SURFACE DEPOSITS. 115 



material. But such terraces do sometimes appear in this 

 deposit, and are of the same character as those observed 

 in the drift and alluvium of other valleys. The accompanying 

 sketch of the valley of Soldier river, just where it joins 

 the great flood-plain of the Missouri river, shows the remains 

 of several successive terraces that were formed while the 

 two valleys were in process of deepening. 



These terraces in the Bluff Deposit, notwithstanding the 

 fact that it is newer than any other deposit except its own 

 alluvium, are certainly of the same age as the other terraces 

 of the same river that have been formed in the drift or any 

 other formation, for they all originated from the same cause, 

 and nearly or quite simultaneously. 



The evidence that this deposit was formed as sediment in a 

 fresh water lake may be summed up thus : The material is 

 very fine and homogeneous, such only as could have been 

 deposited in comparatively still waters. It contains a few 

 shells of fresh water and land mollusks, and no other.* 

 Neither does it contain any marine remains. It is therefore 

 not of marine origin; besides which, no inland deposit of 

 marine origin is known, that has, like this, occurred subse- 

 quent to the drift. The material of the deposit is essentially 

 the same as the sediment of the Missouri river at the present 

 time, as will be seen by referring to the analysis of both 

 in Prof. Emery's report on other pages. This sediment is 

 so abundant now in that river, that if it were possible to 

 throw an obstruction across its valley as high as its bluffs it 

 would become rapidly rilled with essentially the same 

 material that it originally deposited and subsequently in 

 part swept out. This is constantly illustrated in the 

 reservoirs of the St. Louis water-works, which become filled 

 with the sediment of the water taken from the river, so that 

 they must be periodically re-excavated. 



The proportion of sediment contained in the water of the 

 river in its earliest history, was probably somewhat greater 



*No true branchiate shells have been found in the deposit except a few Unios, the 

 greater part being pulmonate gasteropods. 



