There is erosion by many 'agencies'. Caves are formed by water movement, slowly, yet the hypothesised weathered grooves would likely be formed by freeze-thaw - which you wouldn't expect to see an enormous amount of in a cave. It's much easier to suppose that our ancestors were carving these gashes into stones, which were then set vertically, for a good symbolic reason. There's one of them among the Far House stones. One face is covered with cupmarks, two with grooves and the fourth with a teardrop-shaped figure. Although there are several teardrop-shaped cupmarks in the parish the archaeologists consider them natural erosion. They have no problem with the hundred and fifty simple 'ordinary' cupmarks on the other side of the stone, but the teardrop hollow is 'erosion'. Just daftness.