Summary of Ice Age Events
What has been called the "ice age" was actually several ice ages. Continental glaciers advanced and covered large areas of the northern hemisphere many times. Until the 1960s geologists thought there were four major glacial advances and retreats, but new evidence shows there may have been as many as thirty glacial cycles over the past three million years, some more extensive than others. Why the glacial cycles started about three million years ago is not known.
In the last glacial cycle, which peaked about 18,000 years ago, glaciers covered virtually all of Antarctica, Canada, Greenland and Scandinavia, much of northern United States, northern Europe and Britain, and parts of Siberia and Alaska. Glaciers became much more extensive than today in the Alps, Pyrenees, Urals, Himalayas, Rockies, and Andes mountains. They formed on the Hawaiian volcanoes and on some tropical African volcanoes, which are today ice free. Sea level dropped by up to 350 feet.
The ice sheets left scars on the land, scraping away soil, polishing and plucking bedrock, dumping gravelly debris, and cutting out U-shaped valleys. At the terminus, they formed end moraines consisting of unsorted silt, sand, gravel, and boulders. An extensive series of these terminal moraines can be traced almost continuously across the entire United States. Long Island's north shore is made of two of these end moraines, which can be traced all the way to Cape Cod.173 Long Island's south shore is a glacial outwash plain.
In many places, such as New York, Michigan, Wisconsin, and much of Canada, distinctive features called eskers, kames, and drumlins were formed from debris left when the glaciers retreated. An esker is formed when melt water cuts a tunnel under a glacier and the resulting stream lays down a bed of debris. When the glacier retreats the debris is left as a long, sinuous ridge of gravel. Kames are piles of sand and gravel that look like the cone of sand in an hour glass, and are left when dirty melt water drains through holes in the retreating ice. Drumlins are long egg-shaped hills composed mainly of layered gravel deposits, with their long axes parallel to the direction of ice movement. It is not known how drumlins form.
Throughout much of the world extensive deposits of loess were formed. Loess is a deposit of wind-blown dust picked up from glacial outwash. During the ice ages, much of the world was more arid than it is today, although some areas received more moisture. Large lakes were formed in southwestern United States. Lake Bonneville, which covered parts of Utah, Nevada, and Idaho to depths of as much as 1000 feet, was nearly the size of Lake Michigan. Great Salt Lake is but a small remnant of that earlier lake.
Vegetation belts were shifted south of present day positions, so that a tundra belt covered southern Britain, northern France, Germany, Poland, and the United States about the latitude of New York.
After reaching a maximum, the ice sheets retreated by fits and starts, so that Scandinavia was ice free by about 8000 years ago, and Canada by about 6000 years ago.
171 Alastair G. Dawson, Ice Age Earth: Late Quaternary Geology and Climate, Routledge, London and New York, 1992.
172 E. C. Pielou, After the Ice Age: The Return of Life to Glaciated North America, University of Chicago Press, Chicago and London, 1991.
173 Walter Sullivan, op cit, p. 249.