Historical Marker on Red Rock Pass, US-91 South. All highways in Idaho have these historical markers. To see all of them check out the Idaho State Historical Marker Guide. Red Rock Pass is where ancient Lake Bonneville - an inland sea, which covered most of the great basin, overflowed into the Snake Plain. Evidence of this great flood is everywhere along the Snake and Columbia Rivers in the Pacific Northwest. This pass marks the divide between the Great Basin and the Snake/Columbia River drainages.