696 WILLIAM C, ALDEN 
derivation. The limestone pebbles are mostly from the Galena and 
Trenton formations,’ which underlie the drift throughout the greater 
part of the area, and partly from the Niagara limestone whose west 
margin lies a few miles east of the Wisconsin terminal moraines. The 
principal difference in the lithologic composition in the several parts 
of the area consists of a decrease in the content of Niagara limestone 
as one goes westward farther from the margin of that formation, with a 
reciprocal increase in the percentage of Galena and Trenton. Niagara 
pebbles are, however, seen in almost every exposure westward to the 
limit of the drift, the average shown by analyses being nearly 24 per 
cent. of the whole. The presence of this Niagara constituent with an 
occasional pebble of Devonian rock shows clearly the easterly deriva- 
tion of the drift which is in consonance with the evidence of direction 
of the ice movement as shown by striae observed at several places. 
The striae, as shown upon the map, have bearings shifting from 
S. 75° W. east of Rock River and near Rockford, to due west at 
Beloit, N. 62° to 75° W. near Janesville, and N. 35° to 75° W. south- 
east of Monroe, as the ice closed in about the southeast side of the 
Driftless Area. At no place has the writer observed this till sheet 
to be overlain by any deposit other than sand, clayey loam, and loess, 
which could not have been, or probably was not derived from this 
drift by weathering or erosion. 
It is quite possible, if not probable, that this till is underlain in part 
at least by older drift, but that question is not here under discussion. 
So far as the writer has observed there is no good ground for differ- 
entiating the drift exposed at the surface into deposits of more than 
one stage of glaciation. No intercalated soils, weathered zones, 
vegetal or other fossiliferous deposits are known to separate one part 
of this drift from another. Such as have been penetrated in wells or 
otherwise located occur beneath considerable thicknesses of this drift 
and probably represent an earlier stage of deglaciation. 
The drift under discussion is clearly older than that bordered by 
the Wisconsin terminal moraines on the north and east, and it is 
believed to correspond in age with the Illinoian drift sheet which 
covers so large a part of Illinois farther south. ‘There are, however, 
1 The Paleozoic formations are referred to in this article by the names in use for 
them in eastern Wisconsin by the Wisconsin Geological and Natural History Survey. 
