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Lake BonnevilleLake Bonneville was a prehistoric pluvial lake that covered much of North America's Great Basin region. Most of the territory it covered was in present-day Utah, though parts of the lake extended into present-day Idaho and Nevada. Formed about 32,000 years ago, it existed until about 16,800 years ago, when most of the contents of the lake were released through the Red Rock Pass in Idaho. Many of the unique geological characteristics of the Great Basin are due to the effects of the lake. The Great Salt Lake, the world's largest saltwater lake, is a remnant of the original Lake Bonneville. Several levels of the old shorelines are still visible above Salt Lake City, along the Wasatch Front and elsewhere. Its appearance is that of a shelf or bench protruding from the mountainside, well above the valley floor. Four main shorelines are associated with the fluctuating levels of the ancient lake. The Stansbury, Bonneville, Provo and Gilbert shorelines each mark a time when lake level remained constant long enough to deposit massive accumulations of sand and gravel. The Bonneville Bench, at approximately 5100 feet elevation, is part of the preserved ice age shorline. This shoreline marks the highest level attained by the Pleistocene lake approximately 15,500 years ago. During this period, the lake covered over 20,000 square miles (52,000 km²) and was over 1000 feet (300 m) deep in places. About 14,500 years ago, the lake level fell catastrophically as Lake Bonneville overflowed near Red Rock Pass, Idaho. The lake level fell some 350 feet to what is now the next lower bench (the "Provo level") in a flood that geologists estimate to have lasted up to a year. The Provo level is the most easily recognized shoreline feature throughout the valley and is distinguished by thick accumulations of tufa that formed nearshore during the 500 years that the lake was at this level. About 14,000 years ago, the lake started to drop again due to changing climate conditions, and by 12,000 years ago, the lake reached a level even lower than that of the modern day Great Salt Lake. A slight transgression or rise in lake level occurred about 10,900 to 10,300 years ago and formed the Gilbert shoreline. The Gilbert shoreline is the least conspicuous of the major shorelines but evidence of it remains at Antelope Island and in large coastal features, such as the Fingerpoint Spit near the Hogup Mountains. See also References - Green, S. A. and Currey, D. R. (1988). The Stansbury Shoreline and Other Transgressive Deposits of the Bonneville Lake Cycle. In In the Footsteps of G. K. Gilbert--Lake Bonneville and neotectonics of the Eastern Basin and Range Province. Guidebook for Field Trip Twelve, The Geological society of America, p. 55-57.
External link Bonneville
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