Stones in the south-east quadrant at Avebury. Notice any marks? 
Image credit and © moss

Image credit and © moss

Sheep rubbing up against the stones have left a dark stain at the base of each one, the result no doubt of lanolin in the fleeces. The reason the sheep are in the Henge is to keep the grass short – an environmentally friendly way of doing things although the downside is the (hopefully harmless) deposit on each of the stones… and the fact that sitting down and having a picnic can be a hazardous experience!