4i8 



Sierra Club Bulletin 



masonry, always rests, or is capable of resting, at a much steep- 

 er angle than the same material after it has grown old and 

 rotten. If a poultice of acid mud be applied to a strong boulder, 

 it will not be much affected in an hour or day, but if kept on 

 for a few thousands or tens of thousands of years, it will at 

 length soften and crumble. Now, Nature thus patiently poul- 

 tices the boulders of the moraine banks under consideration. 

 For many years subsequent to the close of the ice period very 

 little acid for this purpose was available, but as vegetation in- 

 creased and decayed, acids became more plentiful, and boulder 

 decomposition went on at an accelerated rate, until a degree of 

 weakness was induced that caused the sheerest portions of the 

 deposits, as A B D (Fig. i), to give way, perhaps when jarred 

 by an earthquake, or when burdened with snow or rain, or par- 

 tially undermined by the action of a stream. 



It appears, therefore, that the main cause of the first post- 

 glacial land-slips is old age. They undoubtedly made their 

 first appearance in moraine banks at the foot of the range, and 

 gradually extended upward to where we now find them, at a 

 rate of progress measured by that of the recession of the ice- 

 sheet, and by the durability of moraines and the effectiveness 

 of the corroding forces brought into action upon them. In 

 those portions of the Sierra where the morainal deposits are 

 tolerably uniform in kind and exposure, the upper limits of the 

 land-slip are seen to stretch along the range with as great con- 

 stancy of altitude as that of the snow-line. 



The above-described species of land-slip is followed up the 

 range by another of greater size, just as the different forest 

 trees follow one another in compliance with conditions of soil 

 and climate. After the sheer end of the deposit (A B D, Fig. 

 I ) has slipped, the whole mass may finally slip on the bed-rock 

 by the further decomposition, not only of the deposit itself, 

 but of the bed-rock on which it rests. Bed-rocks are usually 

 more or less uneven. Now, it is plain that when the inequali- 

 ties B B B crumble by erosion, the mass of the deposit will not 

 be so well supported; moreover, the weight of the mass will 

 continue to increase as its material is more thoroughly pulver- 

 ized, because a greater quantity of moisture will be required 

 to saturate it. Thus it appears that the support of moraine 



