Warren Brook in Upton

Taken from the Upton Trail Maps for Warren Brook:

Another place I have been before, this time I entered via Oak Knoll Ln. Stayed left off the trail and then stayed left again when I was drawn downhill, to the lower wetter area. Somewhere in there - a rectangle with a hollow:

But onward: I was walking along a wall looking across, at "A", when I saw some piles on the other side. It was a site, with triangular piles and others, somewhat like a marker pile site. This place was enclosed with stone walls and there were two well formed entrances near the top of the site. 
(A)Entrance 1:

(A)Entrance 2:

I'll come back to showing some pictures of the place but what seems quite interesting is that, later in the walk, I came to another break in a wall (at "B1") and had the hunch to look for rock piles beyond and it worked. With one significant difference: this break was poorly made, obviously just pushed through the existing wall. So [bear with me] if the wall is of the same age in both places then the first site pre-dates the wall and the second one post-dates it.
(B1) Entrance, looking back from inside the site:

See the difference? 
To exaggerate: here some piles from site "A":

and here are some from "B2":

Something of an entirely different sort from what was at "A". 
Anyway, while talking generalities about "A" and the "B"s, let me mention that in the past I found a really nice site in there that I did not see on this walk (second time I have failed to relocate it) but I think it is downstream from "B", near where I put the "?".
[Still have too much to blog about: the details of these places in Upton and of the gully at Kezar hill.]