Concretions Decorate the Grounds of the MetraPark

January 18, 2018


Photographer: Rod Benson 
Summary Author: Rod Benson 

The photo above was snapped from the Rimrocks, looking across Main Street in Billings, Montana at the MetraPark arena, a popular venue for state sports tournaments, concerts, rodeos, etc. As crews began preparing the site for construction in the mid-1970s, they came upon several unusual, large sandstone spheres, called concretions. Instead of breaking them up, a decision was made to incorporate them into the landscaping.

In order for sand to become stone, the grains of sand must be cemented together. Minerals dissolved in groundwater typically do the trick. Calcite (calcium carbonate) is the most common cement for sandstone. But, since calcite can be dissolved by rainwater, sandstone is eroded away fairly easily. However, with the sandstone that concretions are made of the cement is not calcite, but rather a more durable iron compound.

Concretions form when iron carbonate dissolved in groundwater moves through buried sediment. They typically take shape when a mineral precipitates and cements sediment around a nucleus, which in most cases is organic, such as a leaf or piece of shell. The mineral actually precipitates in the pore spaces among the grains of sand. The precipitation may begin around a particular grain or fragment in the rock. As the deposition of iron compound grows around this nucleus, a spherical mass forms. Note that as the iron-cemented rock is exposed to air the iron oxidizes, giving the concretion a rust-color that contrasts with the lighter-colored sandstone it's embedded in. Even when the weaker sandstone around it wears away, the cemented sphere resists erosion. Photo taken in April 2011.

Photo Details: Camera: KONICA MINOLTA DiMAGE Z6; Exposure Time: 0.0080s (1/125); Aperture: ƒ/8.0; ISO equivalent: 50; Focal Length: 31.0mm.