124 PHYSICAL GEOGRAPHY. 



Minnesota, but the greater part of Iowa drift was evidently 

 derived from its own rocks, much of which has been trans- 

 ported but a very short distance, if any, from the place where 

 it originated. Many of the strata of Iowa rocks are soft or 

 friable, and nearly all of them are sufficiently yielding to 

 have been comminuted with facility by the glaciers. This 

 is also true even of many of the granite rocks of Minnesota. 

 We found the greater part of the granite exposed in the valley 

 of the Minnesota river to be too soft and easily disintegrated 

 to be useful even for good common building stone; and at 

 the mouth of the Redwood, a tributary of the Minnesota, we 

 found a cliff of decomposed granite, upward of one hundred 

 feet in height, so soft from top to bottom that it could be 

 crushed to the condition of fine soil in the hand alone. 

 Indeed, it graded imperceptibly info the fertile soil which 

 covered it, but with this exception, the deposit had never 

 been disturbed from its original position. In southwestern 

 Minnesota this friable granite is overlaid by what remains 

 from glacial action of the Cretaceous rocks which were once 

 continuous with those of western Iowa. These Cretaceous 

 rocks are all so soft and friable as to have yielded readily to 

 glacial action, and have evidently contributed largely to the 

 fine materials of the drift. It will be seen then, that the 

 supply of material here for transportation as drift into Iowa 

 was abundant, and well adapted for the production of good 

 soil for our State as well as for the region where it originated. 

 The Drift Deposit of Iowa is so thick and the proportions 

 of its component materials so nearly uniform throughout the 

 State; the soil of that deposit has also a great degree of 

 uniformity. But still it may not unfrequently be observed 

 that an underlying formation has impressed its character 

 upon the soil. We may, perhaps, say in general terms that 

 the constant component element of the drift soil is that portion 

 which was transported from the north, while the inconstant 

 elements are those portions which were derived from the 

 adjacent or underlying strata. For example, in western 

 Iowa, wherever that Cretaceous formation, known as the 



