206 SO-UTHERN CALIFORNIA GEOLOGY 



and an uninteresting science to many people. Con- 

 sideration of the events of the later ones are replete 

 with interest pertaining to human environment such 

 as can hardly otherwise be found. In California it 

 is the story of a live world at work, not of a dead one. 



It is only with the latest portion of the geologic time 

 scale in Southern California that we are particularly 

 interested. This includes the latter days of the Plio- 

 cene, Pleistocene and Recent epochs, those yesterdays 

 of geologic time, which in northern regions, especially 

 in Europe, include the Glacial epochs and are known as 

 the Age of Man. In fact the geologic history of South- 

 ern California practically begins where that of other 

 regions leaves off. Nor will it be an exaggeration to 

 state that most of the great, geologic effects around us, 

 as we see them expressed in the physiograpic environ- 

 ment of Southern California are the records of events 

 that took place in the Pleistocene epoch. 



PERIODICITY OF EARTH MOVEMENTS 



Geologic history teaches us that there have been 

 great cycles of increased earth-making movement 

 alternating with epochs of quiescence during the long 

 course of geologic time, and that the effects of such 

 movement constituted the great barriers which divide 

 the true geologic periods. Although lasting through 

 vast stretches of time, which are incomprehensible in 

 terms of years to the human mind, each movement 

 had a life-cycle of birth, growth and decline of activity. 

 Records and vestiges of these movements are now 

 found as great unconformities and mountain systems 

 of different ages throughout the world. 



Among the cycles of movement of this kind were 

 those that produced the Appalachian mountains at the 



