THE QUATERNARY PERIOD 357 



Advantages and Disadvantages of Glaciation 



Advantages. — As a result of late Tertiary stream dissection, 

 much of what is now the glaciated area of the United States had 

 been converted into a fairly rugged country. Because of the 

 heavy accumulations of drift, chiefly in the depressions, this 

 ruggedness was greatly diminished and, in fact, many districts 

 were actually converted into almost featureless plains. Old 

 lake beds (e.g. that of Lake Agassiz) also are usually very 

 smooth. Thus, agricultural pursuits, transportation, and travel 

 have been made easier. 



Over very extensive areas, such as the upper Mississippi 

 Valley, the soils have been made deeper and richer on the average 

 because the pre-Glacial soils were not only comparatively thin on 

 the numerous hillsides, but also they were sandy or clayey residual 

 materials from which much of the rich (soluble) mineral plant 

 foods had been washed out. The glacial drift soils are usually 

 more uniformly deep and consist of finely ground rocks of many 

 kinds still rich in the soluble plant foods. 



Water-power facilities have been vastly increased because of the 

 development of thousands of waterfalls, rapids, and lakes. Pre- 

 Glacial streams were mostly graded and hence without waterfalls 

 or rapids, while pre-Glacial lakes were almost entirely absent. 

 Lakes, by acting as reservoirs, help much in causing a more uniform 

 flow of streams. In many places such reservoir effect is furthered 

 by artificially increasing the heights of the natural dams, as e.g. 

 in the Adirondacks. Also many large reservoirs can easily be con- 

 structed at comparatively little expense by restoring dams of 

 extinct lakes. 



Large lakes afford cheap transportation facilities, and often 

 have a tempering influence upon the climate. Many lakes fur- 

 nish abundant water supplies for towns and cities, as well as 

 more or less fish for food. 



The benefit of lakes, waterfalls, gorges, etc., from the aesthetic 

 or scenic standpoint would be difficult to overestimate. 



Drift deposits are often used, e.g. clays, for the manufacture 

 of brick, tile, etc., and sand and gravel for various construction 

 purposes. 



Disadvantages. — In some cases the earth's surface has been 

 increased in ruggedness by the drift accumulations, especially in 



