THE WORK OF SNOW AND ICE 237 



sometimes pusli out on the plain beyond. In length they range 

 from a fraction of a mile to many miles. Their thickness is usually 

 measured by scores or hundreds of feet rather than by denom- 

 inations of a larger order, but the variation is great, and exact 

 measurements are almost wholly wanting. The minimum thick- 

 ness is that necessary to cause movement, and this varies with the 

 slope, the temperature, and other conditions. There is also much 

 variation in the thickness in different parts of the same glacier. 

 As a rule, it is thinnest in its terminal portion, and thickest at 

 some point between its terminus and its source. Cliff and recon- 

 structed glaciers are comparable in size to the smaller valley 

 glaciers. Piedmont glaciers may attain greater size. An ice-cap 

 is thickest, theoretically, at its center, and thins away to its borders, 

 but its actual dimensions are influenced by the topography of the 

 surface on which it is developed. The Greenland ice-cap rises 

 about 9,000 feet above the sea toward its southern end, and it prob- 

 ably rises higher in the unexplored center of the broader part of 

 the island. The height of the rock surface beneath the ice is un- 

 known, but it is unlikely that it averages half this amount, and 

 hence the ice is probably very thick in the center. The ice cap of 

 Antarctica appears to rest on high land; its thickness is unknown. 



Limits. The ice of a glacier is always moving forward, but the 

 end of a glacier may be retreating, advancing, or remaining station- 

 ary, according as the wastage exceeds, falls short of, or equals the 

 forward movement. The position of the lower end of a glacier is 

 therefore determined by the ratio of movement to wastage. Its 

 upper end is generally ill-defined. In a superficial sense, it is the 

 point where the ice emerges from the snow-field; but the lower 

 limit of the snow-field is often ill-defined, and in any case is not 

 the true upper limit of the glacier. The snow-field is really an ice- 

 field, covered with snow, and there is movement from it to the 

 tongue of ice in the valley. The ice so moving is, in reality, a 

 part of the glacier. 



The lower end of a glacier is usually free from snow and neve 

 in summer, but toward its upper end it is covered with neve, then 

 with snow, and finally merges into the snow-field without having 

 ceased to be a glacier. The term glacier is, however, commonly 



