THE CONTINENTAL GLACIERS OF THE "ICE AGE" 301 



or swamps since all valleys are characterized throughout by 

 forward grades. The side valleys enter the main valleys as do the 

 branches a tree trunk ; in other words, the drainage is described as- 

 arborescent. In so far as an# portions of a plane surface now remain 

 in the landscape, they are found at the highest levels (plate 16 A). 

 The topography is thus the result of a partial removal by erosion 

 of an upland and may be described as incised topography. Nowhere 

 within the area are there found rock masses foreign to the region, 

 but all mantle rock is the weathered product of the underlying 

 ledges. 



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, more- 

 over, 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 



FIG. 327. -Diagram showing the man- Aiding by the process of deposi- 

 ner in which a continental glacier ob- tion (plate 16B). In SO far as a 

 literates existing valleys (after Tarr). p l ane j s 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 compre- 

 hend. 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 preexisting 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 moutonnees. 

 The glacier gravings. The tools with which the glacier works 



