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Mysteries of the Rock

Just off the Tier 3 driveway, behind a storage shed in a fenced-off area that was once our “Beetopia” garden, lies the largest rock on our campus. Measuring about 18 by 20 inches and 11 inches high, it’s studded with seashells. What is it? How old is it? How did it come to be here?

For answers to these questions, we turned to Ashley Dineen, PhD, who is the senior invertebrate scientist at UC Berkeley’s Museum of Paleontology. We sent her the photo above.

The rock, she explains, is sedimentary, meaning it was formed on the earth’s surface (rather than deep within the earth) by deposits of minerals or organic matter that hardened over time.

“The shells are clam, oyster, and pecten (scallop), and probably some gastropods (mollusks), all clumped together in what we call a ‘shell hash,’ which is usually created by very-high-energy marine environments, like large waves or storms.”

How did it come to be on our campus? Ashley says it may have been here all along, having formed when the area was under water, ages ago. “Your piece was very likely part of a small outcropping of this rock near your school that was dug up or destroyed by development in the area. If this is the case, that would likely make it part of the Briones Formation, from the Late Miocene age – roughly 12 million years old.” (By contrast, the first humans, Homo sapiens, appeared about 300,000 years ago.)

For years our students have been excited to discover fossils in the area behind the school that’s now the Seven Circles Garden.

Our rock, Ashley says, “looks similar to what I've seen at Shell Ridge Open Space,” east of Walnut Creek. She sent us this link to share with families who might want to go fossil hunting at Shell Ridge or nearby Lime Ridge.

Years ago the rock was in the yard behind Room 10. We asked current and former staff members and current staff who went to school here if they remember it. Most have only a vague recollection of it or don’t remember it at all, though one recalls trying to pry shells loose. No one we talked to knows why, when, or how it was moved to its current location.

If you’d like to visit our rock with your child after school, you'll find an opening in the fence behind the shed. Please be extra cautious when walking through the parking lot or on the driveway to reach it.

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This interactive map of the globe from shows what our area – or any area on earth – looked like from 20 million to 750 million years ago. That’s not quite recent enough to show how it looked when our rock was formed, but in geological terms, it’s pretty close!


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