CONCERNING CERTAIN CRITERIA FOR DISCRIMINA- 
TION OF THE AGE OF GLACIAL DRIFT SHEETS 
AS MODIFIED BY TOPOGRAPHIC SITUATION AND 
DRAINAGE RELATIONS? 
WILLIAM C. ALDEN 
Between the limits of the drift sheets of Wisconsin age in southern 
Wisconsin and northern Illinois and the Driftless Area there is exposed 
a deposit of glacial drift of pre-Wisconsin age, having certain 
characteristics and relations which are of some significance in 
their bearing on the question of the criteria to be regarded as 
indicating difference of age in glacial drift sheets—a question of 
prime importance in the study of the Pleistocene deposits. 
The drift under discussion mantles the slopes and uplands on 
either side of Rock and Sugar rivers and extends thence westward to 
the Driftless Area (Fig. 1). In southern Wisconsin the writer’s 
observations were extended westward to the limit of the drift but in 
Illinois they were confined to a belt ten to twenty miles wide on each 
side of Rock River. 
The filling of the main valleys is, in part at least, composed of out- 
wash from the glaciers of the Wisconsin stage and this filling is not 
included in the present discussion. The upland drift consists prin- 
cipally of a sheet of moderately compact, highly calcareous clayey till 
which is practically identical in character and lithologic composition 
throughout the area examined by the writer. 
In places, especially near the surface, the drift is loose and sandy. 
About 80 to 85 per cent. of the pebbles and bowlders imbedded in 
this till, as shown by many observations, and by the average of 71 
analyses made by counting, are from the local limestones. About 
3 per cent. are sandstones, shales, and quartzites, which are probably 
mostly of local origin, and about 14 per cent. crystallines of Canadian 
« Published by permission of the Director of the U. S. Geological Survey. Read 
before the Geological Society of America, Baltimore Meeting, December 30, 1908. 
694 
