THE GLACIERS OF MOUNTAIN AND CONTINENT 265 



below become the natural channels for the outflow of the early 

 glaciers. With a continued lowering of the annual temperature 

 and consequent increased snowfall, the early glaciers become 

 more and more amply nourished. Snow and ice will, therefore, 

 cover larger areas of the upland, and the glaciers will push their 

 fronts farther down the valleys before they are wasted in the 

 warm air of the lower levels. As the valleys become thus more 

 completely invested by the glacier they are likewise filled to greater 

 and greater depths, and they may thus submerge portions of the 

 walls that separate adjacent valleys. Reaching at last the front 

 of the upland area, the glaciers may now be so well nourished at 

 their heads that they push out upon the flatter foreland and with- 

 out restraint from retaining walls spread broadly upon it (Fig. 292). 

 The culmination of the progressive climatic change may ere 

 this have been reached and milder conditions have ensued. If, 

 however, the severity of the climate should be still further in- 

 creased, the expanded fronts of neighboring glaciers will coalesce 

 to form a common ice fan or apron along the foot of the upland 

 (Plate 18 B). This could hardly take place without a still further 

 deepening of the ice within the valleys above, and, probably, a 

 progressive submergence of the lower crests in the valley walls. 



FIG. 293. Surface of a glacier whose upper layers spread with slight restraint 

 from retaining walls. Surface of the Folgefond, an ice cap of southern Norway. 



