Much of this region was covered by a continental glacier that flowed out of Canada approximately 15,000 years ago. It moved as far south as the current Highway 2. By the time it reached this area it was thinning and didn’t do the carving that alpine glaciers do in the mountains. It did, however, move the channel of the Missouri and Milk Rivers. The Milk now occupies the old Missouri River Valley and the Missouri is now further south than it was then. The glacier also covered the area with rich glacial deposits creating the most extensive grain-growing region in this half of the state.
Badlands are found in pockets throughout this part of the state. This terrain is a desert environment created when fire or other disruption destroys the plant cover that would normally protect the surface from erosion. Rainwater hits the stripped surface and compacts it preventing new seeds from taking hold.
The runoff then forms gullies and begins to carve the strange-looking formations. Unusual and strange formations that look like mushrooms are scattered through these areas. They are sandstone that has been carved by the winds.
As you travel through the area, you’ll see hills that have a distinct red banding to them. Often referred to as scoria, their proper name is clinker. Clinker is red sandstone and shale that has had the misfortune of lying above a burning coal seam. The seam may have been ignited by prairie fire or lightning, and “cooked” the overlying formation. As the coal heated it to extreme temperatures, oxygen combined with iron in the rock-forming iron oxide produced the bands of red rust color visible in these hillsides. Clinker is tough stuff and is often used for resurfacing some of the eastern Montana roads.