WEATHERING 33 



tudes, and mountain peaks often owe their jagged shapes, to some 

 degree, to this action, although more largely to that of frost. In 

 regions of deficient rainfall, talus accumulates at the foot of cliffs, the 

 fragments forming the slope having been broken off* by temperature 

 changes. Mountains in desert regions are sometimes almost buried 

 beneath rock fragments and sand, broken from their sides by changes 

 in daily temperature. 



When the heated rocks of arid regions are wet by a sudden down- 

 pour, they cool quickly and are broken asunder. In western Texas 

 blocks 25 feet in diameter are reported to have been rent into 

 several pieces in this way. (Hobbs.) 



3. Mechanical Action of Animals and Plants. — If one observes 

 a cliff" upon which vegetation is abundant, he will see that not 

 only the large but also the small cracks of the rock are filled with 

 roots and rootlets. As these roots and rootlets grow larger they tend 

 to push the blocks of rock apart. The root of the garden pea, for 

 instance, has a wedging force equal to 200 or 300 pounds a square 

 inch. Abundant examples of this wedging process can be found in 

 fertile regions, and are also often seen in cities, where the pavements 

 are frequently broken and tilted by the enlarging roots of trees. 



Plants, earthworms, and burrowing animals open channels through 

 which water from the surface can reach deep down into the soil. 

 Moreover the organic matter carried into the tunnels by the animals 

 is, upon its decay, a source of organic acids which actively attack 

 the rocks and thus hasten the decomposition of those otherwise pro- 

 tected by soil. It is thus seen that the mechanical disintegration of 

 rocks is accomplished both by the agents of the weather and by 

 organisms. 



4. Rain. — The mechanical effect of rain consists in (1) the impact 

 of the raindrops upon the surface, which in the aggregate has a con- 

 siderable effect, as, for example, in gravel deposits where the larger 

 bowlders protect the gravel underneath from the impact of the rain, 

 while that which is not so protected is removed. In the process of 

 time, columns a score or more feet in height may result (Fig. 9). 

 On a small scale, this same result can be seen in almost any soft 

 material after a rain. (2) The mechanical work of rain is seen also in 

 the softening of clay soils, which, on slopes, causes them to creep; 

 (3) in the washing and later deposition of dust from the atmosphere, 

 and (4) in the dissolving of some of the atmospheric gases which may 

 later be used in weathering the rocks. Dew and hoarfrost, being 



