A Day At Mammoth's Ancient Lake

August 13, 2005

By George Shirk
Editor, Mammoth Monthly

                  We were standing on the shore of the old Long Valley Lake and trying to imagine a large body of water extending from the town of Crowley Lake all the way up into Mammoth, then east to the Glass Mountains.
    In the middle of this lake was an island, and on this particular Saturday afternoon in June, we were standing on the island—on the beach, no less.
    How could we tell it was a beach on a lake?
    “A lot of geology is very subtle,” said Dr. Steven R. Lipshie, an engineering geologist with the Los Angeles County Department of Public Works. Lipshie reached to the ground and picked up a perfectly round piece of obsidian.
    Lipshie is a small, athletic man who has studied Long Valley for three decades. He tells lucid stories of the geology of this area, and he's a terrific teacher.
    “Can anyone tell me why these stones are round?”
    The group, on a tour as part of the public outreach program of the Valentine Eastern Sierra Reserve, was baffled.
    “They're round because of wave action,” Lipshie said, and you could have knocked us over with a feather.
    This “beach” is, in fact, high on a bluff on the Resurgent Dome, above and to the west of Hot Creek Fish Hatchery, at an elevation of about 7,400 feet. Around us, the only water we can find is in our water bottles and in the valley below, where Hot Creek meanders on its way to join the Owens River.
    But Lipshie explained that in Pleistocene times, the entire Long Valley caldera was filled with water as a result of the catastrophic Long Valley eruption—an event 750,000 years ago in which the Earth's surface dropped nearly two miles as 160 cubic miles of ash and magma were evacuated over the course of just one week.
    Bishop tuff, the pinkish rock commonly seen along U.S. Hwy. 395 south of Crowley Lake, filled in much of the caldera.
    The rest of it was filled by water, flowing from many of the same Sierra Nevada creeks and streams that we identify today.
    Lipshie gave credit to the late geologist, Roy Bailey, for coming up with the story of Long Valley Lake. At its largest, he said, the lake filled the entire caldera, leaving only the Resurgent Dome as an island.
    As the Resurgent Dome rose, Bailey concluded, it displaced the water in the lake and was responsible for yet another geologic feature with which everyone around here is familiar—the Owens River Gorge.
    Lipshie said the lake, its water displaced by the rising dome, overflowed the lip of the caldera right about where the lake's dam is now. Over the years, this overflow carved the gorge.
    Eventually, the Resurgent Dome island was surrounded by a remnant moat of water. As the Gorge became deeper and deeper,  the lake drained and disappeared.
    Hundreds of thousands of years later, the State of California built  Rte. 203—right along the ancient south moat of Long Valley Lake.

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