Alfred Watkins, Early British trackways, moats… (1922)
“ Where a mountain ridge stood in the path of a ley, the surveyor, instead of building a tump on the ridge as a sighting point, often cut a trench at the right angle and in the path of the ley. This shows as a notch against the sky and makes a most efficient sighting point from below. I have counted eight such artificial notches in the mountain ridge when on the road from Llanvihangel Crucorney to Longtown. Each notch can only be seen on the line of sight, and disappears when a quarter of a mile right or left. ”
