Katie Doyle, age 8, of Kingston, Ont., Canada, for her question:
WHEN DID THE ICE AGE END?
Can you imagine anything cooler to think about on a hot summer day than the ice age? The earth's history includes a number of different ice ages. Scientists tell us a first ice age hit during the Precambrian time more than 1,000 million years ago, with a second during the Cambrian period 600 million years ago. A third came between the Carboniferous and Permian periods between 350 million and 230 million years ago.
Ice Age usually refers to the earth's most recent big freeze which started about 2 million years ago and ended about 10,000 years ago during the Pleistocene Epoch. During this time, glaciers formed in the mountains and polar regions and a series of ice sheets covered much of the northern continents.
Between six and 20 glaciations, or glacier formation formations, built up in Europe and North America. Spreading from the Hudson Bay region, almost all of Canada was covered and much of the northern United States. The Greenland icecap is all that remains of these today.
Ice built up from 8,000 to 10,000 feet thick. In North America, about 5.2 million square miles were covered going southward to the present valleys of the Missouri and Ohio rivers. In Europe it moved 800 miles southward, going almost to Moscow and covering northern England, Denmark and Germany.
During this last ice age, the ice sheets turned so much water into ice that the level of the oceans dropped about 300 feet. As the ice melted, the water went back to the seas and the level went back up.
As the glaciers slowly spread out, they pushed earth and rock that was in front of them. Later as the ice melted, new ridges and mounds had been formed.
Lakes such as the Great Lakes between Canada and the United States had been low places scoured out by the glaciers and then filled up with water as they later melted.
Glaciers also gouged out deep gorges in old river valleys. The Yosemite Valley in California and the fiords in Scandinavia are of this type.
Glaciers also ground many rocks into powder, and strong winds blew the fine dust to great distances. Deposits of this thick, fine silt can be found in Kansas, the Ukraine and in northern China. The silt is called loess.
Modern horses, camels and elephants, scientists tell us, first appeared during the Ice Age. Horses and camels originated in North America and then crossed the Bering Strait to Asia. Elephants, bison, deer and bears that evolved in Asia and Europe crossed the other way and came to North America. Llamas, giant ground sloths and armadillos left North America and moved to South America.