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.

The Work of Glacier Ice.

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.

All these phases of rock wear may be seen about the termini of receding glaciers, on territory which they have but recently abandoned. There can thus be no possible doubt as to the origin of the polishing, planing and scoring.

There are other peculiarities, less easily defined, which characterize the surface of glacier beds. The wear effected is not confined to the mere marking of the surface over which it passes. If prominences of rock exist in its path, as is often the case, they oppose the movement of the ice, and receive a corresponding measure of abrasion from it. If they be sufficiently resistant they may force the ice to yield by passing over or around them; but if they be weak, they are likely to be destroyed.

As the ice of the North American ice sheet advanced, seemingly more rigid when it encountered yielding bodies, and more yielding when it encountered resistant ones, it denuded the surface of its loose and movable materials, and carried them forward. This accumulation of earthy and stony debris in the bottom of the ice, gave it a rough and grinding lower surface, which enabled it to abrade the land over which it passed much more effectively than ice alone could have done. Every hill and every mound which the ice encountered contested its advance. Every sufficiently resistant elevation compelled the ice to pass around or over it; but even in these cases the ice left its marks upon the surface to which it yielded. The powerful pressure of pure ice, which is relatively soft, upon firm hills of rock, which are relatively hard, would effect little. The hills would wear the ice, but the effect of the ice on the hills would be slight. But where the ice is supplied with earthy and stony material derived from the rock itself, the case is different. Under these conditions, the ice, yielding only under great pressure and as little as may be, rubs its rock-shod base over every opposing surface, and with greatest severity where it meets with greatest resistance. Its action may be compared to that of a huge "flexible-rasp" fitting down snugly over hills and valleys alike, and working under enormous pressure.

The abrasion effected by a moving body of ice under such conditions would be great. Every inch of ice advance would be likely to be attended by loss to the surface of any obstacle over or around which it is compelled to move. The sharp summits of the hills, and all the angular rugosities of their surfaces would be filed off, and the hills smoothed down to such forms as will offer progressively less and less resistance. If the process of abrasion be continued long enough, the forms, even of the large hills, may be greatly altered, and their dimensions greatly reduced. Among the results of ice wear, therefore, will be a lowering of the hills, and a smoothing and softening of their contours, while their surfaces will bear the marks of the tools which fashioned them, and will be polished, striated or grooved, according to the nature of the material which the ice pressed down upon them during its passage. Figs. [28] and [29] show the topographic effects which ice is likely to produce by erosion. Plate [XXXIV] is a hill two miles northeast of Merrimac, which shows how perfectly the wear actually performed corresponds to that which might be inferred.