Pocatello in Bannock County, Idaho — The American West (Mountains)
Why is Portneuf Gap Here?
| | Portneuf Gap-Gittins Cutoff Historic Loop | |
Photographed by Cosmos Mariner, July 10, 2024
1. Why is Portneuf Gap Here? Marker
This canyon was cut by the ancestral Bear River around a million years ago. The canyon cuts across the north-south trend of the Bannock Range, following the trend of an east-west fault that formed about 100 million years ago during formation of the Idaho-Wyoming thrust belt. North of the fault and the highway, the rocks of the Chinks Peak area are overturned, forming the upside-down limb of a geologic fold with an amplitude of about 2,000 feet. The basalt flow of Portneuf Valley was erupted about 600,000 years ago from northern Gem Valley near Bancroft. Two lava flows followed the river canyon for about 40 miles. South of here in Marsh Valley the lava flows occupy the center of the valley and the Portneuf River and Marsh Creek are restricted to the edges of the valley.
Erected by Portneuf Greenway.
Topics. This historical marker is listed in this topic list: Natural Features.
Location. 42° 47.742′ N, 112° 20.908′ W. Marker is in Pocatello, Idaho, in Bannock County. It is on South 5th Avenue (Old U.S. 91) 0.3 miles west of North Blackrock Canyon Road, on the right
this page online
Regionally, this marker is in Idaho’s Bear River Country. It is also in the American Mountain West. Globally, it is in North America, the Western Hemisphere, the Western World, and the Anglosphere.
Other nearby markers. At least 8 other markers are within 5 miles of this marker, measured as the crow flies: Gateway to the Pacific Northwest (here, next to this marker); Copper Mines in the Pocatello Formation (here, next to this marker); Utah and the Northern Railway (approx. 0.8 miles away); Port of Entry (approx. 0.8 miles away); Oregon Short Line & Union Pacific (approx. 0.8 miles away); Idaho's 1st Railroad (approx. 4.4 miles away); Fort Hall (approx. 4.9 miles away); Old Fort Hall (approx. 4.9 miles away). Touch for a list and map of all markers in Pocatello.
Related markers. Click here for a list of markers that are related to this marker. Portneuf Gap-Gittins Cutoff Historic Loop
Also see . . . Portneuf River (Wikipedia).
Photographed by Cosmos Mariner, July 10, 2024
2. Why is Portneuf Gap Here? Marker
This is the rightmost of three related markers located at this pull-out on the old highway. Interstate 15 parallels in the background.
Excerpt: The Portneuf River is a 124-mile-long tributary of the Snake River in southeastern Idaho, United States. It drains a ranching and farming valley in the mountains southeast of the Snake River Plain. The city of Pocatello sits along the river near its emergence from the mountains onto the Snake River Plain. The Portneuf River was given its name sometime before 1821 by French Canadian voyageurs working for the Montreal-based fur trading North West Company. The valley of the Portneuf provided the route of the Oregon Trail and California Trail in the middle 19th century. After the discovery of gold in Montana and Idaho, it became a significant stage route for the transportation of people and goods. In 1877 the valley was used as the route of the Utah and Northern Railway, the first railroad in Idaho.(Submitted on March 7, 2025, by Cosmos Mariner of Cape Canaveral, Florida.)
Credits. This page was last revised on February 22, 2026. It was originally submitted on March 6, 2025, by Cosmos Mariner of Cape Canaveral, Florida. This page has been viewed 517 times since then and 100 times this year. Photos: 1, 2. submitted on March 7, 2025, by Cosmos Mariner of Cape Canaveral, Florida.
