Geologic Epochs

The geological processes that created the spectacular cliffs along the northern California coast only recently became known to have begun several hundred million years ago with movement of the earth’s crust. Plate tectonics did not become widely accepted until their contours were serendipitously identified in the 1960s with the world wide deployment of sensitive seismometers, which were actually purposed to detect atomic bomb explosions. Through many years of painstaking data collection, we now know that about 250 million years ago the North American Plate and the Pacific Plate, which had been very gradually moving towards each other, collided. The sea floor crust of the Pacific Plate slipped beneath the continent, heating and melting as it reached the earth's interior creating a subduction zone Then, about 30 million years ago the relative movements of the North American Plate and the Pacific Plate changed from a head-on collision to laterally slipping sideways against each other. This zone of slippage, extending nearly the length of California is the well known San Andreas Fault, which veers about 10 miles off shore under the sea by Point Arena, about 30 miles south of Sea River House, and does not return back onto land until Cape Mendocino, about 125 miles north of us. Along this zone, folding of the sea floor along the margin of the North American Plate resulted in the creation of the Northern California Coast and Transverse ranges, which are composed of remnants of the crushed, crumpled, and folded sea floor sediments and fragments of the edges of the continental plates. Visible in the sea cliffs along the Northern California coast are massive and steeply dipping rock layers, called the Franciscan Formation, which can be seen nearby at Point Arena. These rocks were formed as the Pacific Plate subducted under the North American, sometime in the Late Jurassic period. At the end of the last ice age about 11,500 years ago, the great glaciers that had covered much of the northern hemisphere began melting, causing worldwide sea level to rise as much as 100 feet over the next 5,000 years. Cliffs formed as wave action and sea-level rise eroded weaker rocks and sediment, while the stronger sedimentary rocks became headlands and the sea stacks that dot the many coves along the Mendocino Coast, including Brokeback Cove seen from Sea & River House, and the oft photographed Elk Sea Stacks just 2 miles south on Highway 1.

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