What's That in Kansas

Rock City Canonball Concretions

Cannonball formations in Rock City Park, Kansas
Picture from: Nationalparks  

These roughly round boulders are called "cannonball formations," although they'd be mighty big cannonballs.

They're in Rock City, a park just south of Minneapolis, Kansas

They're made of sandstone that's been cemented together by calcite.  Geologists call them calcite-cemented concretions.  Concretions are sedimentary rocks cemented together by a mineral (like calcite).

Here's what happened. This was originally a layer of Dakota sandstone.  It was porous, so water could flow through it fairly easily through spaces between the individual grains of sand that make up the sandstone.  Some of the water had calcite dissolved in it.

Calcite, you may know, is used to make cement because it can bind together grains of sand and gravel.  As the calcite-containing water was flowing through the Dakota sandstone, something caused the calcite to precipitate out.  It could have been a change in the water temperature, or it's chemical composition.

As the calcite precipitated, it cemented the sand grains around it together.  This created spherical areas of tightly-cemented sandstone within the layer of less-well-cemented sandstone.

When erosion started, it wore away the regular Dakota sandstone faster than the parts that were cemented with calcite.  Eventually, the sandstone on top of, on the sides of, and on the bottom of the concretions wore away, leaving cannonball concretions looking like some strange rocks that rolled onto the landscape.