698 WILLIAM C. ALDEN 
the west (Fig. 3). Judging by the topography of the slopes and 
uplands alone one might say that the drift here was as old as the 
Kansan. Eastward to Rock River there is a little less relief and not 
quite so much dissection of slope, yet there is but little upland left 
and the topography looks mature. There are, however, in several 
places west of Rock River indications in the topography itself that 
the post-glacial drainage development is not as mature as appears 
from a more general view. At quite a number of places small drift 
Fic. 3.—Erosion topography on Galena and Trenton limestone in the Driftless 
Area northwest of Monroe, Wis. Very little if any of the upland plain remains not 
reduced to slope. 
dams in the valleys have diverted the drainage to one side where small 
gorges have been cut through adjacent rock spurs. These drift 
dams are small and their crests are usually well below the general 
upland level. The basins above these dams, however, are clearly not 
due to post-glacial erosion. ‘There is thus positive evidence that the 
valleys were not entirely filled by drift and subsequently re-excavated, 
and hence that the present relief is only in small part due to post- 
glacial erosion. While there is thick drift in many places in the 
valleys, the upper slopes were usually but thinly mantled with drift 
