CH'OLOCJIC HISTORY 21 S 



It is a further anomoly that the older portions of 

 our rivers, as seen in the areas of our valley plains, 

 are apparently the least indented ("canyoned"), in- 

 asmuch as their traces are constantly being filled and 

 abandoned by processes of distributive aggradation. 



So far as I am aware, no geologist has hitherto 

 attempted to point out the distinction between the 

 minor interior faults and the master ones, the latter 

 of which outline and block out the major features of 

 the topography ; or to classify the faults into various 

 belts or groups ; or to suggest the possibility that 

 there is an age sequence to these groups as I have 

 attempted to do in this paper. 



Geologists now seem in agreement that most of 

 the master faulting of Southern California dates back 

 through the Pleistocene epoch, and that it has been 

 more or less continuous through that epoch into the 

 Recent. In some sections the age of the faults may 

 extend back into the Eocene Tertiary. 



GEOLOGIC HISTORY OF THE FAULT SYSTEMS 

 Wrong interpretations might be made of the fault 

 phenomena of Southern California if one is too prone to 

 ascribe to present-day activities many events which 

 have actually happened in the past ; to fail in distin- 

 guishing between events of Pleistocene and Recent 

 times in the geologic history ; or to labor under the mis- 

 taken idea that faulting and seimicity are as actively 

 operative today as they were in the recent past. 



Until quite recently there seems to have been no 

 clearly set forth opinions concerning the ages of the 

 fault systems. Only a hazy idea existed that they 

 were an unclassifiable and indefinite mesh-work 

 of fractures of possibly contemporaneous age. And 



