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|How San Francisco Bay Was Formed
Top Toward the end of the Miocene Epoch, the coastline of northern California lay far inland, perhaps as far east as the present-day beds of the Sacramento and San Joaquin rivers. What is now the eastern section of the Great Central Valley was a gentle plain covered with volcanic debris from the Sierra Nevada, and the future Coast Ranges were a group of offshore islands, which were themselves the remnants of once much higher mountains.
Middle During the late Pliocene Epoch, the coastal mountains rose again, blocking the sea from the plain. At the same time, the Sierra Nevada was being eroded by streams that carried rich sediment into the future Great Central Valley, forming a base for the valley's now very fertile farmland.
Bottom Over the subsequent centuries the ancient volcanic plain has been eroded, bringing into existence the present Sierra foothills. The coastal mountains, after being eroded in the next geologic epoch, rose once more. The mountains are still being eroded, a process creating the familiar outlines of the San Francisco Bay area we see today.
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This page was last updated on 11/14/2017.