Overthrust Geology in the Trilobite Range

Overthrust geology in the Bob Marshall Country taken atop of Pentagon Mountain

Overthrust geology in the Bob Marshall Country taken atop of Pentagon Mountain (Photo Rick and Susie Graetz)

The Trilobite Range is a textbook example Overthrust geology. Countless millions of years ago, as the Northern Rockies were building, slabs of Precambrian (the oldest classification of rocks) the Cambrian sedimentary limestone rose and was pushed eastward over the much younger Cretaceous sedimentary rocks of the Montana prairie. This process is known as geologic thrust faulting. Interpretive signs refer to this region as the Lewis Overthrust. In the simplest of terms, older rocks are now situated above younger ones.

This image shows the many north-south trending ridges in this formation that display the distinct sheer eastern face juxtaposed with a gently dipping west slope.

The Trilobite Range is a part of the headwaters of the Middle Fork of the Flathead River.