132 WORK OF SNOW AND ICE 



are comparable in size to the smaller valley glaciers. An ice-cap 

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

 but its actual thickness is influenced by the topography of the sur- 

 face beneath it. The Greenland ice-cap rises about 9,000 feet above 

 the sea toward its southern end, and it probably rises higher in the 

 unexplored center of the broader part of the island. The height of 

 the rock surface beneath the ice is unknown, but it is unlikely that 

 it averages half this amount, and hence the ice is probably very 

 thick at its center. 



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 waste exceeds, falls short of, or equals forward 

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

 determined by the ratio of movement to waste. Its upper end is 

 generally ill-defined. In a superficial sense, it is where the ice 

 emerges from the snow-field; but the lower limit of the snow-field 

 is 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 its 

 upper end is covered with neve or snow, and finally merges into the 

 snow-field without ceasing to be a glacier. The term glacier is, 

 however, commonly used to mean merely the more solid portion 

 outside (below) the snow-field. 



Movement. The advance of a glacier is too slow, as a rule, to 

 be seen from day to day, but is detected in other ways. If its end 

 advances, it overrides or overturns objects which were in front of 

 it, or it moves out over ground previously unoccupied. But even 

 when the end of a glacier is not advancing, movement of the ice 

 may be established by means of stakes or other marks on its surface. 

 If the position of these marks relative to fixed points on the sides 

 of the valley is noted, they are found, after a time, to have moved 

 down the valley. 



Rows of stakes or lines of stones set across a glacier in its upper, 

 middle and lower portions have revealed many facts concerning the 

 movement of the ice. Generally speaking, the central part moves 

 faster than the sides, and the top faster than the bottom. In 

 Switzerland the determined rates of movement range from one 01 

 two inches to four feet or more per day. Some of the larger glaciers 



