Studies in the Sierra 



421 



period, has yet accumulated so much debris as some of the 

 larger earthquake avalanches which were formed in a few 

 seconds. 



The next species of avalanche in natural order is the annual 

 one, composed of heavy crystaUine snows which have been 

 subjected to numerous alternations of frost and thaw. Their 

 development requires a shadowed mountain side 9000 or 10,- 

 000 feet high, inclined at such an angle that loose fresh snow 

 will lodge and remain upon it, and bear repeated accessions 

 throughout the winter without moving; but which, after the 

 spring thaws set in, and the mountain side thus becomes slip- 

 pery, and the nether surface of the snow becomes icy, will 

 then give way. 



One of the most accessible of the fountains of annual ava- 

 lanches is the northern slope of Cloud's Rest, above the head 

 of the Yosemite Valley. Here I have witnessed the descent of 

 three within half an hour. They have a vertical descent of 

 nearly a mile on a smooth granite surface. Fine examples of 

 this species of avalanche may also be observed upon the north 

 side of the dividing ridge between the basins of Ribbon and 

 Cascade creeks, and in some portions of the upper Nevada 

 Canon. Their denuding power is much greater than that of 

 the first species, on account of their greater weight and com- 

 pactness. Where their pathways are not broken by precipices, 

 they descend all or part of their courses with a hard snout 

 kept close down on the surface of the rock, and because the 

 middle of the snout is stronger, the detritus heaps are curved 

 after the manner of terminal moraines. These detritus heaps 

 also show an irregularly corrugated and concentric structure. 

 An examination of the avalanche pathways shows conclusively 

 that the annual accretions of detritus, scraped from their sur- 

 faces, are wholly insufficient to account for the several large 

 concentric deposits. But when, after the detritus of many 

 years has been accumulated by avalanches of ordinary magni- 

 tude, a combination of causes, such as rain, temperature, and 

 abundant snow-fall, gives rise to an avalanche of extraordinary 

 size, its superior momentum will carry it beyond the limits at- 

 tained by its predecessors, and sweep forward the accumula- 

 tions of many years concentric with others of like magnitude 



