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San Gregorio beach is a wonderful trip for a geology buff. Fanciful sandstone sculptures, turbidite layers, fossil clams embedded in the Purisima formation ... and a clear view of a location where the San Gregorio fault, sister to the San Andreas and Hayward faults, goes offshore.
In addition, San Gregorio Creek creates an estuary between Highway 1 and the tide line, creating all sorts of interesting effects. Stand in the warm creek waters and watch driftwood float downstream, studying the eddies behind the driftwood and the way they change the meandering course of the creek.
Geology of the San Francisco Bay Region (Univ. of Calif. Press, 2006) tipped me off to this beach and its fossils and caves. The book warned that the fossil clams and the caves north of the beach might only be visible at very low tide. But we were there at only a moderately low tide, and there was plenty of room to walk around the headland and see everything there was to see.
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h1: | Whirls of sandstone on the trail down to the beach. |
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h2: | Notice the conglomerate layer up at the top of the cliff. |
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h3: | Fossilized burrows of clams. |
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h4: | Fossil clams shell imprint. |
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h5: | An odd inclusion of rock, full of fossil clams. |
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h6: | No one else seems to notice the fossils. |
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h7: | Turbidite layers, and an amazing amount of driftwood. |
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h8: | Perfect little landslide tongues cover the rock layers in places. |
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h10: | The beginning of a tuff layer, from an ancient eruption somewhere near Lassen. |
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h15: | Follow the tuff layer as it descends to the ground. And note the beginnings of a marine terrace, which extends for quite a distance. But the important thing is where the layered rock ends and the vegetation begins: this was the first of two candidates for where the San Gregorio fault heads offshore. |
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h16: | This was the second candidate for the San Gregorio, just a bit north of the first spot. See the different in vegetation color following the small linear valley extending to the right? Notice the resistant rock layer at the right edge of the photo. If this is the tuff layer again, then the first candidate wasn't the fault. If this is a different layer, then it was. It wasn't obvious from looking. |
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h22: | You can spend a long time standing in the creek studying the ripples. Well, I could anyway. |
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h24: | The estuary. |
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h25: | Looking north, across the amazing collection of driftwood the creek brings in. |
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v5: | Looking back on the end of the rock layers. I may be straddling the San Gregorio fault to take this picture. |