Round here cairns often mark the way, reassuring you that you're on the right path or at the summit. This one is at the top of Beamsley Beacon not far from where we live. The triangulation point in this photo marks the highest point of the hill, but as the walkers leave their stones, the highest point in fact becomes higher than the official record.
There's something interesting about the way these cairnes reflect the many people who have been here at the top and the instinct to change the environment or leave some kind of message.
It reminds me of the idea that you can't observe something without changing it.