If you look on the Oldmap site at Avebury (1889) the river seems to fold back on itself here, or at least may have been enlarged for some reason; the pond? which could have meant dredging. This river after all is a winterbourne one, and the farm would have needed lots of water. Also the spring line, can be followed by the line of wells that appear from the cottages at the entrance to WKLB to the Manor Farm ......

That's the best aerial view of Silbaby I've seen so far; and I agree, it does seem to show a mound lying next to Waden Hill. Whenever I've been there the vegetation has been too thick to have a good mooch around it.

Another thing which bothered me was: the mound doesn't seem to be higher than the road; you walk straight from the road onto the mound, and the road is in a cutting on its northern side.

baz