86 IOWA ACADEMY OF SCIENCES. 



To properly estimate the work in a stage of filling it is neces- 

 sary 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 INTERGLA- 

 CIAL STAGE. 



Between the Illinoian stage of glaciation and the deposition 

 of loess, which accompanied the lowan stage of glaciation, 

 there was a long interval of time during which the surface of 

 the Illinoian drift sheet was subjected to leaching, and weath- 

 ering, 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.* Although the degree of weathering and 

 leaching makes it evident that the interval was protracted, the 

 valley excavation appears to have been comparatively slight, 

 so far as depth is concerned. This is true not only in the 

 region about the lower rapids, but throughout the entire 

 exposed portion of the Illinoian drift sheet. 



The erosion on the lower rapids appears to have been 

 scarcely sufficient to remove the sand filling which occurred 

 during the Illinoian stage of glaciation. It could have 

 amounted to scarcely twenty feet in depth and was mainly in 

 loose material. The limits of the erosion are determined by 

 the level down to which the loess extends. That deposit 

 appears nowhere in situ at a lower level than sixty-five to 

 seventy-five feet above the head of the rapids. Its lower 

 limits, in the portion of the valley above the rapids, are also 

 as great as seventy feet above the present stream. 



A study of tributary valleys in this region has showm that 

 the streams meandered widely and performed a large amount 

 of work, notwithstanding the shallow depth of erosion. For 

 example. Skunk river, in southeastern Iowa, at that time 

 meandered over a width of about two miles (see figure 11), 



*Proc. Iowa Acad. Sci., Vol. V, for 1897, pp. 70-80. Journal ol Geology, Vol. VI, 

 1S98, pp. 171-181 



