Mussel Rock is a rock formation on the coast of

San Mateo County, California San Mateo County ( ), officially the County of San Mateo, is a county located in the U.S. state of California. As of the 2020 census, the population was 764,442. Redwood City is the county seat, and the third most populated city following Dal ...

, offshore from

Daly City Daly City () is the second most populous city in San Mateo County, California, United States, with population of 104,901 according to the 2020 census. Located in the San Francisco Bay Area, and immediately south of San Francisco (sharing its ...

. It consists of one large and numerous smaller rocks of a type known as a stack, where a headland is eroded unevenly, leaving small islands. The rock itself is located 15 meters from the shore of Mussel Rock Park and stands 18 meters tall. The coastal area adjacent to the rock is Mussel Rock Park, consisting of 250 acres of land stretching along the coastline. The park includes a beach area, a

paragliding Paragliding is the recreational and competitive adventure sport of flying paragliders: lightweight, free-flying, foot-launched glider aircraft with no rigid primary structure. The pilot sits in a harness or lies supine in a cocoon-like 'p ...

launch pad, hiking trails, and a parking lot. It is best known for being the closest point to the

epicenter The epicenter, epicentre () or epicentrum in seismology is the point on the Earth's surface directly above a hypocenter or focus, the point where an earthquake or an underground explosion originates. Surface damage Before the instrumental pe ...

of the 1906 San Francisco earthquake, and where the

San Andreas Fault The San Andreas Fault is a continental transform fault that extends roughly through California. It forms the tectonic boundary between the Pacific Plate and the North American Plate, and its motion is right-lateral strike-slip (horizonta ...

enters the

San Francisco Peninsula The San Francisco Peninsula is a peninsula in the San Francisco Bay Area that separates San Francisco Bay from the Pacific Ocean. On its northern tip is the City and County of San Francisco. Its southern base is Mountain View, south of Palo Alt ...

from the northwest. An additional minor fault, the Mussel Rock Fault, was identified in 2000. The area above Mussel Rock consists of steep cliffs with frequent landslides, threatening homes in a subdivision above.

Geology

Geology of the Mussel Rock formation

The Mussel Rock formation itself is a

sea stack A stack or sea stack is a geology, geological landform consisting of a steep and often vertical column or columns of rock in the sea near a coast, formed by Coastal_erosion#Wave_action, wave erosion. Stacks are formed over time by wind and wate ...

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...

period greenstone, from the

Franciscan Complex The Franciscan Complex or Franciscan Assemblage is a geologic term for a late Mesozoic terrane of heterogeneous rocks found throughout the California Coast Ranges, and particularly on the San Francisco Peninsula. It was named by geologist Andrew ...

bedrock, dated 80 to 90 million years ago. Contrasted with the nearby 3 million year old

sedimentary rock Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...

s of the

Merced Formation The Merced Formation is a geologic formation in California, and also in Oregon and Washington state. It is named for Lake Merced, a natural lake on the western San Francisco coastline. Geology California In the coastal San Francisco Bay Area of ...

, the metamorphosed marine basalt comprising the greenstone of the Mussel Rock Formation from the Franciscan complex is more resistant to erosive forces. The Mussel Rock outcropping initially formed around 145 million years ago when movement from Pacific plate forced the

Farallon plate The Farallon Plate was an ancient oceanic plate. It formed one of the three main plates of Panthalassa, alongside the Phoenix Plate and Izanagi Plate, which were connected by a triple junction. The Farallon Plate began subducting under the west ...

southeast,

subducting Subduction is a geological process in which the oceanic lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the ...

under the westward-bound

North American plate The North American Plate is a tectonic plate covering most of North America, Cuba, the Bahamas, extreme northeastern Asia, and parts of Iceland and the Azores. With an area of , it is the Earth's second largest tectonic plate, behind the Pacif ...

. The subduction sheared the uppermost crust of the Farallon plate, and the severed rock accumulated along the North American plate.

John McPhee John Angus McPhee (born March 8, 1931) is an American writer. He is considered one of the pioneers of creative nonfiction. He is a four-time finalist for the Pulitzer Prize in the category General Nonfiction, and he won that award on the four ...

begins his 1993 book, ''Assembling California'', with Mussel Rock. According to McPhee,

Geology of the greater Mussel Rock coastal area

The coastline area surrounding the Mussel Rock formation is at the intersection of several geological formations. The lowermost portion of the

lends

friable Friability ( ), the condition of being friable, describes the tendency of a solid substance to break into smaller pieces under duress or contact, especially by rubbing. The opposite of friable is indurate. Substances that are designated hazardous, ...

marine

sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates ...

to the area, deposited there 2-3 million years ago. The portion of the Merced formation at Mussel Rock is among the older sedimentary deposits, deeper in the continental shelf, pushed to the surface by faulting. The Merced sandstone variably overlies the greenstone of the

. These two bedrock structures are separated at the surface by the

. To the northeast, a thin layer of the Colma formation overlies the Merced.

Erosion and Landslides

USGS 1982-1983 El Nino San Mateo Coastal Reosion Diagram

Due to active erosion, seismic activity, and the loosely cemented nature of the local Colma and Merced formations, the area is an active landslide zone. During ENSO years,

coastal erosion Coastal erosion is the loss or displacement of land, or the long-term removal of sediment and rocks along the coastline due to the action of waves, currents, tides, wind-driven water, waterborne ice, or other impacts of storms. The landwa ...

on the Merced and Colma formations can average up to 2 to 3 feet per year. In addition to erosion via tides and winds, movement along the

San Andreas fault The San Andreas Fault is a continental transform fault that extends roughly through California. It forms the tectonic boundary between the Pacific Plate and the North American Plate, and its motion is right-lateral strike-slip (horizonta ...

and Mussel Rock fault and construction along the clifftops contribute to erosion in the area, exacerbating the risk of landslides.

Ecology

Mussel Rock visitors can harvest

mussels Mussel () is the common name used for members of several families of bivalve molluscs, from saltwater and freshwater habitats. These groups have in common a shell whose outline is elongated and asymmetrical compared with other edible clams, which ...

, but only during the approved season and with guidance from the

California Department of Public Health The California Department of Public Health (CDPH) is the state department responsible for public health in California. It is a subdivision of the California Health and Human Services Agency. It enforces some of the laws in the California Health ...

. The California Department of Public Health regularly tests for

paralytic shellfish poisoning Paralytic shellfish poisoning (PSP) is one of the four recognized syndromes of shellfish poisoning, which share some common features and are primarily associated with bivalve mollusks (such as mussels, clams, oysters and scallops). These shellfi ...

and posts advisory information on its website.

History

The earliest recorded settlement at Mussel Rock was Ompuromo (ocean-flea place) inhabited by the Yelamu tribe of the Ohlone people. Based on carbon dating of archeological findings from a midden excavated at Mussel Rock, the village was estimated to date back to approximately 1500 AD, although Ohlone people are known to have inhabited the San Mateo area since at least 5,000 years ago, and lasted until Spanish colonization in 1776. In 1978, the midden was excavated and found to contain artifacts including debris from lithic tool manufacturing and faunal remains from food items. Unfortunately, only 45 cubic meters were excavated before Daly City began the construction of the garbage sorting facility, Mussel Rock Transfer Station, atop the midden. The adjacent area was used as a landfill from 1958 to 1978, preventing any other nearby archeological excavation. Map of Part of the Coast of California From Point San Pedro Northward

Map of Part of the Coast of California From Point San Pedro Northward

Gallery

Notes

References

* * * * * * * * * *

External links

1982-83 El Niño Mussel Rock Coastal Erosion Map - USGS

Cliff Hangers: 17 houses on Daly City block sliding toward ocean plunge

Environmental History of Mussel Rock
{{authority control Islands of the San Francisco Bay Area Islands of Northern California Stacks of the United States Geology of San Mateo County, California Islands of San Mateo County, California Rock formations of California Uninhabited islands of California Pacific islands of California Daly City, California