WEATHERING 



13 



rock extends to only a slight depth. The greater expansion of the 

 surface produces a strain that frequently causes a layer to break off, 

 sometimes with considerable violence (Fig. <>). In Ixnver Cali- 

 fornia, slabs as much as ten feet long and from eight to ten inches 

 thick have been observed on the southwest side of rock exposures 

 that were produced in this way. Many similar cases may be seen in 

 the arid regions in southwestern United States. Boulders are some- 

 times found in this latitude that show peculiar exfoliation due to 

 unequal heating. This action is more noticeable in fine-grained 



Fia. 6. Exfoliated granite in the Sierra Nevada*, California. Previous glaeiation 

 has removed the loose material, giving the njtenry of heat a better rhance. Korks, Hock- 

 Weathering and Soils, Merrill. (Courtesy The Macniillan Company.) 



than in coarse-grained rocks, and in higher altitudes where the tem- 

 perature changes are great and sudden. \V. 11. Bartlett ' lias shown 

 that granite expands or contracts .(HXMXM.s of an inch per foot for 

 each degree Fahrenheit. Marble changes .<MI<M>I>:><;, while sandstone 

 changes .()()()()()!."> of an inch per degree. Practical applications of 

 this principle have sometimes been made in quarrying and in re- 

 moving rocks in constructing roads. Boulders may be broken up by 

 heating and cooling suddenly. K'ocks are made up of various min- 

 eral crystals that possess different coefficients of expansion. The 



