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Geology of Mammoth Lakes Region The Mammoth Lakes area is a land of fire and ice. Over the millennia, natural forces have raised and sculpted the Sierra Nevada range. During ice ages, massive glaciers moved slowly downward, shaping granite peaks and carving out canyons and lake basins. Perfect examples of this glacial action in the Mammoth area are the jagged spires of the Minarets, the U-shaped valleys of Bloody Canyon and Rock Creek, and the smooth granite domes of Yosemite National Park's high country.

Volcanic forces were at work in the Sierra as well. Dominating the landscape is Mammoth Mountain itself, a dormant volcano that last erupted over 50,000 years ago. Long Valley, which cradles Crowley Lake just southeast of Mammoth, is part of a caldera forged during a highly active period 760,000 years ago. Extraordinary features such as obsidian domes, craters, steam vents, hot springs and even Mono Lake's tufa towers all attest to the region's active geologic past.

Today, we reap the benefit of this natural process in ways other than sightseeing: Mammoth Pacific Geothermal Power Plant generates electricity by harnessing the energy of water heated deep below the earth's surface, and Hot Creek Fish Hatchery uses warm spring waters to maintain optimum thermal conditions for raising young trout.

Devils Postpile National Monument
No feature in the Mammoth area better demonstrates the work of fire and ice than Devils Postpile. The columns of basalt were formed approximately 100,000 years ago when a lava flow slowed and began to cool and crack. Temperatures deep within the lava bed were uniform enough to produce six-sided columnar joints, an ideal configuration within a cooling mass. Columns of four, five or seven sides indicate some degree of temperature fluctuation when the Postpile was formed. A short trail leads to the top of the columns, where the distinct hexagonal pattern of the Postpile is clearly visible, polished to a shine by later glacial action.

Rainbow Falls
Two miles downstream from Devils Postpile, the San Joaquin River tumbles over an abrupt 101-foot drop, sending rainbows of color into the mist. After the easy, 1.5-mile walk to Rainbow Falls from the Red's Meadow area, the roar of the falls and the refreshing mists invite you to stay awhile and enjoy a picnic lunch.

Earthquake Fault
Located off Hwy. 203 on the way to Mammoth Mountain, this sizable fissure is not an earthquake fault caused by a single quake, as the name implies. Geologists now believe it is a system of fractures formed during a series of strong quakes when the Inyo Craters and Inyo Domes erupted centuries ago.

Inyo Craters
One of the last notable volcanic events in the Mammoth-Mono area, occurring 550 to 600 years ago, resulted in Inyo Craters, just north of Mammoth off the Mammoth Scenic Loop. In this explosion, magma stayed well below the surface, heating rock and groundwater to a critical degree and causing a large, triple-barreled blast. Today, rainwater and snowmelt form small lakes at the bottom of two of these huge pits, one 200 feet deep and the other 100 feet deep. The third crater is on top of Deer Mountain, located 400 yards north of the middle crater.

Obsidian Dome, Wilson Butte, Devils Punchbowl
Located near Deadman Summit, a few miles south of June Lake Loop along Hwy. 395, Obsidian Dome is a mile-long mound of black glass, reaching up to 300 feet high. From the parking lot at Obsidian Dome, you can climb up the mound and see the different composition of the obsidian, which is not uniformly black. As you explore the area, keep in mind that the rock can be quite sharp; this is the same material used by the Paiute Indians to sculpt arrowheads, spearheads and knives, their main trade items in times past. The eruptions that formed each of these lava domes 550-600 years ago began with an explosive phase, then exuded slow-moving, sticky lava which oozed to the surface and cooled too quickly to allow crystal formation. Wilson Butte is located along Hwy. 395 just north of Obsidian Dome, and Devils Punchbowl is east of Hwy. 395 just below the June Lake junction.

Lookout Mountain
Located near the northern edge of the Long Valley Caldera, Lookout Mountain rises to 8,352' and provides a 360 panorama of the Mammoth Lakes area, encompassing Crowley Lake, the White Mountains, Glass Mountain, Mono Craters, Mammoth Mountain and the crest of the Sierra Nevada. Take Lookout Mountain Road, following the signs to the summit 3 miles off Hwy. 395. The pumice and obsidian strewn about hint at Lookout Mountain's volcanic origins; it bulged upward approximately 600,000 years ago.

Hot Creek State Fish Hatchery
Located near Mammoth Lakes Airport off Hwy. 395, the Hot Creek State Fish Hatchery raises about 3 million trout each year for planting in Sierra Nevada lakes and streams, and over 20 million eggs for other fish hatcheries throughout the state. Freshwater warm springs - one of the many aspects of geothermal activity in the area - mix with the cold waters of Mammoth Creek to provide ideal temperatures for breeding trout year round. Open daily to the public.

Hot Creek Geological Site
Once Mammoth Creek mixes with hot springs near the fish hatchery it becomes Hot Creek, meandering across Long Valley toward the Owens River and the inlet of Crowley Lake. Along the way, it enters a small gorge at Hot Creek Geological Site, home of bubbling hot springs, a series of fumaroles (gas vents) and hot-water seeps lined with colorful sulfur deposits and mosses. While the brilliant pools and pleasant river canyon make this a popular destination, visitors must exercise caution: the pools and seeps contain scalding water, and water in the creek itself can rise to dangerously high temperatures without warning.

Mono Lake
About 30 miles north of Mammoth, Mono Lake stretches its glassy waters eastward from the base of the Sierra Nevada. The salt waters of this ancient inland sea sustain a unique biosphere where tiny brine shrimp and brine flies feed millions of migrating birds each year. The sweeping views of the glaciated range and the stark Mono Craters make Mono Lake a very special place. Easiest access to Mono Lake is at South Tufa, off Hwy. 120 east, where visitors can meander among the odd-shaped tufa towers. These towers - piled-up mineral deposits formed beneath the water's surface when mineral-rich spring water mixes with salty lake water - are silent witnesses to Mono Lake's diminishing size over the last few decades. The Mono Basin Scenic Area Visitor Center, just north of Lee Vining on Hwy. 395, is the perfect place to learn more about this unique area.

Mono Craters
Resembling a moonscape, the Mono Craters chain stretches from Hwy. 395 near June Lake Loop to Mono Lake. These stark, otherworldly hills are covered with layer upon layer of light-colored pumice from a series of eruptions over many years. The craters can be accessed by dirt roads, but the sandy road surface is not recommended for most passenger vehicles. Panum Crater, on the south shore of Mono Lake, can be reached readily by car, and an easy trail leads up the crater. Repeated upsurges of lava, some 600 years ago, cooled and cracked, forming the odd jumble of volcanic material within the crater. Looking across Mono Lake, visitors can also spy other clues to the area's geologic past: Negit and Paoha islands and dark, flat-topped Black Point on the lake's north shore.

Yosemite - Tioga Pass and Tuolumne Meadows
With its eastern entrance at Tioga Pass just 45 minutes from Mammoth, Yosemite is a favorite day trip for Mammoth vacationers. Relax alongside sparkling Tenaya Lake, walk along Tuolumne Meadows, watch rock climbers and take in the inspiring Yosemite scenery or hiking in Mammoth the many trails. (Tioga Pass is typically closed due to snow from mid-November through May.) Ringed by tremendous peaks - Cathedral, Unicorn, Dana and Conness, to name a few - the Tuolumne area is also home to a series of gigantic granite domes. It's easy to see how it was here, in Yosemite, that John Muir formulated the theory that glaciers carved the Sierra range. A self-guided auto tour brochure provides additional information on regional geology, and is available at the Mammoth Visitor Center on Hwy. 203. Local shops carry scores of guidebooks to the area, too.

Mammoth Ranger Station and Visitors Center
Highway 203, PO Box 148 Mammoth Lakes, CA 93546

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