In all my years I have never...

... got down on my hands and knees to see what goes crunch when out walking in freezing weather.
I don't know if this is the same everywhere (geologically speaking) and you all may know this anyway, but for me it's been quite a revelation.
I first noticed something strange out walking yesterday. Parts of the surfaces of the rough paths seemed to be pushed upwards, by as much as 4 or 5". The space below was made up of columns of ice, and fresh air.
The ground all around here is pretty impervious to water, made up of slate and more slate. So any water simply lies or flows just under the earth, mossy top surface. So if it freezes which it does from the top down it has nowhere to expand to but upwards. What I find fascinating is the form the ice takes. Little columns joined together.
This is of course why road surfaces can suffer in freezing weather.
As an aside, have you ever seen mushrooms forcing their way up through a tarmac path?