Seal Rock

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For the similarly-named Australian coastal location, see Seal Rocks

An old view of the rocks

Seal Rock is a large and prominent rock found offshore at the North end of the Ocean Beach in San Francisco, California, United States (37°46′43″N, 122°30′58″W). It is named for the population of Steller's sea lions (Eumetopias jubatus) and California sea lions (Zalophus californianus) who used to haul out on the rock. Both species are often colloquially called "seals."

[edit] Geology and history of the area

The northern part of Ocean Beach in February, with the Seal Rock

Near the end of the last ice age, this part of the coastline was thought to be between eight and twenty five miles westward of its current position - around five thousand years ago, with the melting of the ice, the sea level rose to its current level.

The coastal features found here today were formed by the actions of waves, wind, and the movement of sand. The geology of this part of the coastline of San Francisco consists of steep rocky cliffs which are punctuated by narrow sandy pocket beaches found at the north end of the city and long sandy stretches at Ocean Beach and Fort Funston to the south. The local features, and indeed the entire San Francisco Peninsula, come from the northern aspect of the Santa Cruz Mountains. Before the urban development seen today the area landward of Ocean Beach consisted of barren sand fields inland to Twin Peaks. The first known map of the area, dating from 1852, was made by the U.S. Coast and Geodetic Survey and describes the coastline here as being entirely unaltered by humans.

The geologic layers beneath Ocean Beach, starting from the top (i.e. the youngest) are:

Placed fill. The placed fill comes from various construction activities that have taken place over the last 100 years or so. The depth of this placed fill varies, but in 1985 it was then estimated to be between 7–16 ft (2.1–4.9 m) deep.
Dune sands. The layer of dune sand has been estimated to be between 13–26 ft (4–7.9 m) deep.
Colma Formation
Merced Formation
Bedrock. The depth to bedrock is estimated to be between 350–400 ft (107–122 m).

The Colma and Merced Formations consist generally of poorly-sorted sandy deposits and were probably deposited during some earlier period of greater sea levels. The Colma Formation is found under the dune sands and also extends inland.