STUDIES FOR STUDENTS. 7 11 



frequently are by no means rare in the drift-covered area, but 

 within the area of the United States, they are not generally of 

 great extent. 



Over wide areas the drift is so thick that the underlying rock 

 is rarely seen except where natural or artificial excavations of 

 great depth have been made. In such cases, excavations afford 

 the only means of knowing the thickness of the drift. In still 

 other regions the covering of drift is so massive that even the 

 deeper valleys and the wells fail to penetrate it to its base. Over 

 considerable areas in Northwestern Minnesota, for example, the 

 older formations are so deeply buried by the drift that they have 

 not been reached by the deepest excavations and borings that 

 have there been made, though this does not mean that they are 

 beyond the reach of deep borings. The drift is also very deep 

 in many parts of Iowa, Illinois, Indiana, Ohio, Michigan, and New 

 York. Depths of something more than five hundred feet have 

 been recorded in a few places. Such thicknesses are rare ; but 

 thicknesses of two hundred or three hundred feet are by no 

 means uncommon. The average thickness throughout the drift- 

 covered area is much less, but no very accurate estimate can be 

 made on the basis of present knowledge. The average thickness 

 for the Upper Mississippi basin has been estimated to be not less 

 than ioo feet. 1 For the eastern part of the United States, this 

 figure is probably much too high. 



The variations in thickness may be great within short dis- 

 tances. One hill may have barely drift enough to cover the 

 rock, while the next may be composed of drift from base to 

 summit. The drift may be thin on the hills, and deep in adjacent 

 valleys, or, less commonly, the reverse may be the case. Thick- 

 nesses varying from nothing to one hundred or two hundred feet 

 are not rare within a single square mile, and sometimes occur 

 within much narrower limits. The natural sections exposed 

 along the sides of valleys, and along the cliffs of seas or lakes, 

 sometimes illustrate the abruptness of the variations in thickness. 

 At one point along the course of a river valley the total face of 



1 Chamberlin : Geikie's "The Great Ice Age," 3d edition, 1894. 



