Geology

One hundred million years ago, in the time of the dinosaurs, when the ocean covered the middle plains of what is now the United States, all the Channel Islands and much of coastal Southern California lay at the ocean bottom. In this darkness, the creation of what we see today at San Miguel Island began.

Sediments brought to the sea by the rivers of the nearby mainland fell slowly through the water to the bottom in a silent oceanic snowfall lasting tens of millions of years. Over this time, various geologic episodes produced multiple sedimentary layers growing many thousands of feet thick.

Then about sixteen million years ago molten rock generated by the subduction of the Pacific Plate invaded the sedimentary strata and vulcanism produced multiple submarine lava flows. Some of these became the northeast shore of San Miguel Island. (The cliffs from Harris Point to Cardwell Point are volcanic, as are Prince Island, San Miguel Hill and Castle Rock.)

While the island was still submerged, roughly ten million years ago, a diatom bloom filled the water and the sediments were full of the skeletons of these creatures. We now know the resulting formations as Monterey Shale, the source rock for almost all Southern California's oil. They outcrop in the passes and canyons of San Miguel's east end. Both the volcanic rock and the Monterey Shale were produced during the Miocene epoch, which ended about five million years ago.

At the end of the Miocene, movement along the San Andreas fault created immense forces that buckled the crust of southern California. These forces raised the mountain ranges of southern Santa Barbara and Ventura counties, depressed the Santa Barbara Channel, and - about three million years ago - lifted a new island from the sea off the coast. Scientists call this prehistoric island Santarosae. It comprised what is now San Miguel, Santa Rosa, Santa Cruz, and Anacapa islands. plus all the areas between them, and much that lies north and south of them. The shoreline of Santarosae was about where we draw the 50-fathom isobath on our charts today.

Movement on the great San Andreas fault not only raised Santarosae from the sea, it also turned it and the Santa Barbara Channel so that they lie east-west. The entire Santa Barbara Channel, the islands, and the Santa Ynez mountains rest on a granitic micro-continent rotated by movement along the fault, turning like a ball bearing between the Pacific plate and the North American plate. As a result, the coastline and mountains in this part of California run more nearly east-west than northwest-southeast as they do in the rest of the state.

At the end of the last ice age - about twelve thousand years ago - the glaciers melted, the oceans rose, today's inter-island passages flooded, and the western tip of Santarosae became the island we call San Miguel.

Contents Page

NEXT