How do we know which way the ice went?

This is a lump of Shap granite perched on a boulder outside the Natural History Museum in Oxford. It was picked up by a university expedition many years ago ... from a beach on the Yorkshire coast. It had been carried there by ice during the Ice Age. It is by finding distinctive rocks like this that we can work out the directions in which the ice moved. I have been told that there is a lump of our local Criffel granite in the middle of Cannock Chase; I am yet to visit.