Our articles this month have been about the very ground beneath our feet – and very interesting ground it is!
Due to the layers of soft limestone close to the surface and the layers of harder sandstone beneath them, we have some unique physical geography here in Middle Tennessee.
One of the most unusual features of our landscape is the prevalence of sinkholes, sometimes called “swallow holes” by the pioneers.
If you have never seen one, then you haven’t lived here very long for they are everywhere! But if you haven’t, let me describe one for you. A sinkhole is a big hole in the ground that appears to get smaller the further down you go, and then it just disappears.
The ground around a sinkhole will be rocky and when it rains, the sinkhole will look like the drain in a bathtub as water disappears into the hole.
Middle Tennessee is full of sinkholes, as is much of central Kentucky, but they can also be found worldwide.
And you don’t want to fall down into one!
But you can grab a flashlight, rope or ladder, and you can do a little exploring – that is, if you are adventurous – because most sinkholes will have a connection to a cave.
Let’s back up a little first. Why do we have sinkholes and how do they form?
Going back into the history of Middle Tennessee, when we were covered by water for millions of years, the oceans receded and the land rose, leaving layers of hard sandstone under layers of softer limestone.
In what is a very slow process, rain in the atmosphere picks up carbon dioxide, creating a mild carbonic acid. This acid gets stronger as it passes through the soil, where it collects more carbon dioxide. The water now gains the power to dissolve and eat away at the layer of limestone.
Starting with cracks in the earth’s surface, the water works its way down to the harder sandstone layers and then stops. It has to go somewhere, so it creates underground streams or rivers, while always eating away at the limestone.
Eventually those underground rivers become caves, but we will talk about them next week.
The cracks in the surface become bigger and as the process of wearing away works, the cracks become holes – sinkholes!
I’ve explored many sinkholes over the years and almost always find a cave.
One of the largest sinkholes I have explored was in the Cato area. It had been used by the family who lived closest to it as a trash dump. In fact, for years people have seen sinkholes as a good way to get rid of old appliances, dead cats and dogs, rusty metal and other garbage.
By the way, this is not the recommended way to use a sinkhole, as it can pollute the local groundwater!
I was joined by a small contingent of Boy Scouts, armed with an array of flashlights and lanterns that would make any spelunker proud. We clambered down through a century of trash to the bottom of the sinkhole, which ended in a large cave.
The floor of the cave was wet – no surprise – and muddy. But we enjoyed our exploration as we searched every nook and cranny, and emerged a few hours later, covered in mud and proud of our efforts.
That sinkhole, by the way, later made the news when its owner turned the cave into the famous “Tennessee Pot Cave.”
Most sinkholes are not so famous.
Sinkholes can be created in soft soils, when rains wash away huge areas underground and then collapse when a heavy person or object goes over it.
Back in the 1880s, a farmer in Smith County who was plowing bottom land by the Cumberland River saw his team of mules and plow just disappear right in front of his eyes! The team had trod over a sinkhole, causing it to collapse from their combined weight. Fortunately, the farmer was able to extract them after a few hours of digging and tugging!