean environments. Take some time to hike parts of Baker Beach or Rodeo Beach to really see these fascinating rocks up close. Learn more about the Park’s geology at: www.nps.gov/prsf/naturescience/geologicformations.htm The PARK Teachers geology program brings teacher candidates to the park to experience place-based inquiry learning. It is accompanied by a web site that helps teachers understand plate tectonics and the Franciscan Complex of the San Francisco Bay Area. The content includes classroom strategies, specific lesson plans and materials, an interactive game of geology, and downloadable teaching tools, guidebooks and posters. If you know teachers or classes that would benefit from our park’s educational resources, get the word out there! www.nps.gov/goga/forteachers/ Paleontological Resource Inventory Fossils have taught us much of what we know about the history of life on Earth. Shells, bones or leaves reveal when and where organisms lived and died, new species arose, and changes in climate occurred. On a grander scale, fossils tell us about the age of the rocks they are in, the movement of land masses and the formation of mountains and seas. The San Francisco Area Inventory and Monitoring Network recently completed a Paleontological Resource Inventory of the park that has identified a number of intriguing facts. First, the tropical radiolarian species found in the Headlands chert probably formed in an equatorial region west of Mexico 100-200 million years ago and then was faulted northward to the current location. Also the Merced Formation of Fort Funston contains the bones and foot prints of large mega fauna of past ice ages such as woolly mammoth, giant ground sloth, mastodon, early horse, and camel. http://science.nature.nps.gov/im/units/sfan/reports/ Executive%20Briefings/Paleo_Inventory_Executive_Briefing.pdf