76 THE JOURNAL OF GEOLOGY. 



tion we find a surface layer of greater or less depth, the boulders 

 of which are hard and fresh, and if beneath this we find another 

 layer of drift, the stony material of which is largely disintegrated, 

 at least in its upper parts, we have good evidence that the 

 surface bearing the disintegrated boulders was exposed for a 

 considerable length of time before the deposition of the over- 

 lying drift, which carries fresh boulders. Since the disintegration 

 of boulders is only one phase of weathering, the limitations 

 of this argument are identical with those already noted in 

 connection with the general argument from differential 

 weathering. 



(7) Differential Siibaerial Erosion. If the drift deposited by 

 one ice-sheet were to be exposed for a considerable interval of 

 time, and if the ice in its subsequent advance failed to reach the 

 limit of its first invasion, the two areas should show different 

 amounts of subaerial erosion, since the one has been exposed to 

 the action of air and water much longer than the other. The 

 line which marks the limit of the later ice invasion should be the 

 line of more or less sudden transition from an area without, 

 where stream erosion has been greater, to an area within, where 

 stream erosion has been less. 



The point here made can not be met by the suggestion that 

 the greater erosion of the outer area was effected by the water 

 issuing from the ice which had retreated to the position now 

 marked by the border of the area of the lesser erosion. So far 

 as we know, such waters would be depositing, not eroding. 

 Furthermore, much of the erosion of the outer area would have 

 such relation to drainage lines that waters issuing from the ice 

 could never have reached the localities where it is shown. 



If the outer and older drift be found to have suffered ten 

 times as much stream erosion as the inner and newer, it is fair to 

 assume that it has been exposed something like ten times as 

 long, if the conditions for erosion are equally favorable in the 

 two regions. The argument has especial weight if it can be 

 found that beneath the newer drift the surface of the older is 

 such as to indicate that it was deeply eroded before the newer 



