The artists' fungus
You might think this was an ancient leather moccasin found in a peat bog, or else a stack of wholemeal pitta breads well past their sell-by date, but it is actually a specimen of Ganoderma applanatum, attached to a fallen tree.
Shelf or bracket fungi like this one are so-called because they grow horizontally and are so hard that a series of them can serve as steps up a standing tree trunk. But since they grow on dead or dying timber that's probably not to be recommended.
This is a long-lived fungus with layers of spore-bearing pores building up beneath over successive years. During the growing season the new under-surface is soft and white but turns brown when scratched. This has led to the use of this fungus as a sort of natural canvas and a tradition has developed in some parts of the world for producing quite astonishing examples of fungal creativity, hence the name Artists' Fungus. Here's one example and here are many more.
Of course they can also be used for notes and graffiti left in situ for other people to find. This one was not yet in the right condition for artistic endeavour but I may return to it later in the year.