We're now on Chief Mountain Highway. Chief Mountain ahead: The peak is easily seen from Montana and Alberta due to the rapid 5,000 foot altitude gain over the Great Plains which are immediately east of the mountain. Despite the mountain's being in Montana, the distinctive eastern face quickly becomes hidden as one travels south into Montana, but remains very easy to see on clear days over much of southern Alberta. Hence, many businesses in the area (notably Lethbridge) have "Chief Mountain" in their name. Chief Mountain is one of the most photographed mountains in the region due to its unique nature.
Chief Mountain is an example of a klippe. It consists of a Precambrian block which rests directly above much younger Cretaceous gray shales. Having an older layer (Precambrian in this case) pushed up to the top is typical of thrust faults. The surrounding portion of the thrust sheet has been removed by erosion leaving behind this isolated block of Proterozoic rock.