Studies in the Sierra 



415 



regularity in the quantity of post-glacial denudation is the dif- 

 ference in the length of time during which dii¥erent portions 

 of the range have been subjected to denuding agents. The ice- 

 sheet melted from the base of the range tens of thousands of 

 years ere it melted from the upper regions. We find, accord- 

 ingly, that the foothill region is heavily weathered and blurred, 

 while the summit, excepting the peaks, and a considerable por- 

 tion of the middle region remain fresh and shining as if they 

 had never suffered from the touch of a single storm. 



Perhaps the least known among the more outspoken agents 

 of mountain degradation are those currents of eroding rock 

 called avalanches. Those of the Sierra are of all sizes, from 

 a few sand-grains or crystals worked loose by the weather and 

 launched to the bottoms of cliffs, to those immense earthquake 

 avalanches that thunder headlong down amid fire and smoke 

 and dust, with a violence that shakes entire mountains. Many 

 avalanche-producing causes, as moisture, temperature, winds, 

 and earthquakes, are exceedingly variable in the scope and in- 

 tensity of their action. During the dry, equable summers of 

 the middle region, atmospheric disintegration goes silently on, 

 and many a huge mass is made ready to be advantageously 

 acted upon by the first winds and rains of winter. Inclined 

 surfaces are then moistened and made slippery, decomposed 

 joints washed out, frost-wedges driven in, and the grand av- 

 alanche storm begins. But though these stone-storms occur 

 only in winter, the attentive mountaineer may have the pleas- 

 ure of witnessing small avalanches in every month of the year. 

 The first warning of the bounding free of a simple avalanche 

 is usually a dull muffled rumble, succeeded by a ponderous 

 crunching sound ; then perhaps a single huge block weighing a 

 hundred tons or more may be seen wallowing down the face of 

 a cliff', followed by a train of smaller stones, which are gradu- 

 ally left behind on account of the greater relative resistance 

 they encounter as compared with their weight. The eye may 

 therefore follow the large block undisturbed, noting its awk- 

 ward, lumbering gestures as it gropes its way through the air 

 in its first wild journey, and how it is made to revolve like a 

 star upon its axis by striking on projecting portions of the 

 walls while it pursues the grand smooth curves of general 



