LOWER RAPIDS OF THE MISSISSIPPI 2 1 



points where the river in rounding a curve has recently encroached 

 upon its bluff there is not an 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 

 50 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 bar- 

 rier, lends support to the view that there has been a slow cutting 

 down of the entire width of the barrier, rather 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 excavation, 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, or one fortieth of a cubic mile. In the rapids 

 under discussion, the barrier is estimated to have suffered a rock 

 excavation to a depth of nearly one hundred feet, or about one 

 fourth of a cubic mile. This difference in amount of work accom- 

 plished is readily accounted for by the earlier date at which the 

 lower rapids began excavation. The excavation, as shown above, 

 appears to have been begun soon after the Kansan stage of 

 glaciation, while the excavation at the upper rapids appears to 

 have begun after the Illinoian and to have been mainly accom- 

 plished 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 



