Looks pretty regular, maybe plough marks? A lot of unsuitable land was brought into cultivation during WWII and after, ans seeing as it's on the surface, it's quite likely.
When i was down at the Mothers Jam the other week, i was sitting atop what Cope calls (i think) the alter stone and thought i saw something similar right next (well 20 feet) away but when i got up close it looked like lichen or something, though there was deffo four parallel indentations (if slight).
Wishful thinking i guess.