M1N0K TRIBUTARIES OF THE MISSISSIPPI. 477 



The breadth of the valley excavation seems also to support the view- 

 that it was produced under a moderate or low rather than a high stream 

 gradient. It seems improbable that a stream which was cutting- down 

 rapidly could have formed a valley several miles in width such as appears 

 along the line of the Mississippi below Muscatine, or along the line of other 

 preglacial valleys occupied by the Mississippi above Clinton. So far as 

 known, the preglacial channels under consideration have furnished no posi- 

 tive evidence of the existence of narrow r trenches cut below the general 

 level of their rock bottoms. A few rock shelves have been found extending 

 out a mile or more, as at Quincy and St. Louis, but these occur at points 

 where the river has, in comparatively recent time, been encroaching upon 

 the rock bluffs, and may, therefore, be a more recent product than the deeper 

 part of the valley. In the present state of knowledge it certainly seems 

 unsafe to cite them in evidence of a preglacial gradation plane standing- 

 above a lower part of the rock bottom. 



MINOR TRIBUTARIES OF THE MISSISSIPPI. 



Since the Illinoian invasion encroached only a few miles upon the dis- 

 trict west of the Mississippi (in the southeastern part of Iowa), it has not 

 greatly influened the course of the western tributaries. The larger western 

 tributaries in southeastern Iowa, as above noted, were temporarily deflected 

 southward along a course immediately outside the Illinoian ice border, but 

 these have regained the courses opened prior to the Illinoian invasion. The 

 tributaries here discussed are, therefore, mainly on the eastern side of the 

 Mississippi. The discussion begins in northern Illinois and the streams are 

 taken up in order southward. 



APPLE RIVER. 



A few streams lie wholly within the driftless portion of northwestern 

 Illinois and adjacent parts of Wisconsin and may, therefore, be passed by, 

 since they maintain their preglacial courses. It is, however, necessary to 

 mention one stream, Apple River, which lies almost wholly within the limits 

 of the Driftless Area, but which has received a marked accession of drainage 

 because of the blocking of a preglacial tributary of the Pecatonica. This 

 diversion occurs just below the village of Millville, as shown in PI. XII. 

 For about 3 miles below Millville the stream is in a gorge but little wider 

 than its bed. The small preglacial Apple River is then entered. 



