Paleontology and geology
At the beginning of the Tertiary, Montana witnessed the final retreat of the sea. About 50 million years ago, a period of tectonic activity began, which folded and faulted the crust, created several mountain ranges, and elevated the state by several thousand meters. From 40-20 million years ago, dry climates prevailed, during which the mountains were eroded and sediments filled up much of the broad mountain valleys. Wet, tropical conditions marked the next 10 million years, and rivers carved down into the older valley fill. Then, beginning about 10 million years ago, a dry, desert-like climate developed over the state, and the modern high plains became established. Tertiary vegetation varied significantly as climates alternated between these wet and dry intervals. Large titanotheres, dogs, and other mammals roamed the landscape.