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. The raised beaches belong to a later discussion of the cause of the Great Ice Age as a whole. The Cause of Glaciation Most geologists believe that a comparatively slight drop in temperature would bring back the glaciers and the ice fields. The German geologist Brückner calculated that summers in the last glaciation were only 4° centigrade, or about 7° Fahrenheit, colder than they are today.[7] What could have caused this slight drop in temperature in the Great Ice Age? Most of the explanations are not satisfactory. One is that the earth happened to pass through a dust laden nebula that reduced solar radiation. Another is a hypothetical decrease in the amount of carbon dioxide in the atmosphere. Other explanations have to do with changes in the altitude of land, shifts in air currents and ocean currents, volcanic eruptions filling the air with dust that screened the rays of the sun. All these adventitious causes would have had to be repeated with the curious and complex rhythm which is characteristic of the waxing and waning of the ice sheets.