Montana Earth Science Picture of the Week

All Glues Are NOT Created Equal

Photo by Jim Schulz

Sand from ancient sea . . .
This photo was taken in near the Missouri River, 40 miles east of Fort Benton (another photo). The layers of especially light colored sandstone found in this area are a part of the Eagle Formation, which owes its existence to sand deposited during the Cretaceous Period along the coast of a shallow sea. In order for the sand to become rock it had to be compressed and cemented. The weight of overlying layers (since been eroded away) provided the compression. As for the cement, most of the sandstone in this area is held together by calcite, which dissolves fairly easily in water. Consequently, the Eagle sandstone erodes quite easily compared to other rocks.

It helps to wear a cap . . .
The unusual toadstool-shaped formations in the photo are sometimes called "pedestal rocks." The light colored stems are composed of more easily weathered Eagle sandstone. The darker capping segments are also part of the Eagle Formation, however their grains were cemented with rust-colored hematite. Evidently at some locations, especially along the top of the Eagle Formation, layers of sand cemented by hematite are common. Compared to calcite, hematite is much stronger cement, and better able to withstand dissolving by rainwater. Because the capping layer was cemented with hematite, it helped these formations survive the erosion that wore down the surrounding sandstone. This darker, tougher, overlying sandstone shielded the weaker stems of sandstone beneath them, allowing the unusual shapes to form. CLICK HERE to see another photo.

A Montana Treasure . . .
Known as "The White Cliffs of the Missouri," the area between Coal Banks (near Big Sandy) and Judith Landing (near Winifred) is packed full of fascinating canyons and other formations caused by the erosion of sandstone. This 47-mile stretch is one of the most popular river trips in Montana, and it is even more interesting for those who have an appreciation of the area's geology, as well as its rich history. (photo)

Below: This photo was taken near Coal Banks. The one at the top was taken two days later, about 30 river miles downstream from the formation shown below.

Terms: lithification, Cretaceous Period

If you are interested in learning more about this area, this book is a good resource: Magnificent Journey: A Geologic River Trip with Lewis and Clark through the Upper Missouri River Breaks National Monument by Otto Schumacher and Lee Woodward


*More about the geology of the White Cliffs Area
*More about the shallow sea
*More about the White Cliffs area
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By Rod Benson
Earth Science Teacher at Helena High School