The chert gravel deposits of the lower Mississippi River valley were formed by glacial and alluvial processes, which tumbled and smoothed stones from all over the drainage of that great basin, from Montana to Minnesota to Pennsylvania and every place in between.
The fossils found in the chert gravels of the the lower Mississippy valley make great finds because they are chert that has been tumbled over the eons.
Chert is a microcrystalline silica mineral that forms when organic material (or their anhydride castings) are replaced by the action of groundwater over the ages. Chert is fairly hard and polishes nicely. The semiprecious gemstones such as agates and jaspers are examples of chert.
A good fossil in crumbly sandstone or limestone is nice and all but not nearly as nice a good fossil made of a nice hard gemstone that was polished by glaciers over eons.
Most of the fossils in the chert gravel deposits are from the Paleozoic Era and more than a quarter billion years old.
These fossils were already fossils before the dinosaurs of the Mesozoic Era evolved.
In the gravel deposits you can also find petrified wood and mammal fossils from much later times in the Cenozoic era.
The Mississippi Department of Environmental Quality has published a PDF guide for Rocks And Fossils Found In Mississippi’s Gravel Deposits
I found this batch of fossils recently in some gravel my mother had put down to form a short walkway beside her house in west Tennessee.
As a boy growing up in the Mississippi Delta, I loved to find fossils in chert gravels. The land itself was nothing but gumbo mud, and there were no gravel deposits or sandy creeks, but there were gravel roads and driveways.
I would walk miles down blacktop roads just to get to a place where new gravel had been spread at the head of a drive. I would often walk on the gravel road on top of the Mississippi River levee.
I remember one time I was walking in the semi-wild pasture land behind the levee, and I found a long muddy waterhole that was drying up, about fifty yards long, the deepest part of some old silted up bayou.
It was where the herds of cows came to drink, and the mud was gouged up with foot-deep hoof prints everywhere.
In the middle where the mud was still wet, hundreds of freshwater pearl mussels over 1 foot in length were coming out of the mud and dying in the heat.
There were hundreds more that were already dead, many rotting and filled with flies, others already clean shells with large white iridescent interiors.
The latter was as beautiful as shell gets, and more than thick enough to be cut into buttons. Even as a child I had good idea how unusually large these shells were and how much these would be worth as raw artisanal material or as natural specimens.
Still, I couldn’t bear to touch a one of the hundreds of extraordinary shells because the die off was so massive and depressing. Each of the shells had many decades of growth rings.
They were already being stomped to pieces by the cows struggling through the mud to get to the last of the water.
Sometimes I think of all the amazing things I saw as a boy playing in the countryside in Mississippi and wonder why I didn’t think to keep one of whatever it was, as if I would always be surrounded by random rattlesnake skins and owl skulls and the like.
Then I go look at the entire room of chert gravel fossils I have collected over a lifetime, and I see why I didn’t keep more.
It’s hard to resist the fossils. They follow me home.