The Sierra Nevada Range 167 



History of the Sierra Nevada 



The accompanying table of successive events of the history 

 of the Sierra Nevada Range is from U. S. Geological Survey, 

 Professional Paper No. 160, by Francois E. Matthes. 



"In the table of geologic time divisions the outstanding 

 events in the geologic history of the Sierra Nevada region are 

 set forth in chronologic order, each referred to its proper era, 

 period, and epoch as definitely as the knowledge at hand per- 

 mits. The figures for the duration of the successive time 

 divisions are taken from the table which the late Professor 

 Barrell compiled from calculations of the age of uranium min- 

 erals from different parts of the world."" The age of these 

 minerals is computed from the ratio of lead to uranium present 

 in them, the ratio at which uranium breaks down and is reduced 

 to lead by atomic disintegration being accurately known. The 

 measures of geologic time are much greater than those which 

 have been current among scientists in the past, but they doubt- 

 less afford much closer approximations to the truth than the 

 shorter measures, for they are of an order of magnitude that is 

 consistent with many geologic facts, notably with the extremely 

 slow rate at which mountains are worn down." 



The Sierra Range a Massive Tilted Block 



Most mountain ranges are carved from great wrinkles or 

 folds in the outer crust of the earth, produced by the buckling 

 of originally flat strata. Both of the ancient mountain systems 

 that formerly occupied the site of the Sierra Nevada Range 

 were of that folded type, but the present range consists essen- 

 tially of a single massive block of the earth's crust that has been 

 dislocated and tilted toward the southwest without appreciable 

 bending or warping. (See fig. 53.) The Sierra Nevada is 

 therefore properly termed a "block range." Great master 

 fractures (faults) extend along the eastern base of the range 



* Barrell, Joseph, "Rythms and the Measurement of Geologic Time," Geol. 

 Soc. of America Bull., Vol. 28. 



