Rough River Lake lies within the “Clifty” area of Pennyroyal Region of Kentucky. The term “Pennyroyal” or “Pennyrile,” used interchangeably, denoted an area exceeding 11,000 square miles. The name derived from a plant of the mint family which grows profusely throughout the area.
The Clifty area comprises the western edge of the Pennyroyal layers have been assigned by geologist to the Chester Series of Late Mississippian Geologic Age, a period estimated to have existed about 300 million years ago.
An abundance of water in the area, both surface and underground, has caused solutioning in the limestone and as a result, created a myriad of sinks and caverns. The most important cavern in the area is Mammoth Cave, the longest cave system in the world with more than 240 miles of mapped passageways. The Pennyroyal plain, extending from northern Tennessee through Kentucky to central Indiana, is an outstanding example of “Karst” (sinkhole) topography and is the most widespread topographic feature of this type in the United States.