• View of the Golden Gate Bridge, taken from the Marin Headlands, looking towards San Francisco at sunrise.

    Golden Gate

    National Recreation Area California

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  • Tunnel Closure

    The Barry/Baker tunnel on Bunker Road will be closed for maintenance during the weeks of 6/2 and 6/9. The tunnel will be open on the weekends. Please use Conzelman Road instead. More »

  • Muir Beach Overlook closure

    The Muir Beach Overlook will be closed for Accessibility improvements and trail upgrades from June 2 through July 21. Alternate viewpoints are available along Highway 1 between there and Stinson Beach.

Limestone FAQ

Limestone hand specimen

Recrystallized limestone hand specimen

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.

 
Limestone exposure at Marin Headlands
Recrystallized pink limestone is found with greenstone basalt at Black Sand Beach in the Marin Headlands.

Did You Know?

Pillow basalt at Point Bonita

The trail to Point Bonita lighthouse is the location of what is likely the earliest detailed geologic map in the state, completed by F. Leslie Ransome in 1893.