Geology

The limestone in Monroe City is part of a blanket of sediments deposited across much of the central United States during the geological time period known as the Mississippian. The Mississippian lasted from about 360 to 320 million years ago, which is about 75 million years before the dinosaurs evolved (dinosaurs have been extinct now for 65 million years).

During the Mississippian, Illinois lay south of the equator thanks to continental drift. Much of the central part of the continent was covered by vast seas that were warm and shallow and very similar to those found in tropical places like the Florida Keys and Bahamas of today. Like those regions, the Mississippian seafloor was carpeted by a multitude of different organisms that produced shells and other hard parts made of the chemical compound CaCO3 (calcium carbonate). As the organisms lived and died, their shells slowly accumulated over the millions of years and as they were buried ever deeper by other sediments, they were eventually converted from loose sediments into the type of sedimentary rock known as limestone.

Limestone is a fairly soft rock.  These layered formations make quarrying and cutting relatively easy.  

We find geodes by the score.

The waterfall with dozens of geodes visible in the rock.

Geode close up