LAND SCULPTURE BY MOUNTAIN GLACIERS 369 



drift site are lowered as deposition goes on upon the lower. The 

 combined effect is thus to produce a niche or faintly etched amphi- 

 theater upon the slope of the mountain (Fig. 391). 



FIG. 391. Amphitheater formed on a drift site in northern Lapland (after a 

 photograph by G. von Zahn). 



The augmented snowdrift moves down the valley birth of 

 the glacier. In still lower air temperatures the drifts enlarge with 

 each succeeding year until they endure throughout the summer 

 season. From this stage on, an increment of snow is left from each 

 succeeding season. No longer entirely wasted by melting, the 

 time soon comes when the upper snow layers will by their weight 

 compress the lower into ice, and the mass will begin to creep down 

 the slope along the course of the inherited valley. The enlarged 

 snowdrift which feeds this ice stream is called the neve or firn. 



Against the sloping cliff which had been shaped by nivation 

 at the upper margin of the snowdrift, that snow which is not of 

 sufficient depth to begin a movement towards the valley separates 

 from the moving portion, opening as it does so a cleft or crevasse 



2B 



