California's western edge has been pushed and shoved throughout its known geologic history. First it was pushed from the west as various oceanic plates were carried into it and then subducted down into the mantle, producing wave after wave of volcanism, mountain-building and erosion. That activity created the rocks of the Franciscan Complex. Then beginning around 25 million years ago,
the San Andreas fault zone grew from southern California toward the north, ripping these rocks into slivers and shoving them northward, at the same time raising them into coastal mountains called the Coast Ranges in northern and central California and the Peninsular Ranges in southern California. Today the fault reaches as far north as Cape Mendocino; beyond there subduction continues today as it does in Oregon and Washington. For more details of this picture, consult the
California geologic map.
The result of all this activity, in a nutshell, is that California's coast is mountainous and active. Cold, moist air off the Pacific Ocean is quickly lifted against the coastal mountains and its water content wrung out as fog. Rocks of the Franciscan Complex form most of the Coast Ranges; granite related to the rocks of the high Sierra Nevada forms the Peninsular Ranges and small parts of the Coast Ranges. Only in a few places—Monterey Bay, Los Angeles and Long Beach—does the gentle sandy shore typical of the east and Gulf coasts occur in California. Here at Baker Beach, rugged Franciscan rocks and fog are displayed to great effect.
For greater detail and many more photos about these rocks and their tectonic story, follow the transect across California devoted to subduction-related rocks.
