Lichen (Lecidella eleaochroma)
I always think that this lichen looks like a child has been drawing with a pencil on the bark of the tree (purely as my drawing ability is so poor and it looks my attempt at drawing but would not want anyone to know!). The lichen forms a crust on the bark of a tree and have blackish edges which merge together to form a larger mass giving the impression continuous wiggly lines. In the centre of each distinct area there are black specs, some more pronounced than others.
Pretty common in south western Britain it grows on all kinds of trees and also on fences. It shows up best on the light coloured, smooth bark of willows and Sallow as well as Silver Birch. It is quite common if you take the trouble to look for it!
Find out more about the nature of Dorset at my website www.natureofdorset.co.uk