Imagine a world in which ice extends farther than the Arctic and Antarctic—a world in which ice sheets cover large chunks of North America, Europe, and Asia. At times, much of the Earth’s surface has been covered by sheets of ice. These periods of Earth’s history are known as its ice ages, with glaciers a prominent feature.
WHAT IS A GLACIER?
A glacier is a slow-moving river of ice, forced to move downhill by its weight. It can be enormous. In an ice age, Earth’s temperature varies, with glaciers pushing forward over land during the cold periods (known as glacial periods) and retreating in warmer periods (interglacial periods).
AN EARLY VIEW
A nineteenth century artwork by a Swedish geologist and naturalist, Oswald Heer, provided an unrealistic picture of large mammals, including mammoths and deer, surviving at the edges of the last ice age. In reality, these mammals lived on steppes (grasslands).
Over millions of years, Earth has moved from warm periods to cold periods and back to warm periods. Scientists don’t know what triggers an ice age, but think it has something to do with gradual changes in the Earth’s orbit around the Sun over millions of years.
An unusual route opens
During an ice age, sea levels fall by as much as 300 ft (100 m) as water becomes locked up on land as ice, instead of flowing out to sea in rivers. As the sea falls, new land appears, sometimes forming a bridge between continents or islands. During the last ice age, land bridges joined Britain to Europe, New Guinea to Australia, and Siberia to Alaska, allowing people to cross from Asia to North America.
Clues from the past
When an ice age ends and glaciers disappear, they leave behind lots of clues that the land was once buried under ice. Glaciers carve away land as they flow, forming deep, U-shaped valleys. When they melt, they leave behind huge boulders, called erratics, often made from a type of rock not found locally.
A blanket of ice
At the peak of the last ice age, some 20,000 years ago, large areas of northern Europe were covered by ice, as well as Greenland and Iceland and parts of the Atlantic Ocean. Europe’s mountain ranges—the Alps, the Pyrenees, the Urals, and the Carpathians—were also covered in ice.
DID YOU KNOW?
Modern humans emerged during the last ice age, living south of the ice sheets. There were also big mammals known as megafauna, including:
■ Woolly mammoths
■ Woolly rhinos
■ Cave bears
■ Cave lions
■ Giant beavers
Many of the large mammals became extinct just after modern humans arrived.