THE ATTACK OF THE WEATHER 



153 



Soil flow in the continued presence of thaw water. So soon 

 as the rocks are broken down by the weathering processes, they are 

 easily moved, usually to lower levels. In part this transportation 

 may be accomplished by gravity slowly acting upon the disinte- 

 grated rock and causing 

 it to creep down the slope. 

 Yet even in such cases 

 water is usually present 

 in quantity sufficient to fill 

 the spaces between the 

 grains, and so act as a 

 lubricant to facilitate the 

 migration. 



Upon a large scale rocks 

 which were either origi- 

 nally incoherent or have . A 



been made so by weather- ' s Si'. M. 'Ill 



/., ,1 -i ^/^".^^'* ? *wfe'si*^w 



ing, after they have be- .. , : .,..^., ;/...,. 



come saturated with FlG - 158 - ~ Talus slope beneath a cliff - 

 water, may start into sudden motion as great landslides or ava- 

 lanches, which in the space of a few moments materially change the 

 face of the country, and by burying the bottom lands leave dis- 

 aster and misery in their wake. 



Within the subpolar regions, where a large part of the surface 

 is for much of the year covered with snow, the underlying rocks 

 are for long periods saturated with thaw water, and in alternation 

 are repeatedly frozen and thawed. Essentially similar conditions 

 are met with in the high, snow-capped mountains of temperate or 

 torrid regions. .For the subpolar regions particularly it is now 

 generally recognized that somewhat special processes of soil flow, 

 described under the name solifluction, are characteristic. The 

 exact nature of these processes is as yet imperfectly understood, but 

 there can be little doubt concerning the large role which they have 

 played in the transportation of surface materials. Such soil flow 

 is clearly manifested under different aspects, and it is likely that 

 by this comprehensive term distinct processes have been brought 

 together. 



Possibly the most striking aspect of the soil flow in subpolar 

 regions is furnished by the remarkable " stone rivers " and " rock 



