Borah Peak
       7 ½’ quadrangle: Borah Peak, ID
       Custer County
       Challis National Forest
       Lost River Range, Rocky Mountains

12,662 ft (3,859m)

Steeply dipping Devonian rock layers on "Chicken-Out Ridge". Summit is to the left. (Photo by Zach Vickery.)

View from beyond Chicken-Out Ridge: light gray Laketown Dolomite at the summit. (Photo by Zach Vickery.)

Bedrock: Laketown Dolomite


Borah Peak lies in a structural horst, a block lifted up during the Eocene Epoch relative to blocks on either side. The horst is bounded by steep faults, the Mahogany Gulch fault to the north, and the Leatherman Pass fault to the south. Within the horst block, the rocks had been previously folded into a syncline-anticline pair during the Sevier Orogeny. The northwest-trending ridge at the summit consists of the Laketown Dolomite, a light bluish-gray dolostone, weathering pale reddish, which bears Silurian fossils including species of Heliolites, Diphyphyllum and Amplexus. The Jefferson Dolomite is younger, and is exposed in a steep limb of the syncline along Chicken-Out Ridge. This unit is a darker bluish-gray to black Devonian dolostone, containing the fossil Favosites.

Folded layers of the Jefferson Dolomite also lie northeast of the summit due to a thrust fault that carried the Laketown northeastward over the Jefferson and younger rocks. The Lost River Range includes other Paleozoic rock units ranging in age from Ordovician to Mississippian, which are unconformably overlain by Tertiary rocks to the northeast. The entire range is part of a larger horst fault block that rose relative to adjacent basins along normal faults during the Tertiary - - typical "basin and range" geology.

The area is still seismically active. A 7.3 magnitude earthquake occurred near Borah Peak in 1983, producing a 21-mile-long fault scarp along the Thousand Springs segment of the Lost River fault, along the base of the mountain. That earthquake also caused ground liquefaction, springs, sand boils and rock falls. Nearby Challis and Mackay suffered considerable damage; two children were killed by material falling from a building in Challis. Photos from the time of the quake can be viewed at the following site:

Surficial Geology: The summit was eroded by glaciers in the Pleistocene into a horn at the center of four arêtes. The cirque to the east contains a tarn pond more than 2000 feet below the summit.

Soil Series: No soil survey has been published for Custer County, but photographs indicate that little to no soil development has occurred on Borah Peak.

Additional Photos:

Climbing sedimentary rocks along Chicken-Out Ridge. (Photo by Zach Vickery.)

Selected References:

  • Link, P.K. and S.U. Janecke, 1999, Geology of East-Central Idaho: Geologic Roadlogs for the Big and Little Lost River, Lemhi, and Salmon River Valleys, in Hughes, S.S. and G.D. Thackray, eds., Guidebook to the Geology of Eastern Idaho: Pocatello, Idaho Museum of Natural History, p.295-334. (Available online at the ISU Digital Atlas, see link below.)
  • Ross, C.P., 1947, Geology of the Borah Peak quadrangle, Idaho: Geological Society of America Bulletin v.58, no.12, p.1085-1160, scale 1:125,000.
  • Wilson, A.B. and B. Skipp, 1994, Geologic map of the eastern part of the Challis National Forest and vicinity: USGS Miscellaneous Investigations Map I-2395, 1:250,000.

Other suggested sources of information:

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