Courtesy of: The Thomas Family

 

Hidden River Cave is located in Hart County, Kentucky, home to some of the longest caves in the world.  The Green River flows through the middle of Hart County and drains most of the surrounding countryside.  Most of the rainwater that falls here flows underground through sinkholes and caves and into the Green River by way of springs.

Five of the ten largest springs in Kentucky are in Hart County.  The water in Hidden River Cave, however, flows into the Green River through the largest spring in Kentucky.  The river that can be seen on the cave tour at Hidden River Cave follows winding cave passages for many miles before it finally flows into the Green River and reenters the daylight world.

Caves form in a terrain known as "karst".  Approximately 40% of Kentucky is composed of karst.  Karst areas are characterized by natural features such as caves, sinkholes, sinking streams, underground streams and other solutional features.  Rocks such as limestone, dolomite, gypsum or marble underlie most karstlands.  These rocks are called solutional rocks because they can be dissolved by acidic rainwater to form caves.

Fossil shell fragments found in limestone are evidence that the rock was formed by the accumulation of layer upon layer of sediment on a shallow sea floor.  These seas covered most of Kentucky more than 300 million years ago.  The accumulated layers of sediment eventually hardened to form the underlying bedrock.  Eventually, the sea receded and the land uplifted allowing the downward flow of rainwater to begin dissolving the bedrock to form caves.  Kentucky's groundwater has been constantly at work dissolving limestone for millions of years.

Hidden River Cave is much younger than the surrounding rock.  It is probably only a few million years old- a mere baby in geologic time!  In this time span, groundwater has had plenty of time to dissolve many miles of cave and some of the largest cave rooms in the United States.

The brachipod (pictured above), such as these found at Hidden River Cave, is Kentucky's State Fossil.