SURFACE DEPOSITS. 117 



been known as lakes to civilized man, but tributaries of the 

 St. Lawrence river would have traversed the regions they now 

 occupy. 



Primary Origin of the Bluff Material. Ascending the 

 Missouri river, we find in Nebraska, Dakota, and even in 

 northwestern Iowa, the source from which the material of the 

 Bluff Deposit was derived. Stretching from here far away 

 towards the Rocky mountains, and bordering the great river 

 on either side, is an immense region occupied by the most 

 friable formations on the continent — those of Cretaceous and 

 Tertiary ages. Seeing these we at once cease to wonder that 

 the waters of the Missouri are muddy, because it is so evident 

 that they could not be otherwise. The Tertiary strata are 

 largely silicious, and the Cretaceous are scarcely less so, but 

 a part of the latter are not only calcareous, but much of it is 

 very nearly pure chalk. It is from the last named strata that 

 the Bluff Deposit has derived its nearly ten per cent of carbo- 

 nate of lime. All these friable strata are even now furnishing 

 abundant sediment to the streams that flow into the Missouri 

 river, but at the close of the Glacial epoch, the fine sediment 

 was, if possible, still more abundant, because then the whole 

 region was strewn with thegrindings fresh from those "mills 

 of the gods " — the glaciers. 



5. OTHER ANCIExNT LACUSTRAL AND MARSH DEPOSITS. 



Besides the Bluff Deposit just described, there are other 

 post-glacial deposits found along the borders of the valleys 

 of the Mississippi, and some of the rivers of the eastern part 

 of the State. So far as yet observed, these are confined 

 within the river valleys or to their immediate vicinity, and 

 evidently originated at different periods in the history of 

 their erosion from the general level that existed at the close 

 of the Glacial epoch, because they are found occupying 

 different levels, varying from almost as high as the general 

 level of the uplands down to a level only a few feet above 

 the highest floods of the rivers at the present time. Some of 



