WEATHERING 



39 



may accumulate in the valleys 

 and on the sides of the moun- 

 tains to great depths, in some 

 cases more than a thousand feet. 

 Where the supply of talus is too 

 great for the stream in the valley 

 to remove, the stream is dammed 

 and a lake is formed (Fig. 14). 

 The form of the crests and cliffs 

 of high mountains is determined 

 to a large degree by cracks which 



have a uniform direction (joints), 



i_ -i t • 1 rn Fig. 14. — Drawing showing a stream 

 as, when the water which tills _ A Za k , * 1 „ „«. «.„ c n. 



' t so dammed by talus as to torm a lake, 



them freezes, the rock is broken Note the angle of the talus slope. 



off along these planes. In tropi- 

 cal regions the fractures of the rock are also 

 important, since they permit the access of water, 

 and chemical decomposition is therefore accom- 

 plished at greater depths at these points. 



It is often possible to state from the shape of 

 the topography in any one locality what the 

 nature of the underlying rock is, but a general 

 rule is impossible, since the same rock is differ- 

 ently affected by the ♦weather under different 

 climates. For example, granite rocks which 

 in a cold climate may be broken into jagged 

 crests, may, in moist, tropical regions, be 

 reduced to rounded forms through the chemical 

 agencies. 



Spheroidal Weathering. — Spheroidal weather- 

 ing results from chemical action and should be 

 distinguished from similar shapes which are pro- 

 duced by exfoliation due to changes in daily 

 temperature (p. 31). When water percolates 

 through the joints (p. 258) and horizontal planes 

 into which all rocks are more or less divided, 

 it attacks with its dissolved gases all the rock 

 surfaces with which it comes in contact; but 

 since the corners and edges of the blocks formed 

 by these joints and planes have a greater 



Fig. 15. — Diagrams 

 showing the effect of 

 weathering upon rock 

 cut into angular blocks 

 by joints (see Figs. 1, 2). 

 The corners and edges 

 are most affected, and 

 the blocks tend to 

 become spheroidal. 

 (Modified after Hobbs.) 



