By John Oliver, President, Historical Society

This month we are looking at the ground beneath our feet!

We have looked at dirt and talked about ancient seas that once covered what is now Middle Tennessee. This week we talk about some of the fossils that we find here.

“Indian money” and geodes are two of the most common seabed fossils found in Trousdale County. These are from the author’s personal collection.

You can find Indian money in just about any pile of creek gravel. The pieces will be brown or light grey in color and look like a small stack of coins piled together. Sometimes you can find an individual “coin” and it will have a hole in the center.

The correct name for these fossils is not Indian money. Instead they are “crinoids” and you are picking up a real piece of history when you hold one in your hand.

Remember that all of Middle Tennessee was once covered by ocean. So some 320 million years ago, during the Upper Mississippian epoch, small ocean plants lived and did by the billions.

As these plants died and were covered up by sediments of mud and sand, they fossilized.

What we pick up (see our photo this week) are the fossilized plant stems of the crinoid plant.

Crinoids had cup-shaped ends and had food-filtering arms as well. They were of a softer tissue and rarely survive the fossil process.

Trousdale County is hardly the only place to find crinoids though.

They are plentiful in parts of Great Britain, where they are called “Saint Cuthbert’s beads.” Poor people in the past would take the small fossils and drill holes in them to make rosary beads.

When asked where Saint Cuthbert’s beads came from, the people in rural England would say they were left on the ground by ancient snakes, or were the “devil’s toes!”

Despite my being told as a child that crinoids were Indian money, Indians in fact did not use them for money.

Other fossils we find from the millions of years that we were covered by ocean water are geodes!

A geode will look like a small fossilized brain and can be of any size! Like crinoids, they are found in creek beds or gravel piles.

A geode (see photo) is especially interesting for what is inside!

They are created when an air bubble or small cavity is formed in the sediment of the ocean floor. They can also be formed by gas bubbles in lava flows.

A shell forms around the air bubble as the surrounding sediment hardens. Water with silica will seep through the shell and into the hollow center.

Again, the process is millions of years long, but after the land rose and the oceans receded and the bedrock weathered away, the geodes were left exposed.

Now back to what is inside a geode – shiny quartz crystals.

I have seen people place large broken and unbroken geode in their gardens as accent pieces. They are sometimes used in garden walls, and I have actually seen a house built with geodes instead of bricks!

Both geodes and crinoids depend upon silica in the ocean water and thousands of years to develop. Middle Tennessee had the silica-enriched water and the right amount of time to create these fossils, and they are just another way that Trousdale County is unique!