86 
IOWA ACADEMY OP 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 Iowan 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 shown 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 2), 
*Proc. Iowa Acad. Sci., Vol. V, for 1897, pp. 70-80. Journal of Geology, Vol. VI, 
1898, pp. 171-181 
