The British and Irish Meteorite Society

Hi,

This is off-topic, but I'm curious. It's definitely not a meteorite.

Does anyone have any idea how a terrestrial boulder, in this case from the local Pendle Grit formation from Beacon Fell north of Preston, gets a crust? The crust is 5 - 10 mm thick and appears to be made from mineral. The boulder sits amongst grass, in a disused quarry, now used as a car park.

The location is here:

(53° 52' 46.4" N 2° 39' 05.7" W)