102 



PHYSICAL GEOLOGY 



tion. Rocks containing a number of minerals are more 

 likely than others to be broken by changes in temperature, 



for the different min- 

 erals expand and con- 

 tract at different rates, 

 thus establishing 

 strains within the rock. 

 Great and frequent 

 changes in the temper- 

 ature of the rocks fa- 

 vor their breaking by 



FIG. 96. Serrate mountain topography. 

 Peaks of granite in the Sierra Nevadas, near 

 Mount Whitney. (Trowbridge.) 



this process, and these 

 conditions are met best in dry rather than moist regions, in 

 low rather than high latitudes, and at high rather than low 



FIG. 97. Serrate mountain topography. Peaks in the Mont Blanc group 

 of mountains. (Tairraz.) 



altitudes. In deserts and on bare mountains, therefore, the 

 process is important, and in many cases the latter are nearly 

 covered by loose, angular fragments that have been broken in 

 this manner from the rocks beneath (Fig. 95), above which 

 sharp, serrated peaks sometimes rise (Figs. 96 and 97). Pieces 

 of rock loosened from steep slopes in this, or other ways, ac- 

 cumulate at the bottom to form piles of talus (Fig. 98). 

 Some of the mountains of the Great Basin region are buried 

 knee-deep with talus. 



