The 'old chalkie' from the 1970s.
Anyone spotted any lately?
I've heard they are rare these days and are mostly confined to Croydon.
I've never had any conclusive answers about how they end up like this, age? leeching of liquids? high bone-meal content? Questions questions!
Maybe it's to do with the sun and they end up like this the same way that chocolate bars do if left in a shop window for too long.