Peter,
Looking at my Ordnance Survey map (Explorer 130, order one from your local bookshop) I find that the end of the straight part of the Avenue is on an 85M OD contour line. This marks the end of a blind valley ending at Lake about 3 Km to the south. These dry valleys are very very common all over the chalk landscapes of S England and only flowed with large amounts of water during and just after the last Ice Age. Now, and during the construction of Stonehenge, there are/were no rivers on Salisbury Plain (The Avon skirts the E edge) only small streams which dry up during the summer. Any rain that falls sinks quickly down to the water table.
Jim.