42 SOIL PHYSICS AND MANAGEMENT 



from three centers of accumulation in Canada. The northwestern 

 half of Europe was covered at the same time. Vast quantities of 

 material of all sizes and all kinds of rocks were transported and 

 deposited when the ice melted, leaving a mantle of boulder clay, 

 drift, or till, varying from a few inches to several hundred feet in 

 thickness. The average depth of the deposit for Illinois, according 

 to Leverett, 1 is about 115 feet. These glacial deposits constitute the 

 material from which the soils were formed over a large area east and 

 north of Illinois, but in the middle west a deposit of loess has 

 buried the drift, producing soils of an entirely different character 

 (Fig. 33). In glaciated Europe the same conditions exist in regard 

 to soils. 



FIG. 34. Limestone boulder showing glacial scratches. Urbana, 111. 



The drift left by glaciers is only one of the important things 

 accomplished by them. The enormous pressure of the ice, 40 pounds 

 per square inch for each hundred feet in thickness, enabled it to 

 wear down hills and fill valleys, especially if they extended nearly 

 at right angles to the direction of the movement. Otherwise it 

 might deepen and broaden them, but on the whole its effect has been 

 to leave the country more nearly level than before. Many regions 

 have been transformed from hilly areas of low agricultural value 

 to undulating or rolling lands well adapted to agriculture. The ice 

 in its movement southward picked up large quantities of detritus 

 of all kinds and sixes and ground it into fine material fitted to form 

 soils. Much of this material was carried from 400 to 1,000 miles 

 or more and during its transportation boulders (Fig. 34) and gravel 

 would rub and grind against each other and against the rock sur- 



