422 



Sierra Club Bulletin 



into a single mass. A succession of these irregularities will 

 obviously produce results corresponding in every particular 

 with the observed phenomena. 



What we may call century avalanches, as distinguished from 

 annual, are conceived and nourished on cool mountain sides 

 10,000 or 12,000 feet in height, where the snow falling from 

 winter to winter will not slip, and where the exposure and 

 temperature are such that it will not always melt off in summer. 

 Snow accumulated under these conditions may linger without 

 seeming to greatly change for years, until some slowly organ- 

 ized group of causes, such as temperature, abundance of snow, 

 condition of snow^ or the mere occurrence of an earthquake, 

 launches the grand mass. In swooping down the mountain 

 flanks they usually strip off the forest trees in their way, as 

 well as the soil on which they were growing. 



Some of these avalanche pathways are 200 yards wide, and 

 extend from the upper Hmit of the tree-line to the bottom of 

 the valleys. They are all well "blazed" on both sides by de- 

 scending trunks, many of which carry sharp stones clutched 

 in their up-torn roots. The height of these "blazes" on the 

 trees bordering the avalanche gap measures the depth of the 

 avalanche at the sides, while in rare instances some noble sil- 

 ver-fir is found standing out in the channel, the only tree suffi- 

 ciently strong to withstand the mighty onset; the scars upon 

 which, or its broken branches, recording the depth of the cur- 

 rent. The ages of the trees show that some of these colossal 

 avalanches occur only once in a century, or at still wider inter- 

 vals. These avalanches are by far the most powerful of the 

 three species, although from the rarity of their occurrence and 

 the narrowness of the zone in which they find climatic condi- 

 tions suited to their development, the sum of the denudation 

 accomplished by them is less than that of either of the others. 



We have seen that water in the condition of rain, dew, va- 

 por, and melting snow, combined with air, acts with more or 

 less efficiency in corroding the whole mountain surface, thus 

 preparing it for the more obviously mechanical action of 

 winds, rivers, and avalanches. Running water is usually re- 

 garded as the most influential of all denuding agents. Those 

 regions of the globe first laid bare by the melting of the ice- 



