Oak Hill, Dover

For the most part I found the walk to Oak Hill to be dull: dry woods and muddy swamps. But I kept seeing outcrops that looked modified, with bits of stone wall connecting or extending them. I saw this propped boulder:

 I only photo'd a couple of rock piles.

These were both along the heights. Both have the shape of a "U" and are about 4 feet across:

I interpret this as an early form of "rectangle with hollow".