The Anaconda-Pintler and Flint Creek Mountains

The mountains surrounding this valley began to form more than 100 million years ago when tectonic forces 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 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 pull-apart. As it did, the Anaconda-Pintler and Flint Creek Mountains to your west rose up from great depth along a gently east-sloping normal fault called the Anaconda detachment fault. It forms the gently sloping mountain front on the west side of this valley. Rocks from beneath this fault form the mountains. The rocks are hard, resistant metamorphic and igneous rocks brought up from deep in the earth’s crust. The rocks above and to the east of this fault are sedimentary rocks that formerly lay on top of those in the mountains; these rocks have been moved 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 Although the Anaconda detachment fault is no longer active, other similar, but steeper, normal faults are, and so the mountains may still be growing.