Friday, June 29, 2012

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. The layered rock of the Monterey Formation, the bleached pages of a book laid up from sediment, 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 in the ocean with lots of microscopic critters lending their shells to the mix. You can see barnacle fossils too. The Laird Sandstone was deposited quickly in riverbeds and is so soft it can be incised to write graffiti - depending on the season - names and pluses inside of hearts. Spring blooms lots of lover pairs. All of this was laid on top of granite that is similar to what made the Sierra Nevada range.

The thing is, this mash up of rocks really happened and is the 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 far end of the beach.

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 Sandstone Formation peals off in slabs and dangerous falling chunks have killed picnickers. All of this tells the languorous and relentless story of change at the margins.

Driving out to the beach we cross the famous San Andreas Fault, now a valley filled in with waters of Tomales Bay. Change on a geologic scale may indeed be languorous, but periodically a lurch occurs as it did in 1906. The epicenter of that lurch is near park headquarters. A fence displaced by sixteen feet remains as a marker and monument.

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