Bearpaw Formation in Montana

January 14, 2022


Photographer: Rod Benson

Summary Author: Rod Benson

This photo was taken along an intermittent tributary of the Missouri River in the Missouri Breaks region of central Montana. The sediments exposed in the cut bank are part of the Bearpaw formation (aka Bearpaw shale). The silts and clays were deposited in the Western Interior Seaway, an inland sea that extended from the Arctic to the Gulf of Mexico during the mid to late Cretaceous period. The part of the sea that covered this part of Montana is sometimes called the Bearpaw Sea, named for the Bears Paw Mountains located 60 miles (100 km) northwest of here.

Over millions of years the sea advanced and retreated across the region, changing the location of the shoreline and the depth of the water. In the meantime, occasional volcanic eruptions to the west spewed ash that winds carried over the sea. The thicker light-colored layer in the photo is ash from one of those eruptions. A few thinner deposits of ash are also exposed in the cut bank. Volcanic ash deposited in seawater changes over time; weathering converts it into a clay material called bentonite. Although it looks like ash from a distance, it feels like a sticky clay that has little resemblance to the ancient ash that settled here.

In addition to bentonite (altered ash) the Bearpaw formation contains a variety of marine fossils and some dinosaurs. Geologists estimate that sediments of the Bearpaw formation were deposited between 75 and 72 million years ago. In places the formation is 1150 feet (350 meters) thick. Outcrops can be found in Montana as well as the Canadian provinces of Alberta and Saskatchewan. As the Western Interior Seaway retreated to the southwest, the Bearpaw shales were covered by deltaic and coastal plains sediments.

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