Montana Earth Science Picture of the Week

Hematite is a better glue

This photo was taken in the White Cliffs region of the Missouri River, 25 miles east of Fort Benton. The layers of light colored sandstone found in this area are known as the Eagle Formation. The sandstone formations in this area owe their 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, throughout most of the sandstone it was calcite, the same mineral that seashells are made of. Calcite is not particularly strong, so the Eagle formation erodes quite easily.

The unusual toadstool-shaped formation in the photo is known as a “pedestal rock.” The light colored stem is composed of easily weathered Eagle sandstone. The darker capping segment is also Eagle sandstone, but its 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, better able to withstand dissolving by rainwater. Because of the capping segment was cemented with hematite, it helped this rock survive the erosion that removed the surrounding sandstone. The capping segment has also preserved the weaker stem of stone beneath it.

Source: Monahan, Glenn and Chanler Biggs. Montana’s Wild and Scenic Upper Missouri River. Missoula, Montana: Northern Rocky Mountain Books, 1997.

Terms: Cretaceous Period, hematite

HOT LINKS

	More about the book (see source above)
	*More about the shallow sea
	*More about the White Cliffs area
	List of past picturres of the week
	Next picture of the week

By Rod Benson
Earth Science Teacher at Helena High School