The classification of the drift deposits is one which has given rise to very divergent views. One school holds that the Glacial Period was marked by increasingly severe conditions of climate followed by a somewhat regular amelioration. Another school holds that it consisted of a series of cold periods separated by warm intervals. It is not contended that in Pleistocene times the ice disappeared completely; for it is pretty certain that on high mountain ranges, and in the Arctic and Antarctic areas, snow-fields and glaciers existed continuously. With cold conditions the ice-covered regions spread from the Polar areas and glaciers descended from the mountains, and with the return of warmer conditions the Polar glaciers and ice-fields decreased in area whilst the mountain glaciers again retreated up the valleys. The problem as to the extent to which such variations in glacial conditions occurred, and the number of times they recurred, can only be settled by a study of the Pleistocene deposits themselves. In this matter theory cannot at present help us.