Approximately 1.5 million years ago, the Missouri River, the Yellowstone River, and Musselshell River all flowed northward into a terminal lake. During the last glacial period, the Laurentide and Cordilleran ice sheets pushed these rivers southward into their present courses. The Charles M. Russell National Wildlife Refuge lies atop these glaciated plains. The glaciers scoured extensive amounts of alluvial deposits from the area, and caused significant erosion of the areas around the Missouri River. These factors left the Russell National Wildlife Refuge rich in readily exposed and recovered fossilized plants and animals.