Lemhi County forms much of eastern Idaho's border with Montana along the Beaverhead Mountains and contains complex geology, rich mineral deposits and tracts of wilderness near the Salmon River. The southern part of the county is within the Basin and Range province with the Beaverhead and Lemhi Ranges and intervening northwest-trending valleys. The Salmon River flowing north from Challis toward Salmon and North Fork, forms the western boundary of the Basin and Range province. The Salmon turns west at North Fork and flows into the River of No Return Wilderness west of Shoup. It joins large tributaries from the south, including Panther Creek and the Middle Fork of the Salmon River, before flowing west into Valley and Idaho Counties.
Much of Lemhi County is underlain by Mesoproterozoic strata of the Belt Supergroup, deposited between 1470 and 1370 years ago. These rocks include the Yellowjacket Formation, Lemhi Group, Swauger and Gunsight formations, and make up most of the northern Lemhi and Beaverhead Ranges and the Salmon River Mountains. Paleozoic sedimentary rocks are exposed in the central Beaverhead and southern Lemhi Ranges, and are mainly limestones. In the Panther Creek area and to the northwest, Belt Supergroup rocks were buried, metamorphosed and intruded by granitic rocks around 1370 Ma.
The area was also subject to intrusion of granitic rocks in Ordovician time, an anomalous age regionally in the Cordilleran orogenic belt.
The Proterozoic and Paleozoic rocks were folded and thrust faulted in Mesozoic time, as part of the Cordilleran thrust belt. A major thrust fault, the Brushy Gulch thrust is exposed northwest of Salmon, and may continue into the Cabin thrust fault system of the Beaverhead Range in Montana.
Eocene volcanic rocks of the Challis volcanic group are found in the southwest corner of Lemhi County, and are cut by northeast striking faults of the Trans-Challis fault system. They are also intruded by coeval Eocene granitic rocks in the Bighorn Crags and Camas Creek areas.
Complex Eocene to Miocene normal faulting produced north-trending basins near Salmon and in the Summit Creek area of the Lemhi Range. Miocene to Recent Basin and Range faulting uplifted the existing mountains.
For more information see the Link and Janecke and Price et al. articles in the Guidebook to the Geology of Eastern Idaho and sections in Rocks, Rails and Trails.
P.K. Link, 10/02
Rocks Rails and Trails: pages
Rocks of Central Idaho
Pictures -Idaho Mountains
Basin and Range Faulting
The Borah Peak Area & the October, 1983 Earthquake
Glaciation in the Mountains of South-Central Idaho
References on Idaho Geology
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