DISCRIMINATION OF AGE OF GLACIAL DRIFT SHEETS 1707 
might be taken as indicative of their later origin. In these cases it 
seems that, even if the weathered surficial part has not been removed 
by erosion, the configuration and structure of the slight elevations may 
explain their lack of weathering. In the first place, the rounded sur- 
faces of the knolls and ridges rising above the plain shed most of the 
water falling on them much more readily than do the surrounding 
plain tracts, especially where the latter have such long gentle slopes 
such as characterize the area in question. The gravel of the knolls 
is particularly loose and open so that such of the meteoric water as 
penetrates, percolates through and out of the gravels very readily and 
has but a comparatively small contact with the surface of the cal- 
careous particles or pebbles. This is true not only because of the 
large interspaces allowing rapid passage of water, but also because the 
total amount of surface exposed by the pebbles is far less than that 
where the material is finely comminuted as the rock-flour matrix of 
the till. The result is that while the waters slowly percolating down- 
ward into the calcareous till on the plain tract may have relatively 
large solvent action, so as to remove the finer particles and even the 
limestone pebbles to the depths indicated above, the limestone pebbles 
of the knolls and ridges continued unaltered beyond a small amount 
of cementation in the lower parts of the deposit by lime carbonate 
carried down from above. On the nearly flat or gently sloping plains 
surface wash seems to have been reduced to the minimum, while the 
slow process of leaching out the calcareous elements and oxidizing 
the ferruginous constituents had its maximum effect. Here the till 
slowly rotted down. Besides the part which was removed by solu- 
tion, some of the finest silt may have been carried away by surface 
wash. ‘This was deposited in the valleys while much of the insoluble 
part of the clay and the more resistant sand grains and pebbles, the 
residuum of the leaching process, was left on the unaltered drift. 
In those situations, where the topography and the constitution of 
the drift were such that the rate of leaching and oxidation exceeded 
the rate of removal of the weathered part by wash, such a residual 
layer was developed. Where the rate of removal was more rapid or 
where the two processes nearly balanced each other, fresh drift is 
exposed at the surface. Where the relations were such that the rate 
of removal lagged behind the process of alteration, but not so far that 
