At the edge of the Pacific Ocean, the beach at Schooner Gulch State Beach near Mendocino, looks as though its been scattered with oversized bowling balls. Almost perfectly spherical, stones like these have caused wild speculation wherever they’ve been discovered, with answers from aliens to dinosaurs, but the answer is actually simple geology.
Best observed at low tide, the so-called bowling balls are actually a geological phenomena known as “concretion”, sedimentary rock formed by a natural process wherein mineral cements bind grains of sand or stone into larger formations. These boulders are the result of millions of years of concretion and erosion, exposing the hard spheres as the mudstone of the cliffs receded around them.
Although rare, this same phenomenon is what created the extraordinary Moeraki Boulders and Koutu Boulders in New Zealand, and Cannonball River in North Dakota, as well as elsewhere in the world.
Know Before You Go
Schooner Gulch Beach is three miles south of Point Arena on California Highway 1