Red Rock Pass

About 14,500 years ago, an earthen dam suddenly broke, beginning one of he largest floods ever recorded in geologic history. Ancient Lake Bonneville, larger in size than Lake Michigan, emptied in a catastrophic torrent. Evidence of the flood such as melon sized gravel is visible along the byway. Today all that remains of Lake Bonneville is the Great Salt Lake. 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.
Red Rock Pass