I guess we all add to erosion, even by touching the stones ,but we can hardly beat ourselves or others up about it, just be a bit careful.

Here's a site report from, mainly, the Whispering Knights, on a single day in 1999. Scary to think this sort of thing accumulates month after month -

Fire: Yes - a ring of scorched grass left by druids banking the fire dish too high, having it burning for too long and not using any insulation in the bottom to prevent too much heat getting through.
Damaged Lichen: Yes
Chipped bits: Yes - no fewer than 11 new chips at the Whispering Knights, one of which appeared to have been hacked off with a bladed instrument, to add to the three new chips I found at the King's Men the previous week
Objects: Yes - coins, small stones, plant matter wedged into the stones
Flowers: Yes - local buttercups
Disturbed soil: Yes
Picnic remains: Yes
Cigarette ends: Yes
Other marks: Yes - runes and lines scored into the surface of the rock and the lichen, notably the capstone, at the Whispering Knights
Ritual remains: Yes. Freshly spilled blood (not more than a couple of hours old) on the Sunday morning, next to a blood soaked thing made of thistle.
Action taken: Tidied up, commiserated and sympathised