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Concrete Evidence

Circular dolomite concretion rock formation on Crystal Cove State Beach with ocean and blue sky with clouds in background

If you’ve ever walked the beach at Crystal Cove State Park in Southern California, you may have noticed a collection of mysterious circular rock objects resting on the shoreline. I’ve always wondered what the natural explanation was for these otherworldly looking, flying saucer-shaped discs scattered randomly around the beach. Are they remnants from times of war, like so many concrete installations that can be found along coastlines all over the Pacific? Crashed UFO’s that visited Earth many years ago? Or just a random formation of rocks?


After a bit of research, I found out that these curious rock formations are known as "concretions", a phenomenon that occurs in different forms all over the planet. Through a natural process called cementation, a sticky substance adheres to a piece of debris, becoming the nucleus, and a sort of rock “tumor” forms around it. Minerals react with water and fuse to the fledgling formation, and over millennia, the rock grows. In some locations, the cementation process produces perfectly spherical, uniform concretions. In the case of Crystal Cove, the predominant mineral is dolomite, which lends to the creation of the specific formations of this area. When the softer substrate that the rock inhabits gives way from erosion, the hard dolomite concretion is released from the sandstone, and it falls or rolls to terra firma below. 


When viewing these formations on the beach, all one has to do is turn around to face the sandstone cliffs, and virgin concretions can be seen lodged in the bluff, awaiting their escape.




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