76 STUDIES FOR STUDENTS 



down the valley from the head of the gravel train, the later is 

 the readjustment. 



It will be seen from this that even where the valley train of 

 modified drift is well preserved and is clearly the highest plain 

 of the kind in the valley at that point, it may be an error to 

 refer it to strict contemporaneity with the ice presence. For 

 aught that we know, it may have been formed several hun- 

 dred, and perhaps a few thousand, years after the ice retired. 

 The difference is trivial in geologic terms, but it may be of much 

 moment in human history. No readjustment of this importance, 

 I think, is known to have taken place naturally during the inva- 

 sion and wide occupancy of this continent by the white race. 

 We should have to draw our geologic lines closer than this to 

 fix the not unimportant period of our people by geologic data. 



Without going farther into the refinements of the case it 

 appears — 



1. That even where the valley train is well preserved and 

 constitutes a broad plain or extensive terrace, and is the highest 

 deposit of the kind in the valley at that point, and its material is 

 indistinguishable in kind and structure from true glacio-fluvial 

 material, there is danger of appreciable error in referring it to 

 the ice age. In the case of relics imbedded in it there are the 

 added dangers already referred to, unless some special evidence 

 excludes them. 



2. That where nothing but isolated terraces are involved 

 much doubt as to the correlation of these is liable to arise legiti- 

 mately. 



3. That where the terraces are below the maximum height for 

 the locality, there is definite ground for suspicion. 



4. That in all cases where there is any chance that the river 

 in its degradational stages has meandered over the locality, the 

 possibility of scour-and-fill gives grounds for serious suspicion. 

 The possibilities of natural deep intrusion by this prevailing 

 process are only limited by scores of feet; indeed, only by a 

 hundred feet, or more. 



The general purport of these combined sources of possible 

 error is to emphasize the very tmpromisi?ig character of valley grav- 



