Quoting u/agssiz95 at the other place:

Geomorphologist working on a PhD near a place with these. Here is my take.

These are cannonball concretions:

You have water flowing through sedimentary rock. The water will alter the rock by precipitating additional minerals making the altered rock more resistant to weathering and erosion. Eventually this weathering/erosion resistant rock is exposed at the surface when the surrounding less resistant rock weathers and erodes away.

The spherical form comes from the way the minerals precipitate and spheroidal weathering. The minerals precipitate outward from a single point forming a rough sphere shape in-situ. When exposed at the surface, nature doesn't like anything with points. The pointed ends of objects tend to weather and erode away faster than the spherical sections of the rock.