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 Mussel Rock formation itself is a sea stack of Cretaceous period greenstone, from the Franciscan Complex bedrock, dated 80 to 90 million years ago.

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.

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.

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

[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.