Geomorphology has many enthusiastic practitioners in Europe and in America, though its place in geography as a whole differs on the two sides of the Atlantic. Many distinguished European geographers began their careers as geomorphologists and later turned to the human aspects of the subject. Many people say that no geographer can work effectively unless he has an adequate training in physical geography, for the whole traditional basis of the subject is the relationship of the earth and man. Fundamental to modern geomorphology is the idea of several cycles of erosion, due to repeated sinkings of sea-level or uplifts of the land so that the same valley may show signs of several active cycles, simultaneous in development although successive in origin. Many hold that ice erodes most fully where it meets most resistance, so that, as E. de Martonne suggested, the result of glaciation in a valley may be the accentuation of gorges.