Ah! the Cove.
Aubrey made a drawing of it but Stukeley believed he got it wrong.
However Stukeley did several sketches which included a Cove half way down the Kennet avenue before finaly giving up on the idea.
Recently Burl has revived the idea in his updated Prehistoric Avebury book.

There is a curious level platform which Could have supported a Cove where the avenue crosses the road.
This spot has interesting astronomical alignments with the moon which mirror those of the Beckhampton Cove.

I have some friends who are setting up a geophysics company, I'll see if I can talk them into going over the site sometime when they are in the area.
PeteG