Beach Bowling: Paleo Style

Aptly named Bowling Ball Beach (part of Schooner Gulch State Beach), the geologic wonders here are plenty and plenty strange. Low tide reveals spherical rocks, two to three feet in diameter, resting in rows within naturally occurring ‘lanes’ within the surf. And if the giant paleo-bowling alley were not enough, just to the north, massive english muffins decorate the sandstone cliffs and wet sand.

The rocks on this remote Northern California beach in Mendocino County are sandstones and mudstones that were deposited on the sea floor during the Miocene (5-25 million years ago).

So just how do these massive bowling balls form?

These round rocks are unusually large concretions. Concretions form when minerals like silica or calcite concentrate around a common center and cement the grains of sand, silt or clay into hardened spheres. The concretions grow larger and larger over millions of years and can withstand weathering better than the surrounding material within the rock formation. Thus as the surrounding rock is eroded away by wind and waves, the concretions remain. Those in the cliff faces fall to the beach, those on the beach surface, largely stay put.

It’s Not a Dinosaur Egg, It’s a Concretion!

Concretions have been mistaken for dinosaur eggs before, most famously in Chechnya in 2012. Dinosaur eggs are much smaller, however and too fragile for so many to remain intact like the rocks at Bowling Ball Beach. Another way you can tell the bowling balls are not dinosaur eggs is that wave patterns in the sand that formed as the sand was originally deposited can be followed from within the “host” sandstone into the round concretions (see photo above).

From Horizontal to Vertical: Formation of the Lanes

What makes this beach really stand out is the combination of both the bowling balls and the amazingly straight and repeating bowling lanes that run right into the cliff face. While all sedimentary rocks are deposited laying fairly flat, these layer cake sandstones and mudstones were tilted upward by 90 degrees. If you look at the map above, you will see the San Andreas Fault is just to the east of Bowling Ball Beach. The uplift and tilting happened by movement and squeezing of the rocks along the San Andreas Fault system. Once tilted 90 degrees, slight differences in erosion of the layers make the lanes really stand out.

English Muffin Rocks

In addition to the natural bowling alley, Bowling Ball Beach features what could be described as a giant game of shuffleboard. Towards the north end of the beach, a bed of gray shale hosts concretions that have been cemented by silica and resemble massive english muffins, five to six feet across. Some rest high up in the cliffside while others have slid down towards the sand like giant pucks in a game of vertical shuffleboard that has lasted for millennia.

Have you visited Bowling Ball Beach or been to another beach with cool geologic concretions and stratifications? Let us know in the comments or on Facebook, and share photos if you have any!

Also, a special thank you to Ted of Geologic Trips for allowing Mobile Ranger to use his photos. Check out his website for a more detailed description of the geologic processes that formed Bowling Ball Beach.