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10 miles W of Mammoth; 50 miles E of Yosemite's eastern boundary

Just a few miles outside Mammoth Lakes, Devils Postpile National Monument is home to one of nature's most curious geological spectacles. Formed when molten lava cracked as it cooled, the 60-foot-high, blue-gray basalt columns that form the postpile look more like an enormous, eerie pipe organ or a jumble of string cheese than anything you'd expect to see made from stone. The mostly six-sided columns formed underground and were exposed when glaciers scoured the valley during the last ice age, some 10,000 years ago. Similar columnar basalt is found in Ireland and Scotland.

Because of its high elevation (7,900 ft.) and heavy snowfall, the monument is open only from summer to early fall. Summer weather is usually clear and warm, but afternoon thundershowers are common. Nights are cold, so bring good tents and sleeping bags if you're camping. Unfortunately, all those beautiful lakes attract lots of mosquitoes. Plan for them.