Sorry LS didn't reply to your Alphamstone etc query . The Alphamstone marking is not in the usual cup and ring tradition if man made it looks like it was tooled . It's use and those of other naturally striking stones ammonite at Stoney Littleton etc is intriguing and an important component of understanding the choice of materials at monuments ,little doubt it is intentional but in artistic terms it's the difference between Duchamp and Da Vinci . One is found art and the other man made and whilst we may conflate the two in studies of art in archaeology there is an important distinction .
Another example you might have mentioned is the use of antural markings within genuine rock art ,look at how the man made markings are enclosed in natural markings . http://www.themodernantiquarian.com/site/6641/craig_hill.html .This is not uncommon and the questiion is did the engravers see these natural markings as convenient frames or whether they maybe perceived as earlier man made markings .