114 PHYSICAL GEOGKAPHY. 



filling of the depression with sediment must have been com- 

 pleted very early after the recedence of the glaciers, because, 

 as shown by the present height of the surface of the deposit, 

 it reached as high a level as that of the highest surrounding 

 land, before the Missouri river, which emptied into and flowed 

 from the lake, had deepened its channel below it sufficiently 

 to commence its drainage.* 



From that time until the deepening of the Missouri river 

 valley had well progressed, the surface of the deposit no 

 doubt existed as a broad, undrained marsh. As the deepen- 

 ing of the valley below the then filled lake progressed during 

 the remainder of the Terrace epoch, the river readily swept 

 out enough of the soft material which its own waters had 

 previously deposited to form its valley in that also, and at 

 the same time caused the deepening of those of the tributary 

 streams. The cutting out of the great valley in this deposit 

 was accomplished, as in the deepening of all valleys, by the 

 vibration from side to side of the stream, alternately occupy- 

 ing every square foot of its flood-plain, and at times hugging 

 the sides until the bluffs were left as steep as they would 

 stand. Cross-cutting by tributary drainage gave the bluffs 

 their serrations, and rains and frosts completed the rounding 

 of their summits and slopes so that they now appear, as 

 they really are, like miniature mountain ranges of dried 

 mud. 



During the deepening of the valley of the great river, few 

 real terraces seem to have been formed in the Bluff Deposit, 

 compared with the number usually found in the valleys 

 of those streams, which have been eroded out of other 



*This deposit now extends a little to the eastward in some places, of what is now the 

 Great Watershed in Iowa, and appears upon some of the upper branches of Raccoon 

 river. This may have resulted however, by the gradual extension upward, even 

 beyond the border of the ancient lake bed, of those branches necessitated by the deep- 

 ening of the principal valleys which took place during the Terrace epoch. It is 

 shown also by the levelings of the Chicago and Northwestern Railway, that at some 

 points the surface of the Bluff Deposit is a few feet higher than the surface of the drift 

 is where that railway crosses the Great Watershed. Further investigations are needed 

 to ascertain what the actual relative level of the surface now is there, and what 

 changes have taken place since the Bluff Deposit was all accumulated, 



