IARIOGILACIOAIL, WAUILILIB SAS (OUR TIEHE WAS SISISIIAAM, 761 
of the preglacial valleys and the stage of development which 
these valleys had reached when the ice invasion occurred. 
The study of physiographic development carried on in the 
eastern part of the United States through the zealous labors of 
Davis, McGee and others has brought to light a strong array of 
evidence indicating that the valley channeling of the region 
under discussion was not begun until after the close of the 
Cretaceous, and that it may have been largely accomplished in 
the latter part of the Tertiary. 
It is evident from the sharply outlined valley borders, and 
other reliefs of the region, that the drainage systems had not 
reached a stage of senescence, though the size of the valleys, 
or the the measure of work which the preglacial streams accom- 
plished, is certainly several times as great as that of the post- 
glacial valleys. The preglacial valleys are not only deeper than 
the present valleys, but were also excavated, as a rule, in a more 
resistant material. The accompanying figure, furnished through 
the kindness of the state geologist of Iowa, serves to show the 
Fic. 1. Cross-section from Sonora, Illinois, to Argyle, lowa, showing old and 
new channels of the Mississippi River. [Iowa Geological Survey. | 
comparative sizes of old and new channels when cut in mate- 
rial of the same degree of resistance. It would apply equally 
well in other large valleys of this region, situated outside the 
limits of the later ice invasion. Within the limits of that inva- 
sion, the post-glacial valleys are much smaller than in the older 
drift territory. Where the present stream follows the preglacial 
channel in the older drift territory, it has usually removed the 
glacial deposits throughout the entire breadth of the preglacial 
valley, leaving only the portion of these deposits that lies below 
