A pale lilac crust fungus, possibly Vuilleminia comedens (waxy crust fungus), growing on a dead branch of an oak tree. I am not sure whether the dark features are part of the fungus or just raised bits of the underlying wood poking through.
A week later it looked like this:
And here is a wider-angle view:
Photos taken in Whiteknights Park, Reading University grounds, Reading, UK, on 2011-01-26 (first one) and 2011-02-03 (second and third).