Please have a look at the most recent image on the Low Thornhope page. It shows a fossil flaw from medium grit sandstone. Does the profile look familiar. It's half of a natural cupmark and the lower bit generally becomes broken in the separation along the grain. It is probably a hailstone pit that has been filled with sediment - but the ancients cannot have known that. So the cupmarks, on stones, are, or were originally, of a similar profile to Silbury Hill, and all its near relatives.