6o 



river for a short distance into Kentucky, thence westward across 

 southern Indiana, Illinois and central Missouri, to a point be- 

 yond Kansas City, where it bends northward across Kansas and 

 . the Dakotas and thence westward along an irregular line a little 

 south of the international boundary to the Pacific Ocean. All of 

 the region north of this line was covered one or more times by 

 great ice sheets, except a small area in southwestern Wisconsin 

 and adjacent portions of Minnesota, Iowa and Illinois, where 

 there appears to have been a sort of island of land surrounded 

 by ice, known as the "Driftless Area," while local glaciers oc- 

 curred south of the glacial boundary at many points in the 

 mountains. 



North of the boundary mentioned, the surface, except for the 

 small driftless area, is covered with a mantle of materials de- 

 posited by the glacier and known as drift. The drift is divided 

 into two main types, the first known as till, and the second as 

 modified or stratified drift. Till is a heterogeneous mass, con- 

 sisting of clay, sand and boulders, frequently known as hardpan. 

 It was deposited mainly directly by the ice, either beneath the 

 sheet or at its margin. The second class of drift includes gravels, 

 sands and other stratified deposits formed by streams leading 

 outward from the ice sheet. It is found chiefly along the valleys 

 which were once occupied by glacial streams, but considerable 

 amounts were also deposited in temporary glacial lakes which ex- 

 isted between the northward sloping land and the retreating ice 

 sheet, while some was laid down as broad wash plains. 



The glaciers which left the various types of drift started in the 

 far north in relatively recent geologic times and spread south- 

 ward to the limits mentioned. Previous to their advance, the 

 rocks were probably deeply weathered and covered with soil, as 

 in the South at the present time, although the extent of the 

 weathering was doubtless somewhat less. The first work of the 

 ice was to remove this soft weathered material. Part was in- 

 corporated with the till and part was carried off by the streams 

 to form clay and sand deposits. Later, after the removal of the 



