By definition—or lack of it—the Pleistocene is rather vaguely bounded, and quite as much at its beginning as at its end. To the paleontologist, the Pleistocene is the time of certain large and picturesque mammals that are now extinct. To the geologist, it is the time of the waxing and waning of the great glaciers. The beginnings and the ends of these two definitions of the Pleistocene do not correspond too closely. We shall use the term as little as possible, substituting the Great Ice Age. The Glacial Hypothesis Appears It is hardly more than a century since science began to realize that large parts of Europe and North America once were covered with glaciers. The discovery came from attempts to explain certain disturbing things called “erratic blocks.” These were large masses of stone—sometimes weighing as much as 10,000 tons—which had no business being where they were, because the native rock in their neighborhood was entirely different. Some of the erratic blocks, for example, should have been hundreds of miles away. The common explanation was that they were water-borne, perhaps by the biblical flood. An American cotton manufacturer accounted for the wearing away and the scratching of such boulders by supposing that they had been embedded in the lower surfaces of icebergs and then swept scraping across the earth by the tumultuous waters on which the Ark had ridden. In 1802 John Playfair, a professor of mathematics at Edinburgh, ventured the theory that the blocks had been transported by glacial ice.[1] This idea had occurred to a mountaineer named Kuhn in 1787, and Saussure echoed it in 1803; they knew and interpreted correctly the moraines of loose stones and boulders which they saw at the foot and the sides of the glaciers. From 1821 to the middle thirties various French, Swiss, and German scientists—Brard, Venetz, Charpentier, and Schimper—discussed and amplified this idea. Though A. Bernhardi, an obscure German professor of forestry, suggested in 1832 that “the polar ice once reached clear to the southernmost edge of the district which is now covered by those rock remnants,”[2] it was not until 1837 that the glacial theory took definite shape. Then Louis Agassiz, speaking before a Swiss society, launched the glacial hypothesis that there had been a period of great cold just before the advent of recent life. By 1840, when Agassiz published his _Studies of the Glaciers_, the idea was pretty generally accepted; he had “added the Glacial Epoch to the geological time-table.” The theory has been much amplified since then.