The Mountains

The ranges of the Rockies have different geologic histories and compositions. The Boulder Batholith, a mass of intrusive granite between Butte [1] and Helena [2], is filled with mineral veins, especially copper and silver. The Absarokas and the Gallatin Range in south-central Montana are blanketed with volcanic rocks from eruptions in and around Yellowstone National Park.

In the far northwestern part of the state, glaciers shrouded the mountains, leaving them relatively rolling and softly shaped. Where glaciers ran down from high cirques into mountain valleys, there is a characteristic straightening of the valley’s path and a U shape to its floor, unlike the V-shaped river valleys. Glaciers also pushed along gravel and soil; these moraines are still visible to the geologically savvy.

The isolated mountain ranges of central Montana are not part of the Rockies. Many of them were formed at the end of the Mesozoic era, about 20 million years after the Rockies rose, when molten granite shot up from the depths of the earth and bowed up the overlying sedimentary formations. Other areas of central Montana were lifted along faults to form high plateaus and buttes.