GLACIERS 197 
in the Sierra and in various parts of the northwest. 
When western Canada is reached, they become more 
numerous, and from here all the way up to Alaska, 
are very common among the mountains. In many 
places, as for instance at the famous Muir glacier, they 
reach the sea (Plate 9). 
The condition necessary for the formation of a glacier, 
is an excess of snowfall over melting; and so in higher 
latitudes, where the winter snowfall is great, and the 
summer temperature low, glaciers may form even at a 
comparatively slight elevation. Even in the same lati- 
tude it may happen that glaciers exist in some moun- 
tains, where the precipitation is heavy, while other 
mountains of equal height, situated in a drier region, 
are without them. ‘This is one of the reasons for their 
absence in the higher valleys of the middle Rockies. 
Characteristics of Valley Glaciers. The valley glacier 
is commonly said to consist of three parts: (1) the 
snow field (Figs. 103, 104, and 108), which is the place 
of accumulation ; (2) the névé, where the change of the 
snow to granular ice, and the movement down the slope 
have begun; and (3) the ice stream (Figs. 103 and 108), 
or glacier proper, which is a mass of ice compacted 
from the snow. 
The glacier may be likened to a stream; for the 
snow field is the vast supply ground, comparable to 
the drainage area of a river, and the glacier is a 
