"Imagine a fairy chain stretched from mountain peak to mountain peak, as far as the eye could reach, and paid out until it reached the 'high places' of the earth at a number of ridges, banks, and knowls. Then visualise a mound, circular earthwork, or clump of trees, planted on these high points, and in low points in the valley other mounds ringed around with water to be seen from a distance. Then great standing stones brought to mark the way at intervals, and on a bank leading up to a mountain ridge or down to a ford the track cut deep so as to form a guiding notch on the skyline as you come up.... Here and there, at two ends of the way, a beacon fire used to lay out the track. With ponds dug on the line, or streams banked up into 'flashes' to form reflecting points on the beacon track so that it might be checked when at least once a year the beacon was fired on the traditional day. All these works exactly on the sighting line."
Alfred Watkins, The Old Straight Track, 1925