By: Steve Roark
Volunteer Interpreter, Cumberland Gap National Historical Park

The geology of our area is unique in that it creates two worlds: a surface world and an underworld of caves, water, and stone. The type of terrain we live on is called "karst" and is characterized by rocky ground, caves, and sinkholes, underground streams, and areas where surface streams disappear into the ground. This type of terrain is the result of the eroding effects of underground water on limestone.

Here’s what’s needed to form a cave: A thick layer of dense limestone near the surface, plenty of rainfall, good groundwater circulation, and eons of time. Limestone (calcium carbonate) dissolves fairly easy in slightly acidic water, which rainwater naturally is. Rainwater percolates along cracks in the limestone, dissolving it slowly and carrying it away. When a large enough crack forms that allows water to flow, the erosion process speeds up and the cracks get wider and deeper until they form cave systems or underground stream channels. Where these stream channels return water to the surface, you have what are called springs. A good clean spring is greatly appreciated by those wanting a good cold drink out of the heart of a mountain, so much so it’s even bottled and sold. When a cave becomes large enough, its roof sometimes collapses near the surface and forms a depression called a sinkhole.

Caves, springs, and sinkholes are all over our area, and many are connected together in a complex array of underground tunnels, cracks, and channels. It makes our area unique, but fragile. These underground water systems are easily polluted from the surface because rainwater enters them so easily. Gasoline leaking from a storage tank or even old cans of paint can seep through the ground into an underground stream and be carried thousands of feet to a well and contaminate the drinking water.

One of the worst ways to poison drinking water is to dump garbage into sinkholes, where rain can carry toxic material into the underground water system. There are a lot of toxins in garbage: mercury and lead from old car and flashlight batteries, acids and poisons from cleaners, furniture polish, paint cans, varnish cans, and hundreds of other products that contain harmful substances. In some cases, only a few parts per million will be enough to cause health problems.

There are other pollution problems of course, such as improperly installed septic systems, improper disposal of pesticides, etc. Pollution is a problem all over the planet, but because of our underground terrain full of holes and channels, water pollution here is a greater concern because it can be carried underground quickly and over great distances. We must be even more careful to see that we do not contaminate our water.