Ice Age was those early periods of the earth’s history when most of the northern part of the earth was covered by a vast sheet of ice.
During the earth’s long history, there have been several ice ages. The earliest was in, what is called the late Pre-Cambrian times, some 700 million years ago. Another ice age occurred during the late Carboniferous and early Permian periods, about 280 million years ago. Finally about 2 million years ago, an ice age began which lasted nearly until our own times. This is known as the Pleistocene Ice Age. Here we shall discuss about the Pleistocene Ice Age only because information about the first two Ice Ages is not available.
The Pleistocene Ice Age consisted of four periods. During each period, the ice formed and advanced southward, then melted back towards the North Pole. This happened four times. The ‘cold periods’ are called ‘glacial ages’ and the warm ones (when the ice melted) are known as ‘interglacial periods’.
The first period of ice came about two million years ago, and is known as Nebraskan. The second period came about 12,50,000 years ago and is called the Kansan. The third one came about 500,000 years ago. It is called the Illinoisan period. The fourth period, known as Wisconsin period, came about 100,000 years ago.
In between these glacial periods, there have been three interglacial periods. These interglacial ages are called the Aftonian, the Yarmouth and the Sangamon Ages. A typical glacial age lasts about 40,000 to 60,000 years, and interglacial age lasts about 40,000 years. Ice of Wisconsin period began to melt about 40,000 years ago and ended some 10,000 years ago. According to geologists, the earth at present may be in an interglacial age.
Geologists have learnt a lot about ice ages by studying fossils. Whole of Canada and one-third of northern United States, as far as New York City and the Missouri River valley, were covered by ice. In places the thickness of ice was from 2400 to 3000 m (8000 to 10,000 ft). In Europe ice covered whole of northern Europe, the British Isles and much of northern Russia. During the Pleistocene epoch, more than 30% of the earth’s surface was covered with ice.
Plants and animals, too, were much affected by the advance and retreat of ice. A number of new animals such as camels, cattle and modern horses appeared during this Ice Age. Many lakes, such as the Great Lake of North America, were also formed during this period.