Red Rock Pass is a low
mountain pass
A mountain pass is a navigable route through a mountain range or over a ridge. Since many of the world's mountain ranges have presented formidable barriers to travel, passes have played a key role in trade, war, and both Human migration, human a ...
in the
western United States in southeastern
Idaho, located in southern
Bannock County, south of
Downey. It is geologically significant as the
spillway of ancient
Lake Bonneville. It is traversed by
U.S. Route 91 at an
elevation of above
sea level, bounded by two mountain ranges; the
Portneuf to the east and the
Bannock to the west.
The pass was cut through resistant
Paleozoic shale
Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especial ...
,
limestone, and
dolomite, and forms a narrow gap in length.
At one time the pass was higher, where the shoreline of
Pleistocene Lake Bonneville stood.
The pass takes its name from the red limestone cliffs which border it. Red Rock Pass has a surface deposit of
calcareous silty
alluvium with
topsoil of dark grayish brown silt
loam
Loam (in geology and soil science) is soil composed mostly of sand (particle size > ), silt (particle size > ), and a smaller amount of clay (particle size < ). By weight, its mineral composition is about 40–40–20% concentration of sand–sil ...
.
Bonneville flood
It is believed that during the last
ice age lava flow
Lava is molten or partially molten rock (magma) that has been expelled from the interior of a terrestrial planet (such as Earth) or a moon onto its surface. Lava may be erupted at a volcano or through a fracture in the crust, on land or und ...
s in the vicinity of
Pocatello began to divert the
Bear River through
Lake Thatcher and then into Lake Bonneville. This sudden influx caused Bonneville to overflow at Red Rock about 14,500 years ago. This overflow caused a sudden
erosion of unconsolidated material on the northern shoreline near Red Rock Pass. As the material gave way, Marsh Creek Valley, immediately downstream, was flooded from wall to wall, and the rapid discharge eroded the pass to its present level. The flood then flowed into the
Snake River Plain
image:Snake River view near Twin Falls, Idaho.jpg, The Snake River cutting through the plain leaves many canyons and Canyon#List of gorges, gorges, such as this one near Twin Falls, Idaho
The Snake River Plain is a geology, geologic feature ...
, generally following the path of the present-day
Snake River
The Snake River is a major river of the greater Pacific Northwest region in the United States. At long, it is the largest tributary of the Columbia River, in turn, the largest North American river that empties into the Pacific Ocean. The Snake ...
to its outlet in the
Pacific Northwest.
Univ. of Utah Media
– Lake Bonneville – accessed 2009-08-19
The Bonneville flood, as it is known, was a catastrophic event. The maximum discharge was about 15 million cubic feet per second (420,000 m³/s), or about three times the average flow of the Amazon River
The Amazon River (, ; es, Río Amazonas, pt, Rio Amazonas) in South America is the largest river by discharge volume of water in the world, and the disputed longest river system in the world in comparison to the Nile.
The headwaters of t ...
, the world's largest river. The speed of flow was approximately , and though peak flow lasted only a few days, voluminous discharges may have continued for at least a year.
References
External links
Huge Floods.com
– Bonneville
– Lake Bonneville
{{Idaho
Landforms of Bannock County, Idaho
Historical geology
Mountain passes of Idaho
Geology of Idaho
Natural history of Idaho