Glaciers are large masses of snow, re-crystallized ice and rock debris that accumulate in great quantities and begin to flow outwards and downwards under the pressure of their own weight. Glaciers form when yearly snowfall in a region far exceeds the amount of snow and ice that melts in a given summer. In this way, massive quantities of material accumulate in relatively small periods of geologic time
It is formed by multi-year ice accretion in sloping terrain. Glacier ice is the largest reservoir of fresh water on Earth, and second only to oceans as the largest reservoir of total water. Glaciers can be found on every continent, including on the greater Australian continent. Glaciers are more or less permanent bodies of ice and compacted snow that have become deep enough and heavy enough to flow under their own weight.
Geologic features associated with glaciers include end, lateral, ground and medial moraines that form from glacially transported rocks and debris; U-shaped valleys and corries (cirques) at their heads, and the glacier fringe, which is the area where the glacier has recently melted into water
Physical effects of the moving glacier is to scour and grind the bedrock surface over
which it travels as it advances, and then to redeposit vast quantities of sand, gravel and silt as it retreats. Further advances and retreats of subsequent ice sheets continue to rework these accumulated glacial sediments.
Scientists who study ancient climates of the earth have identified three distinct periods of glaciation: 1.) a period of glaciation around 600 million years ago about which little is known, 2.) glaciation that effected areas of Africa, India and Australia approximately 275 million years ago, and 3.) the most recent period, which began about 1.5 million years ago and ended (or at least subsided) within the last 15,000 years.
All but small remnants of the continental ice sheets retreated from North America thousands of years ago. Although it may appear that the Ice Age has ended, many scientists argue that our present relatively warm period represents but a brief interlude and that the glaciers may again advance in the future.