Temporary Closure of Waters Around Alcatraz Island
From 7/3 to 9/22 a 500’ marine buffer zone is in effect, closing the perimeter of Alcatraz Island to all private vessels to protect nesting seabirds during America's Cup racing. Tours to Alcatraz continue as usual during the races. More »
Muir Beach (but not nearby Muir Woods) closed July 8-November 2013, but local businesses are open
Though Muir Beach is closed from July 8-November 2013 due to construction, the PELICAN INN IS OPEN. Restrooms and parking are not available at Muir Beach. Pacific Way is closed except to residents. Check back for updates or call (415)561-3054. More »
CAUTION: Post Storm Damage to Coastal Trail
The Presidio Coastal Trail segment just north of the Pacific Overlook and adjacent to Lincoln Blvd remains CLOSED indefinitely. We have posted signage to alert bicyclists and hikers and with information for safe trail alternatives. More »
Where is limestone deposited?
Limestone is a sedimentary rock rich in the mineral calcite, which is made of calcium carbonate. Franciscan limestone is formed mostly from the tiny carbonate shells of single celled marine animals called foraminifera. This type of limestone forms in ocean settings where there is not enough continental mud to dilute the slow “rain” of carbonate shells, and where the ocean is not so deep that all calcium carbonate dissolves into the water before it can be buried in the sediment to form rock. In today’s oceans, all calcium carbonate dissolves below the depth (called Carbonate Compensation Depth or CCD) of about 4 km. Franciscan limestone is thought to have formed on the tops and sides of underwater volcanoes in water less than 4 km deep.
What can the fossil shells tell us?
Scientists have studied foraminifera found in sedimentary rocks. After identifying these fossils, geologists develop an evolutionary sequence of the species. This sequence, called a biostratigraphy, helps geologists determine the age of the rocks. The limestone in the Marin Headlands is too metamorphosed and recrystallized to contain original fossil shells, but other local Franciscan limestone contains fossils that tell us it was deposited from about 125 million ago to about 90 million years ago.
Did You Know?
John Fremont, the explorer, and his wife Jessie Benton Fremont, lived at Fort Mason. Both were abolitionists and their home, once located at the edge of the post, became a center of San Francisco’s intellectual life.