Mussel Rock

The Mussel Rock formation itself is a sea stack of Cretaceous period greenstone, from the Franciscan Complex bedrock, dated 80 to 90 million years ago. Contrasted with the nearby 3 million year old sedimentary rocks of the Merced Formation, the metamorphosed marine basalt comprising the greenstone of the Mussel Rock Formation from the Franciscan complex is more resistant to erosive forces.[4][5]

The Mussel Rock outcropping initially formed around 145 million years ago when movement from Pacific plate forced the Farallon plate southeast, subducting under the westward-bound North American plate. The subduction sheared the uppermost crust of the Farallon plate, and the severed rock accumulated along the North American plate.[4]

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

"Mussel Rock is a horse. As any geologist will tell you, a horse is a displaced rock mass that has been caught between the walls of a fault. This one appears to have got away ... green seas slammed against it and turned white. It was not a small rock. It was like a three-story building, standing in the Pacific, with brown pelicans on the roof ... after a five hundred-mile northwesterly drift through southern and central California, this was where the San Andreas Fault intersected the sea."[5]

— John McPhee, Assembling California (1994)

The coastline area surrounding the Mussel Rock formation is at the intersection of several geological formations. The lowermost portion of the Merced Formation lends friable marine sandstone to the area,[6] 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.[7]

The Merced sandstone variably overlies the greenstone of the Franciscan Complex. These two bedrock structures are separated at the surface by the San Andreas Fault.[8] To the northeast, a thin layer of the Colma formation overlies the Merced.

Coastal erosion diagram, 1982-83 El Niño cycle.

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.[6] During ENSO years, coastal erosion on the Merced and Colma formations can average up to 2 to 3 feet per year.[9][10] In addition to erosion via tides and winds, movement along the San Andreas fault and Mussel Rock fault and construction along the clifftops contribute to erosion in the area, exacerbating the risk of landslides.[9]

The earliest recorded settlement at Mussel Rock was Ompuromo (ocean-flea place) inhabited by the Yelamu tribe of the Ohlone people.[12][13] 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.[14]

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.[14] The adjacent area was used as a landfill from 1958 to 1978, preventing any other nearby archeological excavation.[15]

A map detailing Mussel Rock and the surrounding coastline in 1853.