90 PHYSICAL GEOGRAPHY. 



Every one who examines the drift attentively cannot fail to 

 see that it is composed of more or less finely comminnted rock 

 which existed in other forms prior to its accnmnlation as 

 drift, and that the questions to be considered are: to what 

 extent it has been derived from rocks that now underlie it, to 

 what extent from rocks elsewhere, and by what agency was 

 the comminution, transportation, and accumulation of the 

 material accomplished. 



After long and extensive examinations of the drift in all 

 parts of Iowa, no doubt remains that a large part — probably 

 much the largest — of it was derived from rocks within the 

 limits of the State, and very largely from the rocks that 

 immediately underlie it. It is also just as certain that a 

 considerable part of it, including nearly all its boulders, 

 was derived from the region lying wholly beyond the 

 northern boundary of the State. 



As to the means by which the drift has been accumulated 

 and transported, the greatest number and most important of 

 known facts warrant the belief that it was accomplished 

 through the agency of ice, which, during the Glacial 

 epoch, covered the whole or the greater part of the northern 

 hemisphere, far enough to the southward to reach quite 

 beyond the southern boundary of our State. This former 

 wide-spread glacier has receded until the southern limit of 

 its remnants is now quite within the frigid zone, and they 

 are now producing phenomena there similar to those the 

 effects of which we daily witness in the drift of Iowa. 



The Evidences of the Northern Origin of at least a large 

 part of the material composing the drift, consists mainly in 

 our ability to trace up the boulders it contains to their 

 original ledges far to the northward, and also in the fact that 

 they cannot be so traced in any other direction. This is the 

 most tangible proof; but there is a vast amount of evidence 

 which corroborates this, and nothing which disproves it. 

 It is quite evident also that much of the finer material of 

 the drift, as well as the boulders, is of northern origin, but 

 as its identification would be doubtful or impossible we rely 

 upon the identification of the boulders alone. 



