Above all, geology makes explicit challenges to our understanding of time. It giddies the sense of here-and-now. The imaginative experience of what the writer John McPhee memorably called 'deep time' - the sense of time whose units are not days, hours, minutes or seconds but millions of years or tens of millions of years - crushes the human instant; flattens it to a wafer. Contemplating the immensities of deep time, you face, in a way that is both exquisite and horrifying, the total collapse of your present, compacted to nothingness by the pressures of pasts and futures too extensive to envisage. And it is a physical as well as a cerebral horror, for to acknowledge that the hard rock of a mountain is vulnerable to the attrition of time is of necessity to reflect on the appalling transience of the human body."