50 IOWA ACADEMY OF SCIENCES. 



suberincumbent till and loess are let down bodily. Much of 

 southeastern Nebraska seems to owe much of its roughness to 

 these relations. 



We are now fairly prepared to consider a theory to account 

 for the problematic conditions indicated at the outset. I present 

 it for your criticism. If I mistake not it will explain much, if 

 not all, of the difficulties found. 



We may suppose that the preglacial surface was uneven as 

 in unglaciated areas generally. The advance of glaciers spread 

 over it a blanket of bowlder clay, and left a surface similar to 

 that inside of the Wisconsin moraine. 



Upon this was spread by the flooded streams flowing from 

 the melting ice sheet, either of the same, or some subsequent 

 epoch of the ice age, and also by streams burdened with Tertiary 

 silts and clays from the west, the sheet or succession of sheets 

 of loess. At this stage the surface of eastern Nebraska, 

 western Iowa and northern Missouri, was a silt covered plain 

 similar to that of the lower Mississippi at present. Possibly 

 more uneven and more sloping. We can not conceive that 

 deep rivers were the rule in this work but shallow overburdened 

 streams more like the Platte of the present day, or the Hoang-Ho 

 of China. 



As the amount of water declined the channels would become 

 more contracted as in a low water stage. The beginning of a 

 northward differential elevation and a not improbable lowering 

 of base level by the change in the course of the Missouri river, 

 as the writer indicated in his Missouri report, may have begun 

 a rapid trenching of the water-logged deposits. 



In such conditions the erosion of valleys, we may suppose, 

 went on much more rapidly than later, because of copious springs 

 and great plasticity of the deposits. 



As the drift and loess dried out there would be a relocation 

 or redevelopment of the preglacial valleys, so that the post- 

 glacial streams would approximately correspond to the pre- 

 glacial, as has been pointed out by McGee and others. 



The first erosion was probably largely by ravines, cutting 

 down sooner or later to the underlying till and drawing oif the 

 surplus waters from the loess by springs. Here different sup- 

 positions may be considered. 



If the dryness of the climate was sufficient to render the 

 loess rigid, the springs and streams may have had fair oppor- 

 tunity to erode the drift, not only by corrasion, but by sapping 



