Plate 16.
A. Incised topography within the “driftless area” (U. S. Geol. Survey).
B. Built-up topography within glaciated region (U. S. Geol. Survey).
Characteristics of the glaciated regions.—The topography of the driftless area has been described as incised, because due to the partial destruction of an uplifted plain; and this surface is, moreover, perfectly drained. The characteristic topography of the “drift” areas is by contrast built up; that is to say, the features of the region instead of being carved out of a plain are the result of molding by the process of deposition (plate 16 B). In so far as a plane is recognizable, it is to be found not at the highest, but at the lowest level—a surface represented largely by swamps and lakes—and above this plain rise the characteristic rounded hills of various types which have been built up through deposition. The process by which this has been accomplished is one easy to comprehend. As it invaded the region, the glacier planed away beneath its marginal zone all weathered mantle rock and deposited the planings within the hollows of the surface ([Fig. 327]). The effect has been to flatten out the preëxisting irregularities of the surface, and to yield at first a gently undulating plain upon which are many undrained areas and a haphazard system of drainage ([Fig. 328]). All unstable erosion remnants, such as now are to be found within the driftless area, were the first to be toppled over by the invading glacier, and in their place there is left at best only rounded and polished “shoulders” of hard and unweathered rock—the well-known roches moutonnées.
Fig. 327.—Diagram showing the manner in which a continental glacier obliterates existing valleys (after Tarr).
The glacier gravings.—The tools with which the glacier works are never quite evenly edged, and instead of an in all respects perfect polish upon the rock pavement, there are left furrowings, gougings, and scratches. Of whatever sort, these scorings indicate the lines of ice movement and are thus indubitable records graven upon the rock floor. When mapped over wide areas, a most interesting picture is presented to our view, and one which supplements in an important way the studies of existing continental glaciers ([Fig. 334], [p.308], and [Fig. 336], [p. 312]).