Quite Puzzling

Had a few photos lined up for today but couldn't resist this one.

Went to the Botanics to visit the Witch Hazels as per habit ( see blip for 2nd February). They're at their best just now (although the small Barnstedt Gold specimen by the North Gate seems to have died).

On way out this caught my eye, it's the bark of a Monkey Puzzle tree. My puzzle is that I don't know what the light marks are, they look like lichen. Can anyone enlighten me?