The ancient dam at the north end of former Lake Bonneville
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Red Rock Pass was cut through a sill of resistant Paleozoic shale, limestone, and dolomite, and forms a narrow gap two miles long. At one time the pass was at the shoreline of Pleistocene Lake Bonneville, 300 feet higher. Lava flows in the vicinity of Pocatello diverted the Bear River through Lake Thatcher into Lake Bonneville. The sudden influx caused Bonneville to overflow at Red Rock. Marsh Creek Valley, immediately downstream, was flooded from wall to wall, and the rapid discharge eroded the pass to its present level. The Bonneville Flood, as it isLake Bonneville flood known, was catastrophic. Maximum discharge as about 15 million cfs, or about three times the average flow of the Amazon, the world's largest river. The rate of flow was approximately sixteen-mph, and though peak flow lasted only a few days, voluminous discharges may have continued for at least a year.  Red Rock pass attracts geological groups from miles around, to study the remains of the great forces on nature. (Written by Darrel La Mar Wakley) (Right -Artist L.A. Ramsey's interpretation of Lake Bonneville flooding through Red Rock Pass.)

Portneuf Narrows, a canyon 45 miles northwest of Red Rock Pass, where the flood is estimated to have reached a height of 400 feet.

Left - Lake Bonneville and the flood path.

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(Excerpt and pictures from Idaho State University's Digital Atlas)
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