Friends, these brown round rocks are concretions, a cemented sandstone that is found throughout the badlands.  The Romans made a cement from limestone, volcanic ash and clay.  These elements were all present in the early badlands and it is feasible that a natural cement was formed  from mixtures of limestone (calcium carbonate) ,clays and volcanic ash.  This cement concentrated in a softer sediment and the cannonballs were formed.  Now the softer clay layer is eroding and these concretions are falling out of the walls.

The most interesting are the cannon balls that are 2-3 feet in diameter that are present in the Northern Little Missouri Badlands. In the southern badlands,  smaller cannon balls are often being exposed on slopes of bentonite that has a popcorn texture when dry.  The bottom photo shows a cross section of a small cannonball that has a white nodule in the interior. til Tomorrow MJ