Ronald Turnbull's book called Granite and Grit is one of my very favourite books. It's about the rocks found in the mountainous areas of the UK. He writes so well and it's actually witty; not the dry academic tome you might expect. He notes that whilst we usually associate tors with granite, there are other rock types that also produce tors. We visited the Wain Stones on Bleaklow, which are famous for looking like kissing heads.
Gritstone tors are cracked horizontally first. This is along the boundaries between different depositions because they are sedimentary. He says that they crack vertically underground when water is expelled under high pressure.
Gritstone is very coarse sharp sand. It occurred to me that this would have been made by erosion millions of years ago - ie it was on the surface of the Earth. Then it spent millions of years below the surface and here it is again up on the surface in time for me to see it.
