706 WILLIAM C. ALDEN 
The fact that almost all of the area west of the Rock and north of the 
Pecatonica had previously been reduced to slope gave opportunity 
for considerable erosion there, despite this retardation, but elsewhere 
the slow process of working back into uneroded uplands had to be 
undertaken and as a consequence only the margins of the upland 
tracts have been dissected, while the upland plains, though fairly 
well drained, are still largely untouched by erosion. 
In the relation of the rate of surficial alteration of the drift by 
leaching and oxidation to the rates of removal of the upper part of the 
drift by surface wash under these different conditions prevailing in 
different parts of the area may be found the explanation of the 
apparent discrepancies noted when attempting to apply the criteria 
for determining the age of the drift sheet. The discrepancies are 
apparent rather than real, yet they are none the less confusing. The 
difficulty is to make the proper allowances and draw the right con- 
clusions. 
In the region south of the state line where the extremes of fresh 
or little weathered drift and of drift reduced to a residual condition 
are found at the surface or immediately underlying the loess or loamy 
clay in neighboring exposures, careful examination reveals the fact 
that in many places at least where the drift is so fresh the topographic 
relations are such that the weathered upper part may have been 
removed by slope wash. ‘That it really was so removed seems clear, 
for many, if not all, of these exposures of fresh drift are on slopes, 
and where the exposures extend up the slopes as along roads one often 
finds the fresh drift disappearing under a layer of weathered drift 
higher up. In some places where the road runs along a gentle slope, 
where it does not look as though much wash had occurred, fresh drift 
is exposed at the surface on the down-hill side, while on the up-hill 
side thoroughly leached and oxidized residual till extends to the full 
depth of the shallow cut. It may be that a relation so obvious appears 
scarcely worthy of mention, yet it is one that may be readily over- 
looked in such situations as here where the erosion of the drift occurred . 
before the deposition of the loess, and the latter obscured the slight 
topographic expression that resulted. 
In the cases of the unaltered drift composing the knolls and ridges 
no evidence other than the freshness of the drift was noted which 
