Lichen (Solenospera candicans)
It is hard to believe this lichen is, truly, a living organism; it looks just like a patch of chalk! It is an abundant lichen on hard calcareous rocks, just like we have here on most of the Dorset coast but nationally it is not common.
It forms these white/grey circular patches with a cracked centre. Often these can grow together to form large, crusty patches covering the entire rock substrate making it look as though it has been smothered in chalk dust.
Find out more about the nature of Dorset at my website www.natureofdorset.co.uk