Hi Gail! Thanks for the heads up. Cool specimen, but I suspect it’s a limestone concretion of some sort. Not a vertebrate specimen unfortunately, so no mosasaurs yet! LOL! The weird weathering pattern on the surface suggests there are fossils in it. Looks like corals again or possibly crinoids. Paleozoic most likely… brought in by the glaciers. I’d be interested in hearing the opinions of everyone else though!
Looks solid enough to remove and at least see what is on the other side. If you do decide to move it, please post some more pics. Gotta love a good mystery!
PS: I attached a more detailed SD geologic map for your records just in case you didn’t have it yet.