Wednesday, January 12, 2011

Plate Tectonics

Kehoe Beach is a textbook illustration for geology classes. The Point Reyes Peninsula is as "an island in time".  The whole of the National Park is part of the Pacific Plate and is moving northwest, grinding past the North American Plate. We see lots of evidence at Kehoe. The layered rock of the Monterey Formation, the bleached pages of a book you see when you drop down to the beach at the trail end, match up with rocks 120 miles to the south. This chunk of land is on the move. Further north is the smooth wall of Laird Sandstone laid up on top of the Monterey Formation. The Monterey stone was deposited slowly with lots of microscopic critters lending their shells to the mix. You can see barnacle fossils too. The Laird Sandstone was deposited quickly and is so soft it can be scraped to write graffiti. Which depending on the season when hearts and pluses match up initials. Spring blooms lots of lover pairs scratched into the Laird Sandstone. All of this was laid on top of granite that appears to be similar to what has made the Sierra Nevada range.

The thing is, this mash up of rocks really happened and is backdrop of our recurrent trips to Kehoe. During one lifetime the cliffs seem solid, monumental and permanent but we've seen the slow crumble and slide as alluvial fans of granitic rocky debris slide onto the beach from the far end of the beach. Every year the beach slides toward differentiation. The Monterey Formation is the most solid though it is folded by years of pressure, as though it were rubber bands caught on a treadmill. The perpendicular Laird Formation peals off in slabs and dangerous chunks that have killed picnickers.

All of this tells the languorous and relentless story of change at the margins. Driving out to the beach you cross the famous San Andreas fault, here a valley filled in with waters of Tomales Bay. Change on a geologic scale may indeed be languorous, but periododically a lurch occurs as it did in 1906. The epicenter of that lurch was right here in Point Reyes where you can see a fence displaced by sixteen feet, left as a monument very near park headquarters.

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