198 T. C. Chamberlin — Diversity of the Glacial Period. 



Iowa. The surface within the moraine loop has suffered very 

 slight erosion, except in the vicinity of the streams. Even the 

 channelings of these are sharp-sided and ditch-like. Over the 

 surface shallow sags, without outlet, occur in extraordinary 

 abundance. They are to be numbered rather by tens of thou- 

 sands than by thousands. On the old drift outside of the loop 

 an erosion surface of pronounced type is presented. A com- 

 plete erosion topography has been developed and has sunk 

 itself deeply below the original surface. To appreciate this 

 contrast let any one make a circuit of one hundred miles north 

 from Des Moines upon the later drift, and he will find, 

 throughout, indisputable evidence of freshness and recency. 

 Let him then make a similar circuit of one hundred miles west, 

 south and east on the area of the old drift and he will find a 

 rolling surface with undulations reaching up to 80 feet and 

 more of perfectly characteristic dendritic erosion topography, 

 and upon examination of the stratified portions of the drift he 

 will readily be convinced, if a topographic geologist, that this 

 erosion supervened upon the formation of the drift. On the 

 summits of this rolling land he will find the loess-like silts that 

 cover so much of the earlier drift. Modern topographic sci- 

 ence goes for naught, if evidences of this kind do not signify a 

 prolonged interval between the deposit out of which such an 

 erosion surface has been carved, and the immediately adjacent 

 deposit upon which so little carving has been done. 



To this combination of general erosion and of channel cut- 

 ting, there are to be added evidences of like import from oxi- 

 dation, ferrugination, decomposition and other characteristic 

 forms of weathering. These, in the hands of an expert com- 

 petent to eliminate preglacial and englacial factors, bear testi- 

 mony of great value. The value is certainly almost wholly 

 dependent upon the circumspection, skill and conscientious- 

 ness with which the criteria are applied, but the value is 

 none the less real. When these evidences are found, as in 

 this case, concurrent with general surface erosion and with the 

 special erosion of river gorges, the combination has a value 

 much beyond either factor alone. When to this is added the 

 fact that the drift sheets most eroded and most oxidized, are 

 those which were deposited at low-gradients and without ero- 

 sion advantages at the start, and that in general, they lie in 

 regions that are now lower than the less eroded drift of later 

 date, the case is still further strengthened. When, to the 

 combination, there is also added the fact that the deposition of 

 this low-gradient, now much-eroded, much- weathered deposit, 

 was separated from that of the fresher, little-eroded sheet by 

 an elevation of the surface and an increase of slope which gave 

 a markedly more vigorous drainage, and the still further fact 



