356 ARTHUR C. TROWBRIDGE 



joined the Wisconsin at about its point of entrance to Devils Lake 

 gap. The Upper and Lower Narrows may have been formed for 

 the first time by superimposition during the formation of the 1,200- 

 foot plain, or possibly by adjustment of the streams on sandstone, 

 provided they had been made and filled with sandstone. 



The age of this lower plain and therefore the date of re- 

 excavation of Devils Lake gap are no more proved than is the age of 

 the upper erosional surface. Those who hold that the upper plain 

 is Cretaceous assign a Tertiary age to the younger plain, and those 

 who believe the upper plain to be Tertiary in age naturally assume 

 that the lower plain was formed at the end of the Tertiary period 

 or early in the Pleistocene. There is some evidence in north- 

 eastern Iowa that this plain was intact when the oldest glacial 

 drift was deposited, but this also must be considered to be an open 

 question until more of the field data have been published. At 

 least it is clear that the present Devils Lake gap had its beginnings 

 either in late Tertiary or in early Pleistocene times, which for 

 present purposes is perhaps close enough. 



Long before the monadnocks had been removed from this lower 

 plain — that is, before the second cycle of erosion had reached com- 

 pletion — there was another uplift of the land relative to the then 

 existing seas, and the streams were again rejuvenated. This 

 uplift was much greater than the previous one, for the valleys 

 cut during this third cycle of erosion are much deeper than any 

 which were cut during the second cycle. At this time, too, the 

 main part of the present Devils Lake gap was cut. The bottom 

 of the gap at the beginning of this erosion cycle could not have been 

 lower than 1,200 feet A.T. and the tops of the bluffs were no higher 

 than 1,470 feet; that is, the maximum depth of the valley up to 

 the beginning of the last erosion cycle preceding the deposition of the 

 glacial drift was 270 feet. The maximum depth of the valley at 

 the end of this cycle could be determined, if it were possible to get the 

 altitude of bedrock underlying the glacial deposits in the middle of 

 the gap, a bit of information which unfortunately is not available. 

 The deepest boring into the glacial material within the gap was 

 made by Gustaffson and Prader in 19 14 at a point near the middle 

 of the gap at the north end of Devils Lake, only a few feet above 



