704 WILLIAM C. ALDEN 
north of the state line. ‘The leached top of the till is usually oxidized 
to a buff or brownish tint, but beneath this there is little or no evidence 
of alteration. At a few of the exposures basic crystalline pebbles in 
the weathered zone are considerably disintegrated but most of the 
crystalline and limestone pebbles are fresh, clean, and sound, showing 
little more alteration than in the late Wisconsin drift. Nowhere in 
this part has the residual condition of weathered drift been found so 
well developed or at least so well preserved as it is near Rockford, 
Illinois. 
Taken altogether there is in this area of pre-Wisconsin drift a 
rather confusing lot of differences which must be harmonized if one is 
to reach any definite conclusion as to the relative age of the drift 
exposed. The unaltered drift is practically identical in character and 
lithologic composition throughout, the only differences in the latter 
being such as would be expected from an ice sheet moving in a 
westerly direction across the different rock formations which underlie 
the drift in different parts of the area or immediately to the eastward. 
The only questionable element is the derivation of quartzite ee 
which need not be considered in this connection. 
The differences in the present topography appear to be due very 
largely to differences in the pre-glacial topography which in the north- 
west was only mantled by the drift and not entirely obliterated; and 
in part to a retardation of erosion in certain parts of the area due to 
the relocation of Rock River, following the melting of the Illinoian 
ice sheet. 
The character of the pre-glacial topography depends largely on the 
relations of the limestones and sandstones to the zone of erosion. ‘The 
greater part of the area, that is, the slopes and uplands, is underlain 
by the Galena and Trenton limestones. These and the underlying 
formations have very low dips, curving over the low broad axis of the 
southern end of the Wisconsin island. Going toward this island, that 
is, up the low dip, the edges of the formations are beveled off and 
successively lower formations rise in the zone of erosion. ‘The slopes 
developed on the limestones were rather long and gentle, but where 
the friable St. Peter sandstone rose into the higher levels there was 
more rapid erosion, a steepening of the slopes, and a greater amount 
of dissection was accomplished. This standstone rose well up in 
