The earth probably hasn’t changed much in the last 5,000 years (250 generations). Men have built things on its surface and dug into it and drawn boundaries on maps of it, but the places where rivers, lakes, seas, and mountains now stand have changed very little.
In earlier times the earth looked very different. Geologists call the last great geological period the Pleistocene. It began somewhere between a half million and a million years ago, and was a time of great changes. Sometimes we call it the Ice Age, for in the Pleistocene there were at least three or four times when large areas of earth were covered with glaciers. The reason for my uncertainty is that while there seem to have been four major mountains or alpine phases of glaciation, there may only have been three general continental phases in the Old World.
Both the alpine and the continental ice sheets seem to have had minor fluctuations during their main phases, and the advances of the later phases destroyed many of the traces of the earlier phases. The general textbooks have tended to follow the names and numbers established for the Alps early in this century by two German geologists.
It is the second of these alpine phases which seems to fit the traces of the earliest of the great continental glaciations. In this article, the four-part system will be used, since it is the most familiar, but will add the word alpine so you may remember to make the transition to the continental system if you wish to do so.
Glaciers are great sheets of ice, sometimes over a thousand feet thick, which are now known only in Greenland and Antarctica and in high mountains. During several of the glacial periods in the Ice Age, the glaciers covered most of Canada and the northern United States and reached down to southern England and France in Europe. Smaller ice sheets sat like caps on the Rockies, the Alps, and the Himalayas. The continental glaciation only happened north of the equator, however, so remember that “Ice Age” is only half true.
As you know, the amount of water on and about the earth does not vary. These large glaciers contained millions of tons of water frozen into ice. Because so much water was frozen and contained in the glaciers, the water level of lakes and oceans was lowered. Flooded areas were drained and appeared as dry land. There were times in the Ice Age when there was no English Channel, so that England was not an island, and a land bridge at the Dardanelles probably divided the Mediterranean from the Black Sea.
A very important thing for people living during the time of a glaciation was the region adjacent to the glacier. They could not, of course, live on the ice itself. The questions would be how close could they live to it, and how would they have had to change their way of life to do so.