30 PHYSICAL GEOLOGY 



In such cases, blocks are forced from cliffs, building up slopes of loose 

 fragments at their base, called talus. The formation of talus slopes 

 (Fig. 6) can best be studied in the early spring, when fragments of the 

 rock, loosened as the ice in the cracks melts, fall from the cliffs. 

 These fragments are carried down by their weight until the declivity 

 is too feeble for them to roll farther. When they come to rest they 

 accumulate to form a slope, usually steep, whose angle is called " the 

 angle of repose." The slope of talus (Fig. 14, p. 39) varies from 26 to 

 43 degrees, the angle depending upon the size of the fragments and 

 upon their shape. If the fragments are angular, the slope will be 



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Fig. 6. — Diagram showing the formation of a talus slope and the destruction 

 of a cliff. The successive faces of the cliff A, B, C, D and of the talus A> B, C, D are 

 indicated by dotted lines. 



steeper than if they are rounded, since with the former an early lodg- 

 ment is more likely. The largest blocks accumulate at the foot of the 

 talus slope, the size diminishing regularly to the top. If talus accu- 

 mulates under water the slope will be steeper, since the fragments 

 are, to some extent, buoyed up by the water. 



It is evident that the alternate freezing and thawing of water in the 

 pores and joints of a rock may bring about its complete disintegration 

 unless it is protected by the soil of its own making. It should be 

 remembered, however, that this agent seldom acts alone, but usually 

 serves as an aid to the chemical agencies of weathering (p. 35), by 

 breaking up the rock into small fragments and thus furnishing a larger 

 surface upon which the latter may work. 



Rock Glaciers. — A striking example of the above is shown in the 

 formation under favorable conditions of rock glaciers or " stone 

 rivers " (Fig. 7), many hundred feet in length in regions of severe 

 cold, when great masses of talus are slowly moved some distance down 

 a valley, producing the appearance of a glacier. This is accomplished 

 by the alternate freezing and thawing of the water in the interstices 

 of the talus. 



