ATMOCLASTIC ROCKS 543 



more characteristic at a distance from the mountains, where they 

 also are finer grained and frequently clayey. On the surface of 

 the dry delta the coarse blocks gradually give way to finer, rounded, 

 flat or irregularly worn pebbles, with streams of coarser material 

 between the finer gravel, and with at intervals flat surfaces of luta- 

 ceous material (playas or takyrs) beautifully marked by polygonal 

 sun cracks. 



Movement of Atmoclastic Material. The movement of at- 

 moclastic material is twofold. There is, first, the quiet, more or 

 less continuous creep and flowage of the material under the influ- 

 ence of gravity, and conditioned by the change from coarse to finer 

 material under the disintegrating atmospheric influences. Sec- 

 ondly, there is the sudden movement, exemplified by landslides or 

 rock avalanches. These are very generally conditioned by a satu- 

 ration of the rock debris on a mountainside by water, which thus 

 renders the mass more fluid and lowers the potential angle of sta- 

 bility. A third type of movement may be considered as combining 

 the ciiaracteristics of both of the preceding types, and thus repre- 

 senting a compound of gliding and creeping movements. Sliding of 

 rock and soil as the result of earthquakes may also be considered 

 here. 



Slozv Movements of Rock and Soil. 



Rock and Soil Creep. The creep of soil is always a slow process. 

 It scarcely afifects the surface, which is covered by a blanket of 

 vegetation, beneath which the maximum movement takes place. 

 Gotzinger (18) holds that creep is largely due to the work of satu- 

 rating waters which increase the volume of the mantle rock and set 

 it in motion by forcing the particles upward and outward, this mo- 

 tion being translated into a maximum one down the slope of the 

 surface on which the soil rests. Frost is, of course, an important 

 agent in causing the downhill movement of the soil cover. If such 

 creep takes place over inclined or vertical beds of rock, the upper 

 ends of these beds will suffer overturning or bending in the direc- 

 tion of the creep. 



Solefluction or Rock Flozv. To the more rapid movement 

 of the rock debris saturated with large quantities of water, the 

 name solefluction has been applied ( Anderson-i). This was 

 first described from Bear Island in the North Atlantic (lat. 74>4° 

 N.), where on account of the hard winters much rock debris is 

 produced by mechanical weathering. The melting of extensive snow 

 masses in spring produces a large quantity of water, which trans- 



