I think you'll find its almost the same mechanism as we deal with in old houses when you get salts coming out of old plaster. More likely to be due to a wet winter, now drying out, and drawing salts through the rock. The better the ventilation, the more will form as salts are drawn to the surface. Its a reaction of magnesium in the limestones and shales, with the sulphides in them (pyrite, galena, sphalerite etc) forming magnesium sulphate. I remember my mum finding a lovely bit of it once, many years ago when she came down Smallclough with me as about a 7 year-old. She took it outside and it was raining. By the time we got to the car, she was holding an amorphous lump of rock, minus crystals!!
:smartass:
The Meek Shall Inherit The Earth
... but not the Mineral Rights...