116 PHYSICAL GEOGKAPHY. 



than it is now, and any lake-like expansion that may have 

 existed in it at that time mnst have become so quickly filled 

 as to have occnpied an insignificant part of the time-history 

 of its valley, although the act was an important one in that 

 history. It seems not improbable that the broad lake that 

 occnpied a part of what is now Western Iowa, was mainly 

 filled with sediment while yet the glaciers hovered around 

 the npper conrse of the Missouri river, and were there grind- 

 ing the material which served for the filling. 



That there were formerly other similar lake-like expansions 

 of the Missouri and lower Mississippi rivers, and that they 

 were filled with the same kind of sediment, is not improbable, 

 for it was then in the condition that Carl Bitter aptly terms 

 an " unfinished river." Indeed, it is certain that there were 

 such, for Professor Hilgard, State Geologist of Mississippi, 

 speaks, in his report on the geology of that State, of a 

 similar deposit there, occupying a similar relative position to 

 the Mississippi river that ours does to the Missouri. It is not 

 thought possible that our Bluff Deposit, and that of the State 

 of Mississippi, could have ever been continuously connected 

 as the same deposit, although they were, doubtless, exactly 

 contemporaneous; but it is quite probable that other lake- 

 like expansions besides these two existed between them in 

 the course of the river, their deposits being all alike because 

 all were formed from the same sediment and furnished by the 

 same river from the same sources. 



Such remains of lakes or lake-like expansions in the 

 courses of rivers are common in all parts of the world, and 

 as each river system became " finished," to use Hitter's term, 

 by the deepening of their valleys, the sedimentary filling 

 became dry land. The filling was, of course, most rapid in 

 the case or the muddiest rivers, and those which flowed over 

 formations that are not readily disintegrated, could contain 

 but little sediment. Therefore, their lakes are not filled. If 

 such a river, as the Missouri had emptied into the great 

 northern chain of lakes, they would have become so com- 

 pletely filled with its sediment that they would never have 



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