Ask the Naturalist

Dinosaur eggs on Point Reyes Estero Trail?

March 29, 2013

Tim Hastings wrote to us wondering about “many large round, almost ‘dinosaur-egg’ like rocks dotting the muddy sands” when he was hiking the Estero Trail. Tim’s guess is that the soft rock is susceptible to erosive shaping during the rise and fall of tides; thus the almost uniform rounded, oval shape of these small boulders.

That’s possible, but depending on where exactly these were along the trail, we’re going to guess that these are actually “concretions” — in his Natural History of the Point Reyes Peninsula, Jules Evens defines that term this way:

Concretion Occurs in sedimentary rock when minerals fill the spaces between the grains and solidify, cementlike, forming a spherical shaped mass. The resulting structure is more erosion-resistant than the original rock. Examples are exposed in the Bolinas cliff at the high-tide line and in the cliffs at Drake’s Beach.

It’s possible that the rocks you saw simply weathered into round shapes (I’m still awaiting more expert opinions from some of our geology and Point Reyes experts), but there might well be some of concretions eroded out of nearby cliffs and bluffs as you get closer and closer to the coast. If you took a route like the one Jules Evens did on the Estero Trail to Sunset Beach, then that’s even more likely.

though this isn’t the case at Point Reyes, concretions are often associated with tafoni, the amazing fretwork erosion visible at Salt Point State Park, El Corte de Madera Creek Preserve, Castle Rock State Park, and elsewhere (check out our tafoni around the Bay roundup).

Among the most famous collections of concretions we know about is Bowling Ball Beach up in Mendocino County. Very cool:

Bowling Ball Beach, photo by Josh Hawley
Mendocino County’s Bowling Ball Beach is made of “concretions” of mineral deposits eroded out of nearby cliffs. Creative commons photo by Josh Hawley

Readers, if you have more ideas for explaining Tim’s mystery rocks, let us know!

About the Author

Dan was editor of Bay Nature from 2004 until 2013, when he left to work for SF-based Stamen Design. He is now executive director of GreenInfo Network, a nonprofit mapmaking organization. A onetime professional cabinetmaker, he considers himself a lifelong maker of things and teller of stories. Dan has been working at the intersection of journalism and technology since, at age 16, he began learning reporting, page layout, and database design. His enduring interest in environmental issues crystallized into a career path in 1998 when he assisted former U.S. Poet Laureate Robert Hass in a cross-disciplinary nature writing and ecology course at UC Berkeley, from which Dan received a Masters in English literature. In 1999, he became Associate Editor of Terrain, the erstwhile quarterly magazine of Berkeley's Ecology Center. In addition to editing and art-directing Bay Nature magazine, he was also Bay Nature’s chief technology strategist, fixer of broken things, and designer of databases and fancy spreadsheets. And he was even known to leave the office and actually hike outdoors.