Geology of Montana's Upper Missouri River

The spectacular and varied scenery that floaters see on the Upper Missouri National Wild and Scenic River is directlyrelated to the geology of this part of central Montana. The region’s dry climate, and lack of vegetation, provides excellentopportunities for floaters to view and learn about the geology as they progress down the river.

The rocks of the Upper Missouri region are primarily sedimentary rocks, ranging in age from 70-90 million years old, that were deposited during the Cretaceous period of Geologic Time. Central Montana was covered by an interior seaway that connected the Gulf of Mexico with the Arctic Ocean and which cut North America in half. Thousands of feet of sand, silt, clay, and lime were deposited in the bottom of the seaway by rivers which drained into the sea; eventually these layers of sediment hardened into the layered rocks of today. Dinosaurs roamed the shoreline, and marine animals inhabited the waters; fossils of many of these organisms, including dinosaurs, can be found along the river corridor. About 70 million years ago central Montana was uplifted, and the sea retreated.

About 55 million years ago, during the Tertiary Period, hot magma welled up from great depths, and pushed its way into cracks in the sedimentary rocks. Some of it reached the surface and produced volcanoes, but great quantities of the magma cooled underground in the network of cracks, and formed veins of igneous rocks, called dikes and sills.

The Upper Missouri River then cut down through the entire rock package, and, especially in the White Cliffs, has exposed a spectacular geologic wonderland of 300 foot high white sandstone cliffs that are laced with a complex network of 20 foot thick veins of dark-brown igneous rock.

In the Badlands, a combination of thick weak shale layers and alternating thin layers of strong sandstone combine to produce the hauntingly spectacular badlands which are locally referred to as "The Missouri Breaks".

Most of the rocks in the river’s corridor are quite soft, and succumb easily to erosion. In central Montana’s dry climate, this produces some fascinating scenery which is displayed along the entire length of the river, but particularly in the White Cliffs, and the Badlands. Unusual features such as sandstone arches and "gardens" of toadstool shaped rocks - called pedestal rocks, have awed travelers since Lewis and Clark passed through this area in 1805.

More recently the Upper Missouri River in Montana was impacted by the last Ice Age. Before the Ice Age, the Missouri flowed north through Canada and into Hudson’s Bay. But a massive glacier, flowing southward from Canada, pushed the Missouri River to the south and diverted its flow to the Gulf of Mexico, where it flows today.