SUEFACE FEATUEES. 35 



appearance with the cultivation of the country and the growth 

 of trees upon former prairie surfaces. 



3. DRAINAGE SYSTEM. 



Those two great rivers, the Mississippi and Missouri form 

 respectively the eastern and western boundaries of the State, 

 and receive respectively the eastern and western drainage 

 of it. 



The drainage of Iowa was very properly divided into two 

 systems by Prof. Whitney, the one comprising the tributaries 

 of the Mississippi, and the other those of the Missouri; those 

 of the eastern system running in a southeasterly direction, 

 and nearly at right angles with those of the western system, 

 which latter most generally run in a southwesterly direction. 

 I cannot however agree with Prof. Whitney that the courses 

 of these streams were respectively predetermined by a series 

 of folds or flexures in the strata, the courses of which coincide 

 with those of the streams ; for our investigations have satis- 

 fied us that such flexures do not exist in such positions. The 

 reasons for this opinion may be found in the chapter on 

 General Geology, in that on surface deposits, and in the 

 long plate of sections accompanying this report. In the first 

 of these it is shown that the folds or flexures of strata which 

 really do exist have different directions from the courses of 

 those streams; and in the second it is shown that the streams, 

 particularly those of Western Iowa, have through a large 

 part, and in some cases through the whole of their courses, 

 eroded their valleys out of the incoherent surface deposits 

 alone, and would have run in any other direction just as well 

 if a similar slope had been given to the surface they traverse. 

 To assume the existence of folds coincident with the courses 

 of the streams, it seems necessary to assume also that the 

 disturbance which caused them took place immediately upon 

 the close of the Glacial epoch, and that the drift surface which 

 the waters there rested upon took part in the same folding. 

 Of this we have no corroborative evidence, but we have much 



