Concretions in the White Cliffs Region

Photo: NPS

The White Cliffs of the Missouri River in north-central Montana are known for their undisturbed, raw beauty. The cliffs, the coulees, and the dramatic formations are breathtaking.

One of the unique natural wonders found in the area are concretions. These somewhat spherical masses of sandstone are often found embedded in less durable sandstone. As weather breaks down the softer stone, the concretions appear, oftentimes breaking completely free because the “cement” of a concretion makes it harder and more resistant to erosion.

How do concretions form? It all has to do with the “cement” that holds sandstone together. Minerals dissolved in groundwater typically do the trick, according to the blog of Rod Benson, a former earth science teacher at Helena High School. Calcite (calcium carbonate) is the most common cement for sandstone, but the bonding agent for concretions is a much more durable iron compound. As iron carbonate dissolves in groundwater, it precipitates around a fragment in the rock, such as a leaf, shell, bone, or fossil. The iron compound grows around this nucleus and a concretion begins. As water and wind erode away the calcite-bonded sandstone, the concretions appear.

Concretions can be found in many locations around the world, and in many sizes. The one shown in today’s photo is about the size of a golf ball.