Naturally enough the Great Ice Age was a time of notable changes of climate. Vegetation advanced and retreated widely. The level of the sea rose and fell some hundreds of feet. Whether or not there was more rain and snow—a moot point with science—the many rivers of the world grew in volume, and often in speed, at certain times and became low and sluggish at others. These profound alternations created the river terraces which have aided so much in determining the age of man and his various cultures. As the ocean sank, while the glaciers grew, the slopes of river beds became steeper, and the rivers themselves grew swifter. The turbulent rivers cut deeper channels and carried the displaced materials far down their valleys and ultimately even into the sea. During the cold, dry period at the climax of each glaciation, the dying trees and brush and grasses released their grip on gravels and silts, the intermittent flood waters of the melting glaciers carried away the debris and—because the rivers lost in slope and grew sluggish as the sea level rose—they deposited the gravels and silts in their beds. As the glaciers grew again and the oceans sank, the rivers once more became swifter and more turbulent, cut deeper channels, carried away part of the gravels and silts, and left the rest as terraces. Thus the passing of each glaciation meant the adding of a new and a lower terrace to the river valleys. Four such sets of river terraces are found just outside the areas where the glaciers have been active—in the valleys of the Rhine, the Thames, the Somme, the Isar, and other rivers. [Illustration: THE AGE OF RIVER TERRACES _These simplified sections of a river valley show how successive channels were cut deeper and deeper, leaving the older deposits of gravels and silts in the higher terraces at the sides. The discovery of this process of nature was of the greatest value in determining the age and the succession of the cultures of early man in Europe. The oldest flint tools were found in the gravels of the highest terraces, and the newest in the lowest._] Farther to the south, periods of great rainfall helped to produce similar river terraces in valleys like the Nile. The concentration of masses of ice in central and northern Europe upset the zones of climate of Africa and other parts of the earth and caused great climatic disturbance. Rainfall belts moved far south, and the rain increased in abundance. Such periods of rain are called pluvials. There is still a good deal of argument about whether pluvial periods occurred principally in glacial or interglacial periods. This affects the dating of early man, and it is particularly important to us in the New World.