FOUNTAINS AND STREAMS 253 



pacted.snow which has been subjected to frequent 

 alternations of frost and thaw. They are devel- 

 oped on caiion and mountain sides, the greater 

 number of them, at elevations of from nine to 

 ten thousand feet, where the slopes are so in- 

 clined that the dry snows of winter accumulate 

 and hold fast until the spring thaws sap their 

 foundations and make them slippery. Then away 

 in grand style go the ponderous icy masses, 

 adorned with crystalline spray without any 

 cloudy snow dust ; some of the largest descend- 

 ing more than a mile with even, sustained energy 

 and directness like thunderbolts. The grand cen- 

 tury avalanches, that mow wide swaths through 

 the upper forests, occur on shady mountain sides 

 about ten to twelve thousand feet high, where, 

 under ordinary conditions, the snow accumulated 

 from winter to winter lies at rest for many years, 

 allowing trees fifty to a hundred feet high to 

 grow undisturbed on the slopes below them. On 

 their way through the forests they usually make 

 a clean sweep, stripping off the soil as well as the 

 trees, clearing paths two or three hundred yards 

 wide from the timber line to the glacier meadows, 

 and piling the uprooted trees, head downward, 

 in windrows along the sides like lateral moraines. 

 Scars and broken branches on the standing trees 

 bordering the gaps record the side depth of the 

 overwhelming flood ; and when we come to count 

 the annual wood rings of the uprooted trees, we 



