92 
IOWA ACADEMY OP SCIENCES. 
abrupt face. A large part of the slope is so gradual that it 
has been brought under cultivation. When it is considered 
that the bluff is composed mainly of a firm limestone, the 
height of the rock portion ranging from fifty up to 150 feet, 
with an average height of nearly 100 feet, the prevalence of a 
moderate slope must indicate a long period of excavation. 
But little is yet known concerning the manner in which the 
rock barrier has been cut away, whether by the recession of a 
fall or by the present process of slow cutting across its whole 
breadth. The fact that the old valley below the rapids was 
filled with drift about to the height of the highest part of the 
rock barrier, lends support to the view that there has been a 
slow cutting down of the entire width of the barrier, rather 7 
than the recession of a fall. It seems scarcely probable that 
the till beneath the stream was scooped out to a much greater 
degree below the rock barrier, in the early stages of excava- 
tion, than at the present day. 
COMPARISON WITH THE UPPER RAPIDS. 
The work performed in cutting away the rock barrier, at the 
lower rapids, appears to be several times as great as at the 
upper rapids. In the latter the rock excavation has not been 
sufficient to remove the prominent parts of the barrier. It 
scarcely amounts to an average cutting ten feet in depth. In 
the rapids under discussion the barrier is estimated to have 
suffered a rock excavation to a depth of nearly 100 feet, or 
about one fourth of a cubic mile. This difference in amount of 
work accomplished is readily accounted for by the earlier date 
at which the lower rapids began excavation. The excavation, 
as shown above, appears to have begun soon after the Kansan 
stage of glaciation, while the excavation at the upper rapids 
appears to have set in after the Illinoian and t > have been 
mainly accomplished since the Iowan stage of glaciation. 
THE LOWER RAPIDS AS A CHRONOMETER. 
When this investigation was entered upon by the writer, 
hopes were entertained that the channel across the lower 
rapids vrould furnish a valuable chronometer for determining 
the time since the Kansan stage of glaciation. But from what 
has been shown it is evident that the determination of the 
time is at present very difficult, if not impracticable. It may 
be thought that this channel will furnish a chronometer for 
