Great Falls (Missouri River) - Geological History

Geological History

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, 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 lakes and rivers 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, Yellowstone, and Musselshell rivers flowed eastward around the glacial mass, eventually settling into their present courses. As the ice retreated, meltwater poured through Highwood Mountains and eroded the mile-long, 500-foot-deep (150 m) Shonkin Sag—one of the most famous prehistoric meltwater channels in the world.

The Great Falls themselves formed on a fall line unconformity in the Great Falls Tectonic Zone. The Missouri River settled into a bedrock canyon which lay beneath the clay laid down by Glacial Lake Great Falls. The course of the Missouri in and around the Great Falls has changed very little since then, in comparison to lower regions of the river on the ground moraine that forms much of the upper Great Plains.

The Great Falls of the Missouri River formed because the Missouri is flowing over and through the Kootenai Formation, a mostly nonmarine sandstone laid down by rivers, glaciers, and lakes in the past. Some of the Kootenai Formation is marine, however, laid down by shallow seas. The river is eating away at the softer nonmarine sandstone, with the harder rock forming the falls themselves. Until relatively recently (in geologic time) the Missouri River in the area had a much wider channel, but it has now settled into its current course, where it will continue to cut more deeply into the sandstone.