100 PHYSICAL GEOGEAPHY. 



completed system of surface drainage. That the waters 

 derived from the melting of the receding glaciers modified 

 the surface of the drift to a considerable extent is quite 

 certain; but how far they did so or how much the present 

 condition of the surface between the valleys is due to such 

 modification, and how far it remains now in the condition 

 in which the glaciers left it, seems impossible to determine. 

 There seems very little reason to doubt, however, that much 

 of the surface of Iowa is now essentially in the same condi- 

 tion that it was left at the close of the Glacial epoch. 



We may safely assume that after the recedence of the 

 glaciers, which was, of course, accomplished by a change of 

 climate, and before the surface waters had formed new valleys 

 for the present rivers, or re-excavated for themselves the old 

 valleys which existed before the Glacial epoch, but which 

 became filled by the drift, numerous shallow depressions 

 existed upon the surface. These filling with water from the 

 rains or melting ice, became ponds and lakes, some of which 

 exist at the present day, but the majority have doubtless 

 become drained. If these depressions were longtitudinal or 

 connected in chains, as many of them doubtless were, they 

 would have given initial direction to the future rivers, and 

 become drained by their deepening valleys so that no trace of 

 them would remain. Those lakes and ponds that now exist 

 are mostly found in the regions where the streams have their 

 rise; many of them rest directly upon the watersheds. They 

 have remained because no accumulation of water beyond has 

 sent currents across them to cut channels for their outlet. A 

 part of these have been described under the head of Lakes, 

 and a part of them will be found referred to under the head 

 of Peat, in another part of this report. 



The depressions in the primitive drift surface here more 

 especially referred to, were all comparatively shallow; but 

 one of extraordinary size and depth existed upon the western 

 border of the State. Like the others, it became filled with 

 water as the glaciers receded, and afterwards filled with its 

 own sediment. This sedimentary formation is described 

 under the head of Bluff Deposit. 



