Grew up summers in the Catskill Mts. These walls are very common sight. Was told as a kid the olden times farmers cleared their lands while building these walls with the cleared stones. But after years of hiking and camping in this area [Hunter/Tannersvill/Greene county] I've seen these walls all over the place in grids, deep in the woods where there was never any settlements. The walls themselves are simple enough for even a single person to creat a section of, but the problem is how vast a network of them there are, with no apparent ryme or reason for them or their pattern (at least as can be seen from ground level - and really cant be see from air due to all the trees - a practical jungle forest). Most of these walled in areas of the 'grid' have no entrances like if they were pens for animals. You have to climb over the walls to get into any of the individual grid boxes, boxes and all irregular shapes and sizes of no sense in what 'they' were dividing up. In any case it would have been a great undertaking that I can't see any Europeans doing up in those mountains and no history of there ever having been enough people living in these mountains. An enigma that the locals just pass off as non existent farmers clearing their fields.
I also know of an old quarry in the area of Tannersville. On the old stone (ancient?) road leading to the quarry there is a spot where I noticed what you would describe as maybe natural maybe not, it's so imbedded in the mnt side, but now after watching some of your vids im sure if excavated a bit it will show ancient man made. And at the quarry itself (tiny)is stones with those rounded half holes on the sides of a stone quarrying marks exactly as I've seen in some of your vids. Now this could be also same quarry techniques used back in the 1920s too. I'm no expert. But when I think of all these crazy walls and after seeing your vids I'm wondering if that lil quarry wasn't in fact an ancient one.