DISCRIMINATION OF AGE OF GLACIAL DRIFT SHEETS 703 
two feet, while the limestone pebbles are fresh and sound. ‘There 
are also occasional eskers and knolls of fresh limestone drift rising 
5 to 25 feet above the surrounding plain of weathered drift. The 
relations are such that one might easily be led to the conclusion that 
the old drift sheet was overridden by a later advance of an ice 
sheet, which was thin and caused but little removal of the pre-existing 
drift, and which, on melting, left a scattered deposit of fresh drift upon 
the old. 
East of Rock River where the most youthful topography is found 
the extremes in degree of weathering above noted are less frequently 
seen, there being the more usual combination of oxidation to a dark 
buff or brownish color and fairly complete leaching of the fine cal- 
careous elements from the clayey matrix, but without removal of the 
limestone pebbles, though the surface of such pebbles in the upper part 
of the zone of weathering may be somewhat etched by solution. In 
some exposures the upper part of the drift is reduced to the residual 
condition and a few instances were noted where the matrix was leached 
of lime carbonate to depths of 10 to 15 feet, but this is rare. The 
average depth of surficial alteration east of the river is 4 or 5 feet. 
Were exposures more frequent in the uneroded parts of this upland 
tract the average amount of alteration might be found to be greater 
than this. 
In the tract between Rock and Sugar rivers where the topographic 
development appears considerably more mature, and between Sugar 
River and the west limit of the drift where the amount of dissection 
seemingly approximates that of the Driftless Area, the drift on the 
slopes and crest is generally very thin, often little more than scattered 
pebbles, mostly of the insoluble varieties between the loamy clay soil 
and the weathered surface of the limestone. In many places, how- 
ever, especially on lower slopes and in the valleys, considerable 
thicknesses of drift remain, sometimes 100 to 200 feet or more. 
Where even a few feet of this till are exposed the unaltered part is found 
to be the same highly calcareous drift as seen elsewhere. Tests at 
25 or 30 exposures show the matrix of the till to be leached of its 
calcium carbonate to an average depth of about 4 feet, but the lime- 
stone pebbles are not removed. ‘This depth includes the thin coating 
of brown non-calcareous loamy clay which generally overlies the drift 
