14 FRANK LEVERETT 



firming strongly the above interpretation, that the sand filling 

 occurred during this stage of glaciation. 



In explanation of the small amount of material deposited in 

 the bed of the temporary Mississippi, Professor Chamberlin has 

 suggested to me, that the ground in which this channel was exca- 

 vated may have been frozen at the time of excavation, its situa- 

 tion being on the immediate borders of the ice-sheet, and that 

 this frozen condition of the ground may have prevented the 

 stream from eroding more material than it could readily trans- 

 port. 



The time involved in the valley filling is a question of much 

 interest, but one on which an estimate is very difficult to make. 

 The filling of any given section is not the measure of the full 

 work of the stream, but simply an index of the excess of mate- 

 rial above the limits of transportation by the stream. To prop- 

 erly estimate the work in a stage of filling, it is necessary to 

 compute the amount of material carried through the channel, as 

 well as that deposited in it. It is doubtful if present methods 

 of study are sufficiently refined to enable one to make even an 

 approximate calculation of the time involved. It may safely be 

 affirmed, however, that the filling under discussion progressed 

 slowly, and that the time involved was sufficiently long to affect 

 materially the chronology of the lower rapids. 



Erosion conditions during the Sangamon interglacial stage. — 

 Between the Illinoian stage of glaciation and the deposition of 

 loess which accompanied the Iowan stage of glaciation, there 

 was a long interval of time, during which the surface of the Illi- 

 noian drift sheet was subjected to leaching and weathering, and 

 the formation of a soil. The name Sangamon has been applied by 

 the present writer to the soil and weathered zone formed at this 

 time, and may properly be made to denote the time interval. 1 

 Although the degree of weathering and leaching makes it evi- 

 dent that the interval was protracted, the valley excavation 

 appears to have been comparatively slight, so far as depth is 



'Proc. Iowa Acad. Sci., Vol. V, for 1897, pp. 71-80. Jour. Geol., Vol. VI, 1898, 

 pp. 171-181. 



