CHAPTER IV. 

 THE GLACIAL PERIOD AND ITS DRIFT DEPOSITS. 



REVIEW OF THE GLACIAt, PERIOD IN NORTH AMERICA. 



In the latest geologic period, immediately preceding the Recent and 

 present period in which we live, the north part of onr continent was deeply 

 enveloped in snow and ice. Every year the snowfall was greater than 

 conld be melted away in summer, and its depth gradnally increased till its 

 lower portion was changed to compact ice by the pressure of its weight. 

 This pressure also caused the vast sheet of ice to move slowly outward 

 from the region of its greatest thickness toward its margin. 



Among the proofs of this Glacial ])eriod, it is first to be observed that 

 the surface of the Ijed-rocks in the northern drift-covered portion of the 

 United States, and thence north to Hudson Bay and the Arctic Ocean, bears 

 fine scratches and markings, called stride, like those which are found 

 beneath the glaciers of the Alps. Only one cause is known which can 

 produce markings like these, and this is the rasping of stones and bowlders 

 frozen in the bottom of a moving mass of ice acciTmulated upon the land 

 in a solid sheet of great extent and depth. As these strife are found upon 

 the rocky surface of British America and our own country to a southern 

 limit that coincides approximatelv with the course of the Ohio and Missoui'i 

 rivers, we must conclude that an ice-sheet has covered these regions. 



The superficial material that overlies the bed-rock within the northern 

 glaciated area has everywhere been plowed u]) and \\()rked over l)v the 

 slowly moving ice-sheet, and at its disappearance the greater part of this 

 glacial drift was left in a deposit of clay, sand, gravel, and bowlders, 

 mixed in a confused mass, which is called till. The thickness of the till 

 i;pon much of the bed of Lake Agassiz is from 60 to 200 feet, but in some 

 tracts it is only from 5 or 10 to 20 oi- 30 feet. Throughout nearly all of 



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