Heather Runge, age 10, of Glendale, Ariz., for her question:
WHEN WAS THE ICE AGE?
During the Ice Age, the main centering of ice in North America was near the Hudson Bay area. Ice in this part of the world stacked up to 10,000 feet thick. The pressure of its weight caused the ice to flow westward and southward. It spread over about 5.2 million square miles of land and covered most of North America down to Ohio and Missouri. We call the Ice Age that period of time that started about 1.75 million years ago and ended about 10,000 years ago. We refer to this as the Pleistocene ice age.
There had been two ice ages earlier with one during the Precambrian time more than 600 million years ago and one during the Carboniferous and Permian periods between 350 million and 230 million years ago.
The world had been cooling for more than 65 million years. Slowly glaciers began to form in the polar areas. Scientists have named the European periods of glacier formation as Gunz, Mindel, Riss and Wurm, ranging from the oldest to the youngest.
Scientists don't know exactly when the glacier formation occurred, but they estimate the Gunz glaciation started about 1.2 million years ago. Periods between the glaciations were called interglacials. It was believed that there were only four glaciations and three interglacials but research carried out in the 1960s and the 1970s seems to indicate that the earth may have undergone from six to 20 glaciations during the Pleistocene period.
There were probably four glaciations in North America and they were named Nebraskan, Kansan, Illinoian and Wisconsin. A typical glaciation lasted from 40,000 to 60,000 years while a typical interglacial lasted about 40,000 years.
The last ice retreat, scientists figure, began less than 20,000 years ago, a rather short period of time compared with typical interglacial intervals. It is believed that glaciers are not gone for all time, but perhaps the earth has just entered another interglacial.
When the glaciers were at their height, the ice sheets took up so much water that the oceans dropped at least 300 feet. When the ice melted, water went back into the oceans and filled them to their present levels.
As the glaciers spread out, they pushed soil and giant rocks ahead of them. Scratches were left on the rocks over which they moved. As glaciers retreated, some of the low places filled with water and new lakes were created. This is how the Great Lakes were formed.
Valley glaciers gouged out U shaped canyons in old river valleys. California's Yosemite Valley, a spectacular area, is one of these.
The fiord, a deep canyon partly under water, is another type of unusual area created by retreating glaciers.