These are densely packed crustose lichens, on the bark of a young birch tree in Sheepfield Copse. Groups of lichen species are often consistently associated together, forming recognisable communities. It is probable this is a community, containing several species of Arthonia, that grows on smooth barked trees.
Next time you walk through the copse, pause for a closer look at the trunks of the birch trees there.
Southwick Country Park Nature Reserve 
Thank you for sharing, and nice pairing of images, it captures the entire experience of the seeing of these trees.