Malott: The ''American Bottoms" Region 41 



drainage to changes for which the presence of the ice on the 

 west was responsible. The removal of this glacial material 

 proceeds in -two ways. One of these is the removal of the 

 silt deposits from the floors of persistent pre-glacial stream 

 valleys, and the other is the removal of deposits by the 

 trenching work of post-glacial streams. 



In the cases where the drainage lines were not changed, 

 but were temporarily laked, the process has not been so simple 

 as has often been supposed. The streams have not simply 

 sunk down into the lake-like silts and removed the deposits 

 by broadening their valleys, leaving the local unremoved por- 

 tions as terraces. Plummers and Beech creeks have been 

 at grade level for a long time, but it has been a progressively 

 changing grade level. Their present valleys are wide and 

 the stream channels meander in them. These features are 

 indicative of base level. The presence of the lacustrine de- 

 posits as local terraces of fairly uniform height here and 

 there above the present valleys indicates the Pleistocene level 

 of the streams. The Beech Creek terraces are about the same 

 height above the base level as those of Plummers Creek. The 

 similarity of the Pleistocene terraces along the minor streams 

 near where they enter White River valley suggests a common 

 cause. The large number of terraces 40 to 50 feet high, such 

 as those of Beech and Plummers creeks, are not due to mere 

 local laking of the particular valleys, but rather to a more 

 general laking. The purely local filling of these streams has 

 been insignificant. There are a number of cases, however, 

 of local filling, and the results have been far from uniform. 

 The ponding of the stream valleys in such a manner as to 

 produce a uniform efi'ect in so many cases can be ascribed 

 only to the superior height of the main stream above its 

 former base level. The occurrence of terrace remnants along 

 both forks of White River accordant in height with those in 

 the tributaries is quite common, but such terraces are not 

 so common as those in the tributaries. Thus the terraces in 

 Beech and Plummers creeks are due to the cutting out of the 

 Pleistocene filling, except at locally protected places. These 

 terraces had their cause in the great valley train of Wliite 

 River, which has since been largely removed. The removal 

 Df the Pleistocene silts, then, was not due simply to the cutting- 

 down of individual streams into their deposits, followed later 



