126 



PHYSICAL GEOLOGY 



WEATHERING 



Processes of weathering. As earlier discussions have 

 indicated, weathering is a term applied to nearly all the 

 processes which cause rocks to break up and decay. From 

 the standpoint of general erosion, weathering may be denned 

 as preparation for transportation, for it reduces rocks to 



pieces sufficiently small 

 to be blown or washed 

 away. Furthermore, 

 most of the material 

 moved by wind and water 

 was derived from bed 

 rock in this way. The 

 mantle rock and materi- 

 als dissolved in the hy- 

 drosphere are the most 

 important products of 

 weathering. The princi- 



FIG. 116. A tree growing in an opening pa } processes of Weather- 



in a rock. The growth of the tree has . ,. , . 



pried the parts of the rock apart, and m g were dlSCUSSed in 



enlarged the opening from a narrow connection with the WOrk 

 crack to its present size. Lansing, ,, , j 



Mich. (Macpherson.) of the atmosphere and 



of ground water (pp. 



100-104, 117). Some of the remaining processes may be noted 

 here. The mechanical beating of raindrops disturbs small sur- 

 face particles and causes them to wear one another to slight 

 extent. More important is the work done by plants and ani- 

 mals. The growth of roots in joints and other cracks of the 

 rocks enlarges the openings, and by so doing not only helps 

 directly to break the rocks, but increases the surface exposed 

 to other weathering agents. Root splitting is illustrated by 

 Figure 116. Burrowing animals make openings in the rocks, 

 and in the aggregate bring large quantities of material to the 

 surface, where it is exposed to the attack of the weather. 

 Gravity is regarded usually as an agent of weathering. For 



