Picked up a fossil in limestone on that drive. Turns out it is called a “Death Plate”. Just a hunk of the graveyard that forms when dead animals settle to the bottom.
The closeups show that it’s 
full of little critters that may have been on sea and land and must’ve been from a long, long time ago. There is some crinoid material along with pelecypods and gastropods.
This piece seems to be a part of the Madison Limestone formation (or Madison Group) that stretches from the Black Hills to western Montana and eastern Idaho and and from the Canadian border to western Colorado and the Grand Canyon of Arizona. Most of the Madison Limestones were deposited during Early to Middle Mississippian time (Tournaisian to Visean stages), about 359 to 326 million years ago. (info from Wikipedia)