Why is Portneuf Gap Here?

Portneuf Gap exists because this canyon was cut by the ancestral Bear River around a million years ago. It crosses the north-south trend of the Bannock Range by following 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, rocks in the Chinks Peak area are overturned, forming the upside-down limb of a geologic fold with an amplitude of about 2,000 feet. About 600,000 years ago, basalt from northern Gem Valley near Bancroft erupted into Portneuf Valley, and two lava flows followed the river canyon for about 40 miles. In Marsh Valley south of here, these lava flows occupy the center of the valley, restricting the Portneuf River and Marsh Creek to the valley edges.

Idaho · United States