92 IOWA ACADEMY OF 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 l^apids 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 

 than the recession of a fall. It s«ems 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 

 sufiicienl 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 lowan 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 would 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 



