2 FRANK LEV E RETT 



being at Montrose Island and the foot a short distance above 

 the river bridge at Keokuk. The total descent is 22.17 feet, or 

 very nearly two feet per mile. The rate of descent is greatest 

 in the lower part, there being a fall of about 4^ feet in the 

 lower mile and nearly eight feet in the lower two miles. From 

 Greenleaf's 1 report on "Water Power of the Mississippi and 

 Tributaries," the following data are obtained. " In the first 4800 

 feet from the lower lock there is a rise of 4.21 feet, then 2.22 

 feet in the next 3600 feet, and 1.67 feet in the succeeding 3600 

 feet to the middle lock, making the fall in ordinary low water 

 from a point opposite the middle lock to the foot of the rapids 

 8.1 feet." Above this part, the fall, though not uniform, is less 

 definitely broken into rapids and pools than in the upper rapids. 

 Indeed, there appears to be a rock floor forming the river bed 

 throughout the entire length of the lower rapids. 



Immediately above the head of the lower rapids a deep pre- 

 glacial channel appears, whose floor, as shown by several borings, 

 is 125 to 135 feet below the low-water level of the river. This 

 is filled mainly with blue bowlder clay up to about the level of 

 the river bed. Sand, however, in places, extends to a depth of 

 nearly sixty feet below the surface of the river at low water, as 

 shown by the bridge soundings at Ft. Madison and Burlington. 

 A pool extends from the head of the rapids up to the vicinity 

 of Ft. Madison, nine miles. The depth of the pool in places 

 exceeds twenty feet at low-water stage, thus extending to about 

 that distance below the level of the rock surface in the river 

 bed at the head of the rapids. 



Below the rapids the river for four miles is in a narrow val- 

 ley, in which the depth of the drift-filling is not known. It there 

 enters a broad preglacial valley, which has been found to con- 

 stitute the continuation of that occupied by the river above the 

 rapids, and which no doubt was excavated to a corresponding 

 depth, though as yet no borings have been made which reach 

 its rock floor. The comparative size of the valley of the Mis- 

 sissippi in its new channel across the lower rapids, and the par- 



1 Tenth Census of United States, 1880, Vol. XVII, p. 60. 



