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.
Adolphe Morlot, in 1854, discovered fossils of temperate plants between layers of glacial deposits, and advanced the theory that there had been warm periods as well as cold ones during the Great Ice Age. In his “Notice sur le Quaternaire en Suisse” he suggested three separate glaciations with two warm interglacial periods between. In 1874 James Geikie, the geologist of Edinburgh, brought out his The Great Ice Age and Its Relation to the Antiquity of Man, building upon Morlot’s work; and his Prehistoric Europe, in 1881, expanded the glaciations to six. Yet for thirty more years some stubborn scientists still believed in a single glaciation.
It was not until the turn of the century that the work of Albrecht Penck and Eduard Brückner established the history of the Alpine glaciations on a solid scientific foundation that has endured pretty well till today. They found four major glaciations and named them in neat alphabetical order after four Alpine valleys—Günz, Mindel, Riss, and Würm.[3] They divided the Würm glaciation at first into two periods of activity, and later into a number of smaller oscillations toward the end. There has been some controversy over the subdivisions of the Würm, and one to three Danubian glaciers have been suggested hundreds of thousands of years before the Günz; but the general hypothesis brought forward by Agassiz and the amplifications of his successors are now definitely established. With all this goes much knowledge of the ice sheets that covered Scandinavia, northern England, and Germany as far south as Dresden, and North America from ocean to ocean and down to Long Island and the Ohio and Missouri rivers.