Moqui marbles in Utah. They seem to be the end-result of spherical weathering. Note their distribution along the joints in the sandstone. Looks like a good place to conduct a stoning.
http://i.livescience.com/images/i/000/070/309/original/gsa-cover.JPG?1411070934
Or getting stoned. 
That’s very weird. I never heard of them or saw them in my years in Utah, and I’ve hiked through many of those sandstone areas. The article (and others I checked online) didn’t give the locations of the Moqui beds.
The picture the OP posts shows circular voids in the bed of Moqui Marbles (Not all such pictures do – How Utah's Fantastical Moqui Marbles Formed | Live Science ), which looks similar tro circles caused by “frost sorting”: