Book excerpt: It is not as widely known as it should be that Britain has the most varied geology of any country in the world. This book is a celebration in words and pictures of what its mountains are made of, and how they got there. This in turn determines what they're like to climb, scramble on, or walk over. Why is Skiddaw slate so slippery? How do tors form? Why is gritstone so difficult? Why is Lakeland so picturesque, and the granite lands so grim and forbidding? Geology is destiny, whether it's the rubbishy nature of gullies and screes, the sculpting of valleys by ice or the landslip weirdness of Quiraing on the Isle of Skye. British mountains contain many interesting and different ingredients: gneiss and granite and gabbro; limestone and sandstone; schist and slate; the product and the debris of tectonic shifts, volcanoes, earthquakes and glaciers over many millennia. This book explains all this to the layman, from an expert but personal perspective, and will add immeasurably to the fun and satisfaction to be gained from any day in the hills.