302 EARTH FEATURES AND THEIR MEANING 



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 indi- 

 cate the lines of ice movement and are thus indubitable records 

 graven upon the rock floor. When mapped over wide areas, a 



FIG. 328. Lake and marsh district in northern Wisconsin, the effect of glacial 

 deposition in former valleys (after Fairbanks). 



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). 



It has been customary to think of the glacier as everywhere 

 eroding its bed, although the only warrant for assuming degra- 

 dation by flow of the ice is restricted to the marginal zone, since 

 here only is there an appreciable surface grade likely to induce 

 flow. Both upon the advance and again during the retreat of a 

 glacier, all parts of the area overridden must be subjected to this 

 action. Heretofore pictured in the imagination as enlarged 

 models of Alpine glaciers, the vast ice mantles were conceived to 

 have spread out over the country as the result of a kind of viscous 

 flow like that of molasses poured upon a flat surface in cold 

 weather. The maximum thickness of the latest American glacier 

 of the ice age has been assumed to have been perhaps 10,000 feet 

 near the summit of its dome in central Labrador. From this 



