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.
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