Red Rock Pass Geological Site
About 14,500 years ago, ancient Lake Bonneville overflowed at this site.
A dam of alluvial fans between Oxford Mountain to the west
and the Portneuf Range to the east
suddenly eroded releasing Lake Bonneville from the Great Basin
into the Snake River system.
The Peak flow was about one million cubic meters per second at the pass,
or about 500 times the maximum discharge on the Snake River at Idaho Falls, Idaho.
Ancient “bathtub ring” shorelines up to 1,000 feet above the valley
floor are readily visible in the Salt Lake Valley.
Evidence of the flood is visible thoughout southern Idaho with
areas of scoured bare bedrock (“scabland”)
and deposits of boulders (“melon gravel”) marking the flood path.
After about 11,000 years, Lake Bonneville receded to become the Great Salt Lake.
Highly saline and only 40 feet deep, it is but a shadow of
giant fresh-water Lake Bonneville.
Red Rock Pass is the geographic northern extremity of the Bonneville drainage
basin, and was also designated by the early Latter-day Saint leaders as
the northern edge of the proposed State of Deseret.
North of here, water flows to the Snake, Salmon, and Columbia rivers,
on the way to the Pacific Ocean, but south of here it flows into the Great Basin and the Great Salt Lake.
South of the monument in Red Rock Pass, the house-sized limestone blocks
were jostled during the breakout of the Bonneville flood.
The uneven topography northwest of the monument is a landslide which flowed
into Red Rock Pass after it was deepened about 400 feet during the flood.
Ancient cave formations are found in the flat-lying limestone of
Red Rock Butte immediately north of the monument.