_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.
The raised beaches and the submerged beaches were obviously caused primarily by the lowering and raising of the sea level and not of the land. There were land movements, of course—as there are even now—but they were either too small or too irregular to account entirely for the systematic arrangement of old beaches in many parts of the world.
There has been much controversy about other glacial matters, but there can be no question that the submerged beaches and the land-bridges were a by-product of glaciation. The great masses of ice—estimated to have averaged half a mile to two miles thick in North America and somewhere between those figures in northern Europe—depressed somewhat the parts of the earth on which they lay; the rest of the land tended consequently to rise a bit, though not enough to account for the now sunken beaches and for the land-bridges that united Africa and Europe, England and the Continent, and Alaska and Siberia at various times. It was the immense amount of sea water drawn up and locked in the glaciers that reduced the area of the ocean and created new shore lines and the land-bridges. Estimates of how much the seas were lowered from their present level range from 70 to 1,800 feet; the best are 200 to 300 feet. There is still enough glacial ice to raise the ocean more than 100 feet if it all melted.