Geology major of long ago: opinion - Native American grinding holes. When holes are eroded into rock, it's more likely to be the kinds of rock which are mostly grains cemented together with some kind of calcium; the rock in the picture looks like a metamorphic rock, normally a tough rock which wouldn't wear away much. And holes from natural rainfall happen more frequently on a REALLY flat surface - this picture makes it look like the water would have drained off, instead of sitting in a puddle and softening the rock. That's the normal start of the process - a very slow process - which will result in those eroded holes.
Rocks like Mormon Rocks are full of holes that formed on nearly vertical surfaces, or even underneath an overhang, but the erosion was a little different there. In those cases, the rock was likely not as hard/cemented, and the main agent of erosion is wind-driven water and wet sand, frost expansion from water that seeped in, or small burrowing creatures. Those start as small holes and may eventually widen to form arches with a hole in the center, like Rainbow Arch at Lake Powell; that's also a sandstone formation.
It takes a lot of patience to make that many grinding holes in a big slab of metamorphic rock; my hat is off the the ancients!
