Scott Morrison, age 11, of Johnson City, Tenn., for his question:
HOW LONG DID THE ICE AGE LAST?
When ice covered a major part of the earth's'large land regions, we had what is called the ice age. Scientists believe that there have been a number of ice ages, and that each of them has lasted several million years.
The earliest ice age was recorded during the Precambrian time, which started more than 600 million years ago. The next important ice age took place in the early Cambrian Period, just about 600 million years ago.
Then another ice age occurred during the Carboniferous and Permian periods, about 350 million and 230 million years ago. The most recent ice age was during 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" is most often said to be the Pleistocene. This one lasted about 1.74 million years.
About 3 million years ago, in the late Tertiary Period, the earth became cooler. 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.
Then glaciers formed and great continental ice sheets also developed. The ice sheets 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 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 European and North American ice melted during the Pleistocene ice age, the 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 moved back, 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 and river valleys. Some of these, including Yosemite Valley in California, rate as spectacular landscapes. Others, called fiords, are partly under water. Many of these fiords are in Scandinavia.
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.