356 



HISTORICAL GEOLOGY 



water is much greater, and that in a short time, geologically con- 

 sidered, the American Falls will be dry. 



The drainage of the upper Ohio River Basin has been well-nigh 

 revolutionized as a result of glaciation. By comparing the pre- 

 Glacial drainage map (Fig. 224) with one showing present-day 

 drainage, the principal changes will be readily understood. The 

 pre-Glacial upper Ohio flowed northward from Beaver, Pennsyl- 

 vania, instead of southward, as 

 at present, and, between Beaver 

 and Sharon, the direction of pre- 

 Glacial drainage has been ex- 

 actly reversed. Also, all of what 

 is now known as the drainage 

 area of the upper Allegheny 

 River passed northward through 

 two pre-Glacial streams. The 

 drainage changes were caused by 

 ice occupancy and deposition of 

 heavy drift across the north- 

 western portion of Pennsylvania. 

 Another well-studied example 

 of important drainage change is 

 shown by the accompanying 

 map (Fig. 225) of part of north- 

 ern Illinois. The pre-Glacial 

 Rock River flowed southward 

 into the Illinois River, instead 

 of southwestward into the Mis- 

 sissippi as at present. 



Even such large rivers as the 

 Mississippi and Missouri were 

 sometimes notably shifted out of their pre-Glacial channels by 

 the invasion of the ice sheets. Thus the Missouri River, which 

 formerly followed what is now the James River Valley in eastern 

 South Dakota, was forced many miles westward to its present 

 course across the state. 



The above cited cases are sufficient to illustrate the general 

 principles of drainage modifications due to glaciation, the two 

 chief factors having been (1) actual presence of the ice or (2) 

 heavy drift filling in pre-Glacial valleys. 



Fig. 225 

 Pre-Glacial drainage (dotted lines) 

 of a part of northwestern Illinois. 

 (Modified after Leverett.) 



