DISCRIMINATION OF AGE OF GLACIAL DRIFT SHEETS 697 
certain differences in the topography, amount of erosion, and depth 
and degree of surficial alteration of this drift in different parts of the 
area, which are very confusing when one attempts to apply the usual 
criteria for the discrimination of the age of drift sheets and which 
might be, and indeed have been considered indicative that this drift 
represented more than one stage of glaciation. It is the purpose of 
this paper to point out the differences and to suggest an interpretation 
of them which may perhaps find application in other areas. - 
ra 
Fic. 2.—Erosion topography on Galena and Trenton limestones thinly covered 
with Illinoian drift three miles east of Monroe, Wis. Very little if any of the upland 
plain remains not reduced to slope. 
The first of these differences to be considered are topographic. 
The character of the valley bottoms is to be neglected as due to sub- 
sequent filling, only that of the slopes and uplands being considered. 
In that part bordering Sugar River and extending thence westward to 
the Driftless Area the surface is much dissected with reliefs of 200 to 
300 feet. At no places does a square mile of the upland remain and 
the drift-mantled slopes show drainage lines at intervals of 10 to 30 
rods, which often notch the crests of the divides (Fig. 2). The 
topography as a whole closely resembles that of the Driftless Area to 
