Cannon Ball Concretions
During the Paleocene, the sea receded from the State of North Dakota leaving sediment deposits consisting of sandstones and mudstones. Which coagulated into perfectly round cannonball like formations during the eroding water movement. Fossils found in these cannonball concretion rocks leave a history of the animals, minerals, and plants that inhabited the sea during this period.
You can find Cannonball Concretions in the North Unit of Roosevelt National Park along the Little Missouri River that runs through the park.
Don't forget to say hello to the prairie dogs while you're searching for cannonballs.