SURFACE FEATURES. 55 



have very little length, for the adjacent upland is mainly 

 drained by creek-branches of the Big Sionx, some of the upper 

 subdivisions which reach aronnd behind the faces of the 

 bluffs, and drain the surface-water back from their very 

 brows, and carry it by their own channel to the main stream. 



At rare intervals, along about fifteen miles above the mouth 

 of the river, the Cretaceous strata are found exposed in the 

 face of the bluffs of the Iowa side. No other strata are 

 exposed along all that part of the valley of the Big Sioux 

 which borders our State, except a single exposure of Sioux 

 Quartzite at its extreme northwestern corner. None of these 

 strata, as before remarked, can be said to have characterized 

 that part of the valley below the vicinity of Sioux Falls. 

 Some good mill-sites may be secured along that portion of the 

 Big Sioux which borders Lyon county, but below this the fall 

 will probably be found insufficient or the location for dams 

 insecure. 



With the exception of the drift- streams of northwestern 

 Iowa, the waters of which are usually clear, that of the 

 rivers hitherto described has generally a stained appearance, 

 even at low- water. When flooded, all of them carry out 

 much sediment suspended in the water, which sediment is 

 derived mainly from the .surface deposits through which they 

 flow, but none of them can be said to be constantly muddy 

 as the water of the Missouri river is. All these streams are 

 comparatively small; but an idea of their actual and relative 

 length may be obtained from the geological map accompany- 

 ing this report. 



Tlie Missouri river. The only discussion of this remark- 

 able river that can be given in this report, must relate alone 

 to the characters of the stream and its valley along that part 

 of its course which goes to form the western boundary of 

 Iowa. We have no information of the amount of water 

 annually flowing past the State of Iowa in this great river, 

 because no detailed hyclrographic survey of it has been 

 made along our border. 



It is, however, one of the muddiest streams on the globe, 



