PREGLACIAL DRAINAGE. 311 



the course of the river since preglacial times except in the 

 vicinity of Louisville, Kentucky, where it now flows over a 

 rocky barrier constituting the Falls which obstruct navigation 

 at this point. There is, however, a deeply buried channel 

 south of Louisville filled with the coarser silt bought down by 

 glacial floods from the melting ice which reached and crossed 

 the river for some distance above. 



Coming to the Mississippi River two preglacial channels 

 are of special interest. One lies just west of Minneapolis, 

 following a wide shallow depression dotted with small lakes 

 and entering the valley of the Minnesota a short distance 

 above Fort Snelling, where the present Mississippi joins the 

 valley. This preglacial gorge is now filled with glacial debris 

 to a depth of 200 feet or more. The present Mississippi 

 after plunging over the Falls of St. Anthony occupies a narrow 

 rocky gorge, which it has worn since glacial times, to its 

 junction with the main valley. 



A similar broad, deep preglacial valley channel of the 

 Mississippi now filled with glacial debris is found west of 

 Keokuk, Iowa. This became so completely filled with till 

 that the river was forced to seek its present channel, which 

 passes over rocky rapids at Keokuk, compelling the govern- 

 ment to construct there a canal with locks for the sake of 

 navigation. (See Fig. 95). 



In Professor I. C. White's report upon Pike and Monroe 

 counties, Pennsylvania, t he gives an account of no less than 

 twenty-three channels which have been buried by glacial 

 debris. Among these that of the Wallenpaupack Creek is 

 the most striking. At present this creek empties into the 

 Lackawaxen at Paupack Falls, where it descends 260 feet in 

 a mile. But on ascending the creek two or three miles, to 

 the vicinity of Tafton, the course of a preglacial valley can 

 be easily recognized, leading into Kimball's Run, and join- 

 ing the Lackawaxen at Kimball's Station. This channel is 



t Ibid, G B , pp. 52-63. 



