74 STUDIES FOR STUDENTS 



cases that can be held to be exempt from this suspicion are 

 terraces that can be shown to be remnants of the original glacial 

 flood plain and can be discriminated demonstrably from rem- 

 nants of some one of the many flood plains that succeeded this 

 original one in the history of the subsequent erosion of the 

 valley deposits. Except quite near the former ice edge, where 

 the true glacial plain is demonstrably connected with a distinctive 

 head at the ice edge, this discrimination is practically impos- 

 sible. In many cases the terraces themselves bear characters 

 that awaken the suspicion that they do not represent the true 

 glacial plain. In many cases they do not stand at consistent 

 heights, and in many others they bear a suspicious config- 

 uration. 



In the immediate vicinity of the ice edge from which the 

 gravel trains originated, it is possible to identify with confi- 

 dence the original glacio-fluvial plain, if due circumspection is 

 exercised, but I think that it is only within a few miles of the 

 parent ice border and in a few special cases of other types that 

 the conditions are favorable for a trustworthy reference of 

 buried relics, to the ice age. 



Adjustment plaijis as sources of deception. — The chief source of 

 misplaced confidence by even glacialists of ability and experi- 

 ence lies in mistaking a post-glacial adjustnie?it plaifi for a true 

 glacio-fluvial plaiji. It is a familiar observation that the true 

 glacial flood plain rises rapidly as its head at the ice edge is 

 approached. There is for this the obvious reason that the 

 waters issuing from the ice edge are usually heavily overloaded 

 with detritus, and on this account they build the stream bot- 

 toms up to a high gradient. When the ice retires and the 

 waters become purer a readjustment of the gradient follows. 

 This is accomplished usually by cutting down the high gradient 

 at the head and redepositing the material so removed at some 

 point of lower gradient below. Later this adjustment becomes 

 the subject of a new adjustment by which the gradient is again 

 lowered in the upper part, and the material removed in so doing 

 is redeposited still farther down the valley, and so on by a pro- 

 gressive series of partial readjustments until the whole gradient 



