I just don't know. I wish I did.
Oddly enough, there are many "cup marks" in the granite boulders on the ridge east of my house.
I have no idea if they're some kind of very odd [and very specific] type of erosion or what.
The area was absolutely thick with Indians, at one time so maybe they made them *but* I also have a greenstone hammer/ax/celt found near here that is referred to "archaic" in era.
It's a ground tool, not chipped or flaked like the usual Indian artifact, although there's plenty of chipped/flaked arrowheads around.
I often wonder if the ax/hammer didn't have more to do with the "Kennewick man" type of inhabitants than "Indians".
Be all of that as it may, the "cup marks" are always full of acorn and walnut shells because squirrels and chipmunks seem to love placing nuts in the "cup marks" and then crack them.
What does it "mean"?
Ain't gotta clue.....:)