Page:Popular Science Monthly Volume 43.djvu/331

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but there are many other conditions where it is obscure. Let us see how some of these obscurities arise.

In the United States and in British America there is a vast district of country covered with glacial drift. In a period known to geologists as the Glacial epoch deep snows and gigantic accumulations of ice extended from a region far to the northward down into the United States, nearly to the mouth of the Ohio River. The margin of this great ice field stretched from this central point eastward and northward to the Atlantic Ocean, and westward and northward to the Great Plains, while the Rocky Mountains were covered with great ice fields. This enormous ice sheet was ever working southward, and ever melting along its southern boundary. As it moved southward it plowed the mountains, dug down the hills, and generally filled the valleys with the débris; and it spread over much of the great area a vast sheet of rounded gravels, sands, and clays; and it fed the streams from the border of the ice sheet with fine silt that was distributed along the valleys to the Gulf of Mexico. This glacial flour is now recognized as the loess of the South. Since the disappearance of the great ice sheet the glacial formations that were made by it cover much of the dry land. Now, these glacial formations, being composed of incoherent bowlders, gravels, sands, and clays, are pretty easily distinguished from the underlying, more indurated rocks; but rains, brooks, creeks, and rivers have been at work carving new valleys, and remodeling the bluffs, hills, cliffs, and mountains of all the country, and in the process have distributed over the land formed by the glacial ice extensive bodies of overplacement. This overplacement is incoherent, like the glacial formations. There is no difficulty in distinguishing the overplacement from the primeval foundation, but there is great difficulty in distinguishing it from the glacial formations, and it requires nice powers of observation to always make the distinction with certainty. The criteria for distinguishing overplacement from the original glacial formations have been gradually discovered and formulated in the last few years.

In 1882, by act of Congress, the Geological Survey was authorized and directed to make a geological map of the United States. The survey entered upon this work in different parts of the country. Among many problems before it, one of the more important was that of mapping the glacial formations, and in order to do it two things were necessary: First, it was necessary to distinguish the glacial formations from modern overplacement; second, it was necessary to study the history of the glacial action and the various structures which the ice produced, for there are many—such as moraines, osars, kames, and bodies of till, sand, gravel, and bowlders; and it was sought to discover the history of their forma-