water as a mechanical agent. 233 



Glaciers. 

 1. General Features and Formation of Glaciers. 



1. Nature of glaciers. — Ordinary glaciers are accumulations of ice of suf- 

 ficient size to flow down from snow-covered elevations. They are ice-streams, 

 100 to 1000 feet or more in depth, fed by the snows and hoar frost of exten- 

 sive areas above the limits of perpetual frost. The half-compacted snow of 

 the source is the neve of the Swiss, the firn of the Germans. These fields 

 stretch on from 1000 to 7500 feet below the snow-line, because they are 

 masses of ice so thick that they are not entirely melted during the summer 

 season. Some of them extend down between green hills and blooming banks 

 into open cultivated valleys. The extremities of the glaciers of the Grin- 

 del wald and Chamouni valleys lie within a few hundred yards of the gardens 

 and houses of the inhabitants. 



Each glacier is the source of a stream made from the melting ice. The 

 sub-glacial stream begins high in the mountains, from the waters that descend 

 through the ice; finally, it gushes forth from its crystal recesses, a full tor- 

 rent, and hurries along over its stony bed down the valley. 



An avalanche is a mass of ice, snow, water, mud, and stones sliding with 

 crashing sounds from some point high up on the side of a mountain ; a glacier 

 is ice flowing slowly from a perpetual source. Between the two there are 

 small glacier patches, lodged in steep valleys, called hanging -glaciers that 

 never move far enough to gain a descent. 



As in the case of rivers : (1) glaciers depend for formation and size on 

 the amount of precipitation, and on the size of the drainage area; (2) they 

 take possession of all the valleys of a mountain-region and flow down slopes 

 of all angles ; (3) the ice-streams of the upper valleys combine, like so many 

 tributaries, to make the large ice-courses or trunk-glaciers ; (4) they suffer 

 loss from evaporation. 



But unlike rivers : (1) glaciers require for origin a region extending above 

 the limit of perpetual snow ; (2) they require for commencement of flow 

 a large accumulation of the material of a stream ; (3) the conditions for 

 increase are best when the yearly precipitation is largely snow. Moreover, 

 (4) the drainage areas are always small compared with those of rivers. The 

 Aletsch, the longest glacier of the Alps, and according to Tyndall the grand- 

 est, ends in less than 15 miles ; and no glacier outside of Greenland and the 

 Antarctic region exceeds 60 miles in length. Further, (5) they often have 

 confluent heads in a snow and ice region, and may have nearly universal con- 

 fluence over a continent, as in a Glacial era. 



The limit of perpetual snow, above which lie the snow-fields of the source, 

 is in general near the line of 32° F. for the mean temperature of the summer. 

 But it varies with the precipitation ; for if this is small, the snows of winter 

 may be mostly melted by the heat even of a cool summer, and the limit may 

 be much above the summer line of 32° F., while, on the contrary, if very 



