Montana Earth Science Picture of the Week

This photo, taken by David Lopez of the Montana Bureau of Mines and Geology, was featured on the bureau’s 2004 Montana Geology Calendar. It shows unusual formations called “hoodoos” that are located several miles west of Pryor along the western edge of the Crow Indian Reservation. Hoodoos are pinnacles or pillars of rock that formed when a layer of rock more resistant to erosion overlies a layer of less durable rock. These hoodoos were formed because the tough layer of reddish sandstone cemented with iron oxide capped a layer of sandstone (tan) that was not so well-cemented. The cap of tougher sandstone caused weathering and erosion to cut downward along fractures in the formation. helping to preserve the pillars.

Which way did it flow? . . .
That Greybull Sandstone that forms these hoodoos was deposited as sands in an ancient river channel about 100 million years ago. Geologists have traced over 60 miles of the ancient river channel on the surface in the area between Lodge Grass and Fromberg. In most places the Greybull Sandstone exhibits excellent cross-bedding, a characteristic that can be used to determine which way the ancient river flowed. Since the cross-bedding (shown in photo) forms along dunes and ripples that migrate downstream, geologists can use them to determine which way the river was flowing. Cross-bedding in the Greybull channel suggests that the river flowed westward before turning northward about 15 miles west of Pryor.

Diagram courtesy of U.S.G.S.

Term: (river) channel