20 AGRONOMY 



The delta at the mouth of many large rivers is made up of 

 these finer particles, and consequently delta soils are among 

 the most fertile in the world. 



Work of glaciers. Another most effective agent in grinding 

 up the bed rock, but one that is no longer active in our coun- 

 try, was the great ice sheet that ages ago spread from colder 

 regions southward over a large part of North America. One 

 or perhaps several of these extended their movements to cen- 

 tral Pennsylvania and the Ohio River valley, and westward 

 nearly to the Rocky Mountains. In the northern part of the 

 United States the ice sheet was a mile or more thick and 

 moved over the country with irresistible force, breaking off 

 great fragments of rock as it went, and now advancing with 

 a cold season, now retreating with a warm one, ground the 

 immense rocks to bowlders, the bowlders to pebbles, the peb- 

 bles to sand, and the sand to powder. The interior of Green- 

 land is still covered with such an ice sheet, and similar 

 accumulations of ice in the form of glaciers are found in 

 Switzerland, the Rocky Mountains, and other elevated parts 

 of the world, where the process of grinding up the bed rock 

 in this way may still be witnessed. Upon the final retreat of 

 the ice sheet the country over which it moved was left strewn 

 with vast deposits of rock fragments, and these, sometimes 

 intermingled, sometimes sorted out by running water into 

 beds of clay, sand, and gravel and thrown up into hills and 

 ridges, between which were many small lakes and marshes, 

 cover much of the country in the northeastern states. The 

 topography of a glaciated country is quickly recognized by 

 the geological student. 



Modifications of the bed rock. It is believed that not only 

 has the mantle rock been derived from the bed rock by 

 processes already noted, but that much of the bed rock itself 

 was originally formed from harder rock that was first weath- 

 ered into small particles and later consolidated by various 



