Glaciers are made up of fallen snow that is compressed into a large, thickened mass of ice over many years. They form when snow remains in one location long enough to undergo the transformation into ice. What makes glaciers unique is that they move, their own weight causing them to flow much like slow rivers.
Some glaciers are as small as football fields, while others can be over a hundred kilometers long.
Since the mid-nineteenth century, scientists and naturalists have been closely studying glaciers. In this photograph from 1894, two men approach a yawning crevasse. For safety, they use rope harnesses, attaching themselves to more stable ground further away from the crevasse. Even today, people doing fieldwork on glaciers take these precautions. An Ice Age is an extended period when cool temperatures allow the polar ice to advance into the lower latitudes.