In 1785, James Hutton, the father of geology, voiced the idea that the present holds the key to the past [source: Erickson]. This adage meant that though a major glaciation hadn't covered the world for tens of thousands of years, it had left behind clues to its character and activity. What did the rounded hills known as drumlins have to do with the ice age? Where did these erratic boulders come from?
Scientists like Louis Agassiz were familiar with glaciers, or snow that compacts so tightly that the bottom layer turns to ice. When the boulders in the Jura Mountains in Switzerland were traced back to the Alps, 50 miles (80 kilometers) away, glaciers explained these geologic anomalies that covered Europe and North America. What started as anomalies ended up as insights into what the ice age was like.
The ways in which some rocks were polished smooth and why some showed different layers allowed geologists to measure how thick the glaciers and ice sheets were. Using grooves on the sides of mountains and layers in the rocks, Agassiz and other scientists were able to determine that the glaciers and ice sheets present during the ice age were about 1 mile (1.6 km) thick [source: Imbrie].
This evidence of glacial activity showed just how much ice there was -- about one-third of the world was under thick ice, for a grand total of 17 million cubic miles (71 million cubic km) of glacial ice [source: Gosnell]. Antarctica, which already had an ice sheet, had 10 percent more ice than it does now [Gosnell].
What really set the ice age apart was the amount of ice in the Northern Hemisphere. In North America, ice covered Canada south through the central United States, extending from New York to Washington State. In Europe, ice covered Scandinavia, Ireland, Germany and western Russia. In North America alone, glaciers covered 10 million square miles (26 million square km), or about 13 times the area that they cover today [source: Erickson]. To form these massive ice sheets, the water was sucked out of the oceans, causing sea levels to drop about 350 feet to 400 feet (107 meters to 122 meters) [source: Imbrie].
The glaciers weren't static. In fact, they've often been described as bulldozers [sources: Erickson, Yasuda]. They advanced and receded in an undulating motion, leaving behind piles of rocks and other glacial till (natural debris that glaciers leave behind). And even though the ice wasn't everywhere, the glaciers affected the rest of the continent. The outskirts of the glaciers turned into arctic deserts, and a wind of dust called loess covered the land, created by the grinding motion of the moving glaciers.
The glaciers also preserved the fossils of plants and animals who lived through this chilly time. The global temperatures during the last major ice age were about 10 degrees Fahrenheit (5.6 degrees Celsius) lower than they are today [source: Erickson]. It may not sound like much, but when we look at the adaptations that animals of the time made, we know it must have been cold. Based on fossil evidence, we know that woolly mammoths, bison, wild horses, musk oxen, caribou, lions, antelope and the short-faced bear all roamed the land. They adapted to the cold temperatures by storing up fat reserves and growing specialized coats. The musk ox, for example, has shaggy hair two feet (0.6 m) long and underwool that is the most effective insulator of any animal fur [source: Barton].
How did it get so cold that animals needed hair a few feet long? How do ice ages start anyway? We'll tackle some of the theories on the next page.