Cathy Van Hosen, age 10, of Beaumont, Texas, for her question:
HOW LONG DID THE ICE AGE LAST?
When ice covered a major portion of the earth's large regions of land, we had what is called the ice age. Scientists believe that there have been several main ice ages, and that each of them has lasted several million years.
The earliest ice age occurred during the Precambrian time, which started more than 600 million years ago. The next important age took place in the early Cambrain Period about 600 million years ago.
Yet another ice age occurred during the Carboniferous and Permian periods, between about 350 million and 230 million years ago. The most recent ice age occurred at the time of the Pleistocene Epoch which started about 1.75 million years ago and ended just 10,000 years ago.
The period most people refer to as the "Ice Age" usually is said to be the Pleistocene ice age. This ice age lasted about 1.74 million years.
About 3 million years ago, in the late Tertiary Period, the earth became colder. Glaciers started to form in the mountains and polar areas. This was not a sudden change. The world had been cooling down since about 65 million years ago. But this last cooling was marked by a series of ice sheets over parts of the northern continents.
When the glaciers formed, great continental ice sheets also developed. They grew thick and flowed downward from their centers.
In North America, the main center of ice was near Hudson Bay. Ice piled up from 8,000 to 10,000 feet thick. The pressure of its weight caused the ice to flow westward and southward. It spread over about 5,200 square miles and covered most of North America down to about the present valleys of the Missouri and Ohio rivers.
In Europe, the Scandinavian Peninsula was the center of glaciation. Ice piled up about 10,000 feet thick and flowed southeast about 800 miles, almost to Moscow.
The European ice also covered northern England, Denmark and Germany. It spread over an area about half the size of that covered by glaciation in North America.
At their height, the ice sheets turned so much water to ice that the level of the oceans dropped at least 300 feet. When the ice melted, water flowed back into the oceans and filled them to their present level.
As the glaciers slowly spread out, they pushed soil and loose rocks ahead of them like giant bulldozers. They left scratches called striae on rocks over which they moved. Soil and rocks left behind when the ice melted formed mounds and ridges called moraines.
As the glaciers retreated, the low places they had scoured out filled up with water, forming lakes such as the Great Lakes. Valley glaciers gouged out U shaped gorges in old river valleys. Some of these, including Yosemite Valley, rate as spectacular landscapes. Others, called fiords, are partly under water.
The glaciers ground some rocks into powder. The wind picked up this fine dust and blew it far and wide. Thick deposits of this fine silt, called loess, are found in Kansas. The Ukraine and northern China also have large areas covered with loess.