Within the general area covered by the ice, there is an area of several thousand square miles, mainly in southwestern Wisconsin, where there is no drift. The ice, for some reason, failed to cover this _driftless area_ though it overwhelmed the territory on all sides.
[Illustration: WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V., PL. XXXIII.
The North American Ice Sheet, at the time of maximum development.]
[Illustration: WISCONSIN GEOL. AND NAT. HIST. SURVEY. BULLETIN NO. V., PL. XXXIV.
View from the north of the Owl's Head, a hill two miles north of east of Merrimac, which has been shaped by the ice. The side to the left is the stone side.]
Plate II shows the limit of ice advance in the area here described. The region may have been affected by the ice of more than one glacial epoch, but the chief results now observable were effected during the last, and the others need not be considered.
As the edge of an ice sheet, or as the end of a glacier, retreats, the land which it has previously covered is laid bare, and the effects which the passage of the ice produced may be seen. In some cases one may actually go back a short distance beneath the ice now in motion, and see its mode of work and the results it is effecting. The beds of living glaciers, and the beds which glaciers have recently abandoned, are found to present identical features. Because of their greater accessibility, the latter offer the better facilities for determining the effects of glaciation.
The conspicuous phenomena of abandoned glacier beds fall into two classes, (1) those which pertain to the bed rock over which the ice moved, and (2) those which pertain to the drift left by the ice.
_Erosive work of the ice._--_Effect on topography._--The leading features of the rock bed over which glacier ice has moved, are easily recognized. Its surface is generally smoothed and polished, and frequently marked by lines (striæ) or grooves, parallel to one another. An examination of the bottom of an active glacier discloses the method by which the polishing and scoring are accomplished.
The lower surface of the ice is thickly set with a quantity of clay, sand, and stony material of various grades of coarseness. These earthy and stony materials in the base of the ice are the tools with which it works. Thus armed, the glacier ice moves slowly forward, resting down upon the surfaces over which it passes with the whole weight of its mass, and the grinding action between the stony layer at the base of the ice and the rock bed over which it moves, is effective. If the material in the bottom of the ice be fine, like clay, the rock bed is polished. If coarser materials, harder than the bed-rock, be mingled with the fine, the rock bed of the glacier will be scratched as well as polished. If there are bowlders in the bottom of the ice they may cut grooves or gorges in the underlying rock. The grooves may subsequently be polished by the passage over and through them of ice carrying clay or other fine, earthy matter.