SURFACE FEATURES. 51 



The whole course of Soldier river , except that part which 

 traverses the great flood-plain, is through the region occupied 

 by the Bluff Deposit. It has cut its valley down through this 

 deposit to the drift, along a great portion of its course but 

 nowhere far into it. It has no exposure of strata along its 

 entire course, and is consequently, a characteristic stream of 

 the surface deposits unmodified by underlying strata. 



Little Sioux river. Under this head are included both 

 the main and west branches of that stream, together with 

 Maple river, which is also one of its branches. Like Soldier 

 river, both the West Fork and the Maple run their entire 

 courses — except their upper branches which are mere prairie 

 creeks upon the Drift Deposit — through the region occupied 

 by the Bluff Deposit, and are so similar to the Soldier in all 

 respects, that they need no separate description. The main 

 stream, however, has its rise near the northern boundary of 

 the State, and runs a great part of its course upon the Drift 

 Deposit alone, before it enters the region occupied by the 

 Bluff Deposit, which it does in the southern part of Cherokee 

 county. That portion- of the valley of the Little Sioux, from 

 its source to the place where it enters this region, may be 

 regarded as a typical drift-valley as they occur in Iowa, for 

 nothing but this deposit is to be seen within or around it, and 

 so far as is now known, it is not at all modified by the under- 

 lying strata whatever they may be. 



The two principal upper branches near its source in 

 Dickinson and Osceola counties, are merely small prairie 

 creeks with gravelly beds and banks, and shallow, indis- 

 tinctly defined valleys. Upon entering Clay county the 

 valleys begin to have considerable depth, and after their 

 confluence there, the valley soon reaches a depth of about one 

 hundred and fifty feet. The depth continues to increase so 

 that the part of the valley that has a westerly course along 

 and near the boundary line, between Clay and Buena Vista 

 counties, reaches a depth from the general prairie level, 

 which is here quite distinct, of nearly two hundred feet. The 

 valley here apparently cuts across the ridge which constitutes 



