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Joe T. Fallinin Campground at Mackay Reservoir

Geology of the Lost River Range & Big Lost River

Basin and Range FaultingStretching 70 miles from Arco to Challis, the Lost River Range contains 8 of Idaho’s 10 highest mountain peaks. Seven of these peaks are over 12,000 feet!
These mountains are typical of a “Basin and Range” landscape. This formation is created when the earth’s crust is stretched so that it cracks apart, creating large faults. Along the faults, the mountains are lifted up from pressure beneath the crust as the valleys subside. Look on the regional map at the campground entrance to identify the series of Basins and Ranges as you travel east/northeast of Mackay.
In 1983, the Mt. Borah Earthquake demonstrated the “Basin and Range” process is continuing today. The fault can be seen from Doublesprings Road about 15 miles north of Mackay Resevoir. Look for the linear, white crack along the bottom of this photograph.

Mt. Borah and its Earthquake Fault
Mt. Borah, Idaho’s highest peak at 12,662 feet, is particularly difficult for the last 1,000 feet elevation because of loose rocks and steep slopes. Chicken-Out Ridge is appropriately named for the knife-edge ridge you have to walk to get to the summit. 
If you want to hike these rugged peaks, plan for an early start with good weather and plenty of daylight, and take lots of water and good boots!
Geology of the Big Lost River
Located at the base of the Lost River Range, Mackay Reservoir captures water from the Big Lost River before it disappears into the Snake River Aquifer at the eastern edge of the Snake River Plain. Along with flows from the Little Lost River and Birch Creek, the water makes its way through layers of porous basalt and gravel that serve as a giant sponge absorbing and transporting these rivers underground about 130 miles to the southwest. The waters eventually surface at Thousand Springs on the Snake River in Hagerman Valley, southwest of Twin Falls, Idaho.

Geology of the Big Lost River

Last updated: 02-05-2013