The city of Great Falls 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 city lies at the southern reach of the Laurentide Ice Sheet, a vast glacial sheet of ice which covered much of North America during the last glacial period. Approximately 1.5 million years ago, the Missouri River flowed northward into a terminal lake. The Laurentide Ice Sheet pushed the river southward. Between 15,000 BCE 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 River flowed eastward around the glacial mass, settling into its present course. As the ice retreated, meltwater from Glacial Lake Great Falls poured through the Highwood Mountains and eroded the mile-long, Shonkin Sag, one of the most famous prehistoric meltwater channels in the world.