Through the last 100 million years, volcanism and glaciation formed the mountains we know today as the Boulder-White Clouds. From about 51 million years ago until about 40 million years ago volcanic activity known as Challis volcanics contributed to the formation of the Boulder-White Clouds. Challis volcanic rock rests on a mountainous surface of sedimentary rock more than 248 million years old. Basalt and rhyolite lava erupted during this 11 million year period of volcanic activity. Tuffs, or rock formed from volcanic ash of Challis volcanics, vary in color, including lavender, light green and several other pastel colors. Such rock is discernible in the eastern White Clouds’ tundra rising above 10,000 feet.
Along with rock formed from Challis volcanism, Idaho batholith granite and limestone are common throughout the Boulder-White Clouds. An eight-mile wide by 38-mile long highly mineralized belt runs north-south through the center of these two mountain ranges. It consists of colorful sedimentary rock such as argillite, quartzite, limestone and conglomerate rock.
A major Idaho fault, the Dillon Lineament, runs northeast from the Snake River Plain through the Boulder-White Clouds into Montana. The epicenter of the 1983 Borah Peak Earthquake lies on this fault.
Between 1.8 million and 11,000 years ago, glaciation shaped the Boulder-White
Clouds. The U-shaped valleys, saddle-like gaps between two peaks, sharp
ridges separating adjacent valleys, and polished, striated rock outcrops
are testimony of the glaciers that once occupied this area. These glaciers
also created most of the 120 lakes of the Boulder-White Clouds and formed
moraines, or conspicuous deposits of boulders, stones and other debris,
in the shape of a mound or ridge. Railroad ridge, west of the East Fork
of the Salmon River in the Boulder-White Clouds, is a 10,400-foot glacial
moraine with rare endemic alpine flowers and 900-year-old white bark pines.