Montana Governor and First Lady's Math and Science Initiative
Lone Mountain
Mile Post 47 on US Highway 191 near Big Sky
Geologic processes have created a winter wonderland
for skiers and snow boarders on Lone Mountain,
the prominent peak that rises above Big Sky. Some
geologists think that if the mountain was cut in half, there
would be a Christmas tree pattern of igneous rock within layers
of sandstone and shale. Magma from deep in the earth rose
up along a vertical crack, then spread out laterally between
the layers of sedimentary rock. The magma never erupted as a
volcano, but crystallized into an igneous rock called dacite before
it reached the surface of the earth. The “trunk and branches
of the tree” are dacite. The igneous rock and adjacent baked
sedimentary rocks are much more resistant to erosion than
softer sedimentary rock, which has eroded away to leave the
mountain standing tall. Glaciers, landslides, and rock fall have
greatly modified Lone Mountain, sculpting bowl-shaped cirques
high on Lone Mountain. The cirques contain rock glaciers,
which are piles of rock with year-round ice in their cores. Like
ice glaciers, they slowly creep down the mountain slopes.
The rugged mountains to the northwest of Big Sky are the Spanish Peaks, composed of metamorphic rock that is billions of years older than the igneous and sedimentary rocks on Lone Mountain. These extremely old rocks were brought up from the depths along an enormous fault zone that extends from the Tobacco Root Mountains to the town of Gardiner north of Yellowstone Park.
A Brief History
The mountains provided a spectacular backdrop for
visitors to one of the Gallatin Canyon’s many dude ranches.
The ranches offered visitors adventure and a taste of the Old
West. They rode horses, hiked mountain trails, fished the area’s
trout streams, and reveled in the solitude of the mountains. The
ranches were immensely popular before World War II, but their
popularity diminished in the 1950s until now only a few remain.
