The Missouri River lies atop the Great Falls Tectonic Zone, an intracontinental shear zone between two geologic provinces of basement rock of the Archean period which form part of the North American continent, the Hearne province and Wyoming province. Approximately 1.5 million years ago, the Missouri River, 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 lakes and rivers southward. Between 15,000 and 11,000 BCE, the Laurentide Ice Sheet blocked the Missouri River and created Glacial Lake Great Falls. About 13,000 BCE, as the glacier retreated, Glacial Lake Great Falls emptied catastrophically in a glacial lake outburst flood. The current course of the Missouri River essentially marks the southern boundary of the Laurentide Ice Sheet. The Missouri, Yellowstone and Musselshell rivers flowed eastward around the glacial mass, eventually settling into their present courses. As the ice retreated, meltwater poured through the Highwood Mountains and eroded the mile-long, Shonkin Sag—one of the most famous prehistoric meltwater channels in the world.