STTBFACE FEATURES. 43 



usually steeper than they are when it is eroded out of the 

 more sandy and gravelly drift of a part of northern Iowa. 

 If the valley is eroded out of the Bluff Deposit, its sides may 

 be either gently sloping, or even occasionally so steep that a 

 man can climb them only with great difficulty. The peculiar 

 physical properties of this strange deposit are described 

 upon subsequent pages. 



The majority of the streams that constitute the western 

 system of Iowa drainage run, either along the whole or a 

 part of their course, upon that peculiar deposit described in 

 another part of this report under the head of Bluff Deposit. 

 As these streams seldom reach any more solid material, their 

 banks and beds are always muddy; the stream itself occupy- 

 ing a narrow, tortuous ditch in usually a narrow flood-plain, 

 and having banks so steep and muddy that they are almost 

 impassable to man or animals except by bridges. Other 

 portions of some of these streams and the whole of some 

 others of the western system, as well as the upper, branches 

 of all those of the eastern system, rest upon and within the 

 Drift Deposit alone. 



As the last named deposit is not quite so uniform in its 

 composition in all parts of the State as the Bluff Deposit is, 

 the characters of the beds and banks of the streams which 

 run upon it are a little different in different parts of the State. 

 Thus as sand and gravel are more prevalent in the drift of 

 northern than in that of southern Iowa, the beds and banks 

 of the streams in the first named portion of the State, are 

 frequently pebbly and firm, while those upon the same 

 deposit in southern Iowa are sometimes nearly as muddy, 

 and possess much the same characters as those do which run 

 through the Bluff Deposit. The immediate beds, however, of 

 all streams upon the drift are usually gravelly; the gravel 

 being the residue from the large amount of the finer drift 

 material that has been removed by the streams to form its 

 valleys. 



From the foregoing description of the beds and banks of 

 the streams of western Iowa, it will be properly inferred that 



