Concretions in the Monterey Formation
Visitors to Crystal Cove often ask about the hamburger bun-shaped or UFO shaped rock “formations” littering the
beach especially in the vicinity of the Beaches Cottage. These features range in size from a few inches to more than 12 feet in diameter. Look carefully in the cliff
towering over the side yard of the cottage.
You will see several examples embedded in the Monterey. Geologists call these bodies concretions. Although
small examples are common, large specimens like many here are relatively rare - although do occur elsewhere in
California such as along the Santa Barbara Coastal Plain and at Bowling Ball Beach at Schooner Gulch near Point
Concretion formation is a fascinating story. A favorite hypothesis is that the rounded masses are like tumors growing in the body of an organism. If you are
unfortunate enough to get a splinter under your skin, your body reacts. The site may become infected by bacteria or other microorganisms, and a “tumor” forms
walling off the splinter from healthy tissue.
The Monterey also grew “tumors” by being infected. How? The Monterey formed in a mile plus deep marine environment. Clay and other fine-grained sediment such as diatom and calcareous plankton tests (skeletons) accumulated to form an ooze (a marine sediment with more than 30% organic material). Heavy rain and flooding brought wood to the beaches. Some drifted out to sea, became water-logged, and sank into the ooze.
These “splinters” became infected with bacteria which interacted with the sediment and the wood to produce swelling. The ooze with the nodules ultimately turned to stone.
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