Glacial Period, or Ice Age, a time when continental glaciers, resembling those now on Greenland and Antarctica, covered almost a third of the world's land surface. The period probably began about 2,000,000 or more years ago and continued to within 10,000 or 15,000 years of the present time. In the geologic time scale, the Ice Age corresponds mainly to the Pleistocene Epoch of the Cenozoic Era and is sometimes called the Pleistocene Ice Age. Geologists have found evidence for several earlier glacial periods, the last one occurring more than 230,000,000 years ago, late in the Paleozoic Era.

The Pleistocene Ice Age was not a period of constant glaciation. Rather, it was marked by periodically advancing and retreating ice. In North America and Europe there were at least four great advances, called glacial stages. Each was followed by a long, relatively warm interglacial stage, during which the ice melted and receded. Brief periods of relative warmth, during which the glaciers halted their advance or temporarily receded, occurred during glacial stages.

The North American and European glaciers advanced and receded at about the same times. The North American glacial stages—from oldest to most recent—are called the Nebraskan, Kansas, Illinoian, and Wisconsin. The glacial stages studied in Europe's Alps are called the Gnz, Mindel, Riss, and Wrm.

Scientists do not know whether the Ice Age is really over; the present period, some have speculated, may be another interglacial period to be followed by yet another coming of the ice.

Except for a few areas, little is known about the thickness of the glaciers. In Scandinavia the ice is believed to have been at least 8,500 feet (2,600 m) thick; in New England, more than 5,000 feet (1,500 m).

Changing Conditions

Scientists have learned that Ice Age climates changed from time to time, especially on lands in the middle latitudes. During, periods of advancing ice, the average temperature of the earth was probably about 10 to 15 degrees F. (6 to 8 degrees C.) lower than it is today. During periods of retreating ice, it was as high as today or higher.

Changes also occurred in the levels of the sea and the land. So much water was bound up in the glaciers that the level of the sea dropped, at times by as much as 300 to 400 feet (90 to 120 m). As a result, many shallow straits, such as the Bering Strait and the Strait of Dover, became dry land. In addition, the land beneath the glaciers was pushed down by the weight of the ice. After the glaciers receded, the land began to return to its former level; some of this land is still rising today.

A number of Pleistocene animals became extinct, including the saber-toothed tiger, mammoth, woolly rhinoceros, certain musk oxen, and the giant ground sloth. Among the animals that survived were the horse, bear, wolf, and dog. Humans also were able to survive.

Effects of Glaciers On the Land

Regions that were covered by Ice Age glaciers usually have distinctive land and water features. In some places, such as New England and Scandinavia, the moving ice eroded the land to bedrock. The receding ice left smoothly rounded hills and wide, U-shaped valleys. In other places, such as the glaciated plains and prairies of the United States, the ice left thick deposits of eroded material. Among the landforms created by this material are moraines, drumlins, eskers, and kames.

Streams were frequently turned into new channels, old lake beds filled, and new ones created. Because of their recent geologic origin, glaciated regions are often poorly drained and have many marshes, streams, and lakes.

Causes of the Ice Age

Scientists believe that the Ice Age was caused by large-scale changes in the amount or distribution of heat energy in the earth's atmosphere and oceans. Various theories explain how the changes could have resulted from variations in one or more of the following:

  • The amount of energy emitted by the sun.
  • The shape of the earth's orbit, or the angle or direction of the earth's axis with respect to its orbit.
  • The composition of the atmosphere, temporarily altering the amount of solar radiation reaching the earth's surface and the amount of heat trapped by the atmosphere.
  • The amount of land near the earth's poles or the average elevation of the land above the sea, affecting snowfall accumulations, wind patterns, and related phenomena.