258 GEOLOGY, 



of other orders, but the variation is great, and exact measurements 

 are almost wholly wanting. The minimum thickness 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 a glacier. As a rule, it is thinnest in its terminal 

 portion, and thickest at some point intermediate between this and its 

 source, but nearer the latter than the former. Cliff and reconstructed 

 glaciers are comparable in size to the smaller valley glaciers. Piedmont 

 glaciers may attain greater size. 



An ice-cap is theoretically thickest at its center and thins away to 

 its borders, but its actual dimensions are influenced by the topography 

 on which it is developed. The Greenland ice-cap is known to rise about 

 9000 feet above the sea, and it probably reaches considerably higher 

 than this in the unexplored center of its broad dome. The height of 

 the land surface beneath is unknown, but it is unHkely that it averages 

 half this amount, and hence the ice is probably 5000 feet or more thick 

 in the center. There is reason to think that it is much thicker in Antarc- 

 tica. 



Limits. — The ice of a glacier is always moving forward (neglecting 

 temporary halts), but the end of a glacier may be retreating, advan- 

 cing or remaining stationary, according as the rate of wastage is greater, 

 less, or just equal to the forward movement of the ice. The position of 

 the lower end of the 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, since there must be move- 

 ment from the granular mass of ice beneath the snow to make up for 

 the waste below, and the moving ice beneath the snow-field which feeds 

 the tongue of ice in the valley is just as really a part of the glacier as 

 the more consohdated portion in the valley below. If a definite upper 

 Hmit for an alpine glacier is to be named, it should probably be the 

 Bergschrund, a gaping crevasse, or series of crevasses which sometimes 

 open near the precipitous slope of the peak or cUff where the snow-field 

 lies. The Bergschrund is formed by the moving of the lower part of 

 the snow-field away from the portion above. 



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

 summer, but, traced toward its source, it first becomes covered with 



