At the edge of the Pacific Ocean, the beach at Schooner Gulch State Beach near Mendocino looks as though it’s been scattered with oversized bowling balls. Almost perfectly spherical about two or three feet in diameter, 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 phenomenon 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, Cannonball River in North Dakota, Valley of Balls in Kazakhstan, as well as elsewhere in the world.
A peculiar story stands behind the Schooner Gulch name. The story has it that one night a schooner was stranded on the beach, but the following morning visitors could find no evidence of the vessel on the beach.