Concretion Near Billings, Montana

July 29, 2008


Provided by: Rod Benson, Helena High School
Summary Author and Editor: Rod Benson

The broken sphere of sandstone is a "concretion" that was once embedded in the Eagle Formation laid down during the Upper Cretaceous period between 100 million and 65 million years ago. Eventually, the less durable sandstone that serves as its pedestal will erode away and the sphere will tumble down. The Eagle Formation is composed of sands that were deposited when this area was near the shore of the ancient Western Interior Seaway, which stretched from the Arctic to the Gulf of Mexico.

Apparently, a concretion forms as an iron compound starts to be deposited around some sort of nucleus within sandstone. What serves as this nucleus is not well understood. As the deposition of iron grows around this nucleus, a concretion forms a spherical area within a layer of sandstone. The concretion is quite resistant since it has been cemented with hematite, an iron compound. Photo taken on March 23, 2008 near Billings, Montana.

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