Ice ages influenced landscape from "summary" of Principles of Geology, Volume 1 by Charles Lyell,Sir Charles Lyell
The Earth's surface has been greatly modified by the action of ice, during a series of successive periods, to which the name of "glacial epochs" has been applied. These epochs were followed by long intervals, during which the temperature of the climate was higher than at present. The effects of ice on the land are found in every part of the globe. They consist of smoothed and polished surfaces, with scratches and grooves, called "striae." These striae indicate the direction in which the ice moved. The ice transported immense quantities of loose materials, such as boulders, gravel, sand, and clay, which it deposited in ridges and mounds, forming what are known as "drifts." The drifts vary in thickness from a few inches to several hundred feet. They contain fragments of rocks different from those on which they rest, showing that they must have been brought from a distance. The erratic blocks found in many parts of the world are also evidence of the action of ice. These blocks are sometimes of enormous size and have been transported for many miles. Some are perched on the tops of hills, far above the level of any existing glaciers. The marks of glaciers are seen in mountain valleys and on plains, in regions now free from ice. In the northern parts of Europe and America, there are vast tracts of country where the surface is covered with drift, and where the rocks are smoothed and striated. These tracts extend over many thousand square miles. The drift is sometimes several hundred feet thick, and in some places, it forms hills of considerable height. The presence of erratic blocks in regions now free from ice, the occurrence of boulders on the tops of hills, and the polished and striated surfaces of rocks, all prove that the land has been subjected to the action of ice. The effects of ice ages on the landscape are undeniable, and they continue to shape the Earth's surface to this day.
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