Geology
Like nearby Yosemite Valley, Hetch Hetchy was sculpted by glaciers as recently as 10,000 years ago, though the Hetch Hetchy glacier was more recent and larger than the one in the paleo-Yosemite Valley. Today the Hetch Hetchy area is drier.
On the upper portion of the valley, beyond the reservoir, there is evidence of relatively young lava flows. One recent flow formed the Little Devils Postpile which, as the name suggests, is a smaller version of the Devils Postpile near Mammoth Lakes to the southeast. Both formations are excellent examples of columnar basalt, a phenomenon that results from contraction of basaltic lava as it cools (forming hexagonal columns). Similar formations are found in the Giant's Causeway in Northern Ireland, and the New Jersey Palisades in the United States, as well as other places throughout the world.
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