Anaconda-Pintler and Flint Creek Mountains

Montana Highway 1, MP 1, Anaconda rest area

The mountains surrounding this valley began to form more than 100 million years ago when tectonic forces compressed the earth’s crust and forced layers of underlying sedimentary rock eastward along great thrust faults. The faults stacked flat slices of rocks on top of one another to form high mountains similar to the Andes or Himalayas today. Molten rock was injected beneath the surface and cooled to form large masses of granite, called the Boulder Batholith, that are now exposed on the east side of the Deer Lodge Valley. Then, about 50 million years ago, the earth’s crust in this region began to be pulled apart. The crustal rocks were broken and the Anaconda-Pintler and Flint Creek Mountains to your west were separated from the Deer Lodge Valley by a gently east-sloping normal fault called the Anaconda detachment fault that forms the gently sloping mountain front on the west side of this valley.

Rocks that form the mountains are hard, resistant metamorphic and igneous rocks brought up from deep in the earth’s crust. The sedimentary rocks that formerly lay on top of those in the mountains slid eastward and downward along the detachment fault and now lie buried beneath the Deer Lodge Valley. As the valley dropped and the mountains rose, the valley filled with thousands of feet of younger material derived from the eroding mountains. Although the Anaconda detachment fault is no longer active, other similar, but steeper, normal faults are, and so the mountains may still be growing.