18 SOIL COMPOSITION, CLASSES AND TYPES 



but denuded slopes may lose several feet in a year. Not all 

 the alluvium carried by rivers is deposited on either side of 

 their courses before the sea is reached. Indeed, it has been 

 estimated that the Mississippi river carries into the GuK of 

 Mexico each year an amount of sediment equivalent to a pile 

 of sand and mud one mile square and 268 feet thick. What 

 becomes of all this material? 



When in a well drained condition, alluvial soils are usually 

 rich and productive. Many soils are kept productive because 

 of rich sediments deposited on them. The soils in the Nile 

 Valley in Egypt are splendid illustrations of this natural method 

 of fertilization. 



Glacial soils or ''glacial driff soils are so called because the 

 material composing them was transported, mixed and built up 

 through the action of glaciers or huge ice sheets many thousands of 

 years ago. Though this seems a long time, nevertheless these are 

 regarded as young in comparison with residual soils. 



The Stoiy of the Glaciers. — ^Among the many wonderful 

 chapters in the history of the earth recorded in rocks, mountains, 

 hills and valleys, that of the Great Ice Age is comparatively easy to 

 interpret.. Abundant evidence points to the fact that at some re- 

 mote time the greater portion of the northern part of North Amer- 

 ica was covered by great sheets of ice resulting from the accumu- 

 lation of snows due to climatic changes. These great ice sheets 

 moved southward and invaded the northern part of what is now 

 the United States. In the central portion the ice extended as far 

 south as the Missouri River and southern Illinois, Indiana and 

 Ohio (Fig. 9). 



The Ice Age, especially so far as the north central portion of 

 the United States is concerned, does not mean the invasion and 

 melting of just one immense ice sheet or glacier, but it includes at 

 least five great and separate ice periods. During some of these 

 periods, notably the first ones, the ice sheets extended farther 

 west and south than during other periods. The effect of the Kansan 

 Drift, one of the first invading ice sheets, is best observed in Kansas, 

 hence the name. This is supposed to have occurred not less than 

 400,000 years ago. The last invasion, made during the Wisconsin 

 Glacial Period, is estimated to have occurred 20 to 50 thousand 

 years ago. There is strong evidence to indicate that between 

 each of the five ice periods or stages the chmate was tropical 

 or semi-tropical. 



