196 SOUTHERN CALIFORNIA GEOLOGY 



Floating- wood blocks which illustrate the up-and- 

 down isostatic movements of fault blocks. (Tabor) 



the alternations of picturesque highlands and produc- 

 tive valley plains and various other apparent peculiari- 

 ties of the landscape that give individuality to South- 

 ern California. 



The extent of the vertical movement of the great 

 fault blocks, as I have shown on previous pages, often 

 aggregates 20,000 feet or more. Such conditions may 

 be seen along the borders of the San Gabriel, San Ber- 

 nardino and San Jacinto Ranges. These blocks are 

 clear, large-scale demonstrations of the theory of iso- 

 static adjustment. 



To fully appreciate the vertical movements which 

 they have undergone, we must remember several im- 

 portant facts. The first of these is that the granitic 

 rocks which compose any one of these uplifted fault 

 blocks, such as the San Gorgonio Ridge of the 

 San Bernardino Range, which at present stands 

 over two miles above sea level, were probably made at 

 depths of twenty miles or more beneath the surface 

 of the earth ; also that these granites were once cov- 

 ered by an equal thickness, or overburden, of rock ma- 



