Have you ever seen a stone doorstep outside an old house, worn to a hollow in the centre? Have you gone for a country walk, and followed a path which has been made by the feet of people and animals taking the easiest route through the grass between two points? Did you notice that the bare earth was lower than the grass on either side? How about the area under your children’s swing? Unless you have laid a special surface, I bet there is a neat little groove in the ground under the seat where they scuff their feet. If you visit a prehistoric stone circle, you may not notice the same effect in action, but it is there all the same.

I took part in an interesting piece of archaeology when I was working in Nottinghamshire. There was a small stone circle, like a miniature Stonehenge, on a hillside in the Peak District some way from Nottingham. It was on a piece of land to which the public had access. Walkers, picnickers and ad-hoc campers wandered around the stones and lit the odd fire in the centre. Every year, the man for whom I was working was paid to come up and monitor the monument. Using a surveyor’s level, he had to lay out a grid inside and around the circle of stones and measure the height above sea level of the ground at the same points on that grid each year. My exciting job was to hold the measuring staff on the various points while he took the readings. (This was of course before GPS.) And every year, the ground level had been worn down over most of the area by one or two centimetres. The damage was imperceptible to the naked eye, but in ten years, that prehistoric monument would have been worn away by ten to twenty centimetres, just by people walking on it.

And that is why we are not allowed to walk around inside the stone circle at Stonehenge or the Ring of Brodgar, all thousands and thousands of us tourists who visit them every year.