THE MISSISSIPPI VALLEY. 4(35 



Illinois, is in striking contrast even with Pleasant Valley and the portion of 

 the Mississippi immediately below, though these valleys are much narrower 

 than the valley above the point where this deflection occurs. 1 



Pleasant Valley, as interpreted by Udden, appears to mark the line of 

 a preglacial stream which discharged eastward, carrying probably the 

 drainage of the district now tributary to Duck Creek, which enters the 

 Mississippi opposite the western end of this valley. Possibly it carried the 

 drainage of a valley heading near Fairport, Iowa, a few miles above Mus- 

 catine, or the course of that drainage line may have been eastward along 

 the present lower course of Rock River, reversed. Udden locates the 

 divide near Fairport because of the height of the rock bluffs at that point, 

 their elevation above the river being about 175 feet, and because of the 

 eastward slope of the valley floors of the small tributaries of the Mississippi 

 in the section immediately east of Fairport. It appears from the study of 

 these tributary valleys that the preglacial rock cutting had extended nearly 

 to the level of the present stream before entering Pleasant Valley or the 

 lower course of Rock River Valley. The principal part of the rock cutting 

 along the Mississippi below Rock Island has been in the immediate vicinity 

 of Fairport, where for a space of perhaps a mile it may have reached a 

 depth of 150 feet. The amount of rock excavation appears to be greater 

 in the divide near Fairport than in the one crossed near Leclaire, yet the 

 rock barrier has been more effectually removed at the Fairport than at the 

 Leclaire divide. This may be due in part to the more yielding nature of 

 the rock near Fairport, where it consists in the main of soft Coal Measures, 

 and in part to longer cutting at Fairport. 



The question of the date of the deflection of the Mississippi across 

 the Leclaire Rapids is of much interest, though as yet a fully satisfactory 

 solution has not 'been reached. Were the course of the present stream past 

 Leclaire the only one to be considered, a study of the work accomplished 

 by it might afford a means for estimating the time required in the excava- 

 tion compared with that of the excavation of channels whose date has 

 already been established. But the fact that the energies of the Mississippi 



'Attention has already been called to a temporary course of the Mississippi across eastern Iowa, 

 but this course seems to have been abandoned during if not prior to the Iowan ice invasion and did 

 not persist so long as these courses through the abandoned channels connecting the Mississippi and 

 Rock River valleys. 



MON XXXVIII 30 



