I have always assumed the avenues were heading down to the river, though that only works if the path of the river has moved.
I agree with Rhiannon about a path from the cove to the 3rd circle, however I suspect the farmer would disagree for the same reason that there is a sign at the gate leading into the main site from the 3rd circle saying "Circle visitors only". The way in by the car park has an honesty box on it, so ppl coming to the circles via the imaginary footpath (as most would) won't pay...