LAKE CHELAN 419 



of a river may be a few feet only or a few scores of feet ; that of 

 a glacier may be thousands of feet. It is this greater size, vol- 

 ume, and weight which makes glacial ice behave like water. In 

 such large masses ice is plastic, accommodating itself to inequal- 

 ities of its bed, flowing with some freedom, spreading out and 

 contracting, much as water does. 



A word of caution must here be interpolated. The channel 

 of a river, in which its water flows, must not be confused with 

 its valley, which it drains. The above comparison refers to the 

 channel of a river, not to its valley. 



• Glaciers in mountain regions commonly head in amphithea- 

 ters or cirques — basins lying directly at the heads of canons, 

 under the shadow of the summit cliffs. An amphitheater is 

 surrounded on three sides by vertical walls or steep slopes, down 

 which the ice and snow slide in avalanches, accumulating in 

 the bottom. The effect is precisely like that of a waterfall. 

 The falling snow and ice dig a hollow or depression at the foot 

 of the steep descent just as water does. Such amphitheaters 

 are found at the heads of all glacial gorges in high mountains, 

 and today are found to contain small alpine lakes in place of 

 the ice which once occupied them. From its head in the amphi- 

 theater the glacier moves down the gorge, scouring and cutting 

 the bottom and sides as it travels. The ends of the mountain 

 spurs are planed off instead of being trimmed to sharp, angular 

 points, as is done by streams in gorges cut by them. If the 

 bottom of the canon be uneven, if it contain abrupt elevations 

 and depressions, the glacier flows over them as water would 

 flow over similar obstacles in. its channel, gradually cutting 

 them away. Where the descent becomes abruptly steeper the 

 ice, in bending to follow the surface, is commonly cracked, form- 

 ing a network of crevasses, making travel over its surface very 

 difficult and dangerous. 



Where the main glacier is joined by a branch, the bed of the 

 branch is commonly found to be at a higher level than the bed 

 of the main glacier, because being larger and heavier the main 

 glacier has greater cutting power; indeed, in many cases the 

 beds of small branches are hundreds, or even thousands, of feet 

 higher than that of the main glacier to which they are tribu- 

 tary. The parallelism between the glacier and the river in their 

 channels is further illustrated by this fact. The surface of the 

 ice in the main glacier and in the branch must have been at the 

 same level, although the bottoms, as stated above, differ greatly 



