Digging for coal and fossils
In the mid 1800/s some fossils were found in a tributary of the Illinois river, called “Mazon Creek.” They had simply weathered out of the ground, but it wasn’t until coal mining moved in, that the true extent and value of the Mazon Creek fossil deposit was discovered.
Here are some articles about the Mazon Creek fossils.
This last article is rather lengthy, and it talks about the increasing difficulty of finding Mazon Creek fossils.
And here’s an article about the Tully Monster, which has only been found in Mazon Creek.