222 McGee and Call— i L&ee of Des Moines, Iowa. 



the ice was retarded along the ridge, and the pressure was 

 reduced, thereby not only diminishing the rate of erosion, but 

 heaping up an unusual thickness of morainic debris along the 

 ridge in what may be styled a submedial morain. (2) The 

 attenuation of the ice brought the fine debris undoubtedly dis- 

 seminated throughout the lower portion of the ice to the sur- 

 face in unusual quantities, thereby facilitating superficial melt- 

 ing along the ridges, and thus determined the course of a 

 supra-glacial stream. (3) The rapid melting of the ice along 

 such lines so reduced its thickness and diminished its pressure 

 upon the subjacent surface as to divert thither all sub-glacial 

 water, which accordingly formed sub-glacial streams coincident 

 and finally confluent with the supra-glacial rivers. These 

 streams formed canons in the ice, and when eroded through, 

 either formed basins or extended their corrasion into the sub- 

 jacent deposits, according to the slope of the surface. In either 

 case, when the canons were long, the streams so deeply corraded 

 their beds before the bounding walls of ice disappeared as to 

 permanently retain the water-ways in the ridges over which 

 they were first defined; while, when the canons were short, the 

 streams left the ridges as soon as they reached the margin of 

 the ice. 



Passing now to the region shown in the accompanying map, 

 we find to the eastward a typical as in which the ice-canon was 

 too short to define a water-way, and about the confluence of 

 the rivers a plateau which formerly existed as an ice-bound 

 basin, and through which the rivers corraded their valleys 

 before the final disappearance of the ice. In such a basin the 

 loss was deposited, just as was all of that of eastern Towa ; the 

 coldness of the waters and the low temperature of the air being 

 attested by the depauperate shells found imbedded in it. Here, 

 however, a re-advance of the glacier occurred before the ice was 

 melted from the plains east of Capitol Hill and west of Walnut 

 Creek, which disturbed, contorted and broke up the superficial 

 portions of the loss, and mingled its materials with the clays 

 Iders of a super-imposed sheet of drift; the re-advance 

 being too slight to completely remove the loss even from ex- 

 posed localities. 



Apropos to the re-advance of the ice-sheet here suggested is 

 Upbam's discovery that Des Moines lies approximately in the 

 course of the southernmost lobe of the great terminal moraine.* 

 Now, if the phenomena are coincident, as is forcibly suggested, 

 it follows that, as has already been urged by Ohamberlin,t 

 this moraine was formed, not during an independent ice-period, 



*Xmrh Armu.il Import of the Geological and Natural History Survey of 



