Duxbury Reef

January 16, 2026
Late afternoon, sunny and cool.

This was not my first visit to Agate Beach, but it is the first time I’d really taken a look in the wonderful tidepools on Duxbury Reef. This was part of a College of Marin Flex Session, and I really enjoyed this visit. The reef is a remarkable, beautiful place, and I saw loads of interesting organisms.

We started out by climbing to the top of the bluff that overlooks the reef, for a magnificent view. Duxbury Reef is one of the largest shale reefs in North America, stretching nearly three miles along the coast of Marin County, just west of Bolinas. Formed from Miocene-epoch Monterey shale, this rocky shelf is renowned as one of California’s top tidepooling destinations.

When we made our way down to the beach, the first animal I saw was a Common Raven foraging amongst the rocks. As we headed north through the pools, Black Tegula were abundant, along with many tegula shells inhabited by both Blueband Hermit Crabs. Sunburst Anemone were common in this southern section of the reef, along with Rockweed and Common Coralline algae. Dire Whelk and Rough Limpet were present, along with some Giant Green Anemone. We also saw a partial Pacific Purple Sea Urchin endoskeleton/test.

A bit further north, we found Red-banded Transparent Shrimp, along with rocks covered in Pacific Acorn Barnacle and California Mussel. We saw abundant Aggregating Anemone, along with more Sunburst and Giant Green anemones. Gastropods included Angular Unicorn, Checkered Periwinkle, Scale-sided Piddock, Common Jingle, and more Dire Whelk. On an open patch of flat sand, I saw the tiniest crabs I’ve ever seen: Striped Shore Crab about 1 cm wide, and we found Calliarthron tuberculosum (algae) in some of the pools.

Farther down, I found one gorgeously red Urtibrina rimicola, along with Mossy Chiton, Rough Keyhole Limpet, and Leafy Hornmouth. (Some of these rocky intertidal critters have such fantastic names). Here, we also found Gooseneck Barnacle in amongst the California Mussel, an Owl Limpet. We also found a second species of hermit crab: Grainy Hermit Crab. In one of the deeper pools, we finally found some living Pacific Purple Sea Urchin, along with a Pacific Rock Crab, plus more Aggregating Anemone. During our walk, we also found a couple of Owl Limpets, a rare and protected species that is often poached. Gentle reminder to please not take or harm wildlife when tidepooling, as many of them are delicate, and at risk of extinction.

We didn’t stay too long at the reef, as we were quickly losing the light, but even though we were only there for about an hour, we saw a wide variety of species. I’m looking forward to future visits to the reef.