ARE THERE EVIDENCES OF MAN IN THE DRIFT? 317 



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 accu- 

 mulations 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 cen- 

 tral 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 debris ; 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 dryland. 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 over- 

 placement. This overplacement is incoherent, like the glacial 

 formations. There is no difficulty in distinguishing the overplace- 

 ment 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 author- 

 ized and directed to make a geological map of the United States. 

 The survey entered upon this work in different parts of the coun- 

 try. 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 vari- 

 ous 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- 



