GEOLOGIC HISTORY 211 



fault systems of Pleistocene time are related to 

 them. Likewise it is probable that each cycle of load- 

 ing and unloading of ice may have produced a corres- 

 ponding Hyrcenian cycle of earth movement, and that 

 each of these cycles may be related to one of the di- 

 rectional groups of faulting herein described. 



It was during the great Glacial Epochs — close of 

 Pliocene Time and the beginning of the Pleistocene — 

 that the last, great mountain-making movements of 

 Southern California took place. Nearly every high- 

 land and lowland which we now see around us was 

 largely made or added to during these epochs. Even 

 our Dominguez Range, which we speak of as "re- 

 cent," was made in the last half of the Pleistocene. 

 In terms of human chronology it is hundreds of 

 thousands of years old. Most of the great master 

 fault lines and fault scarps, and all of the master high- 

 lands and rift valleys of Southern California were 

 either made or rejuvenated in Pleistocene time. 



Beyond doubt we are now living in an interglacial 

 epoch of decreasing structural activity, due to the re- 

 moval by melting of the ice of the last great glacial 

 load. Furthermore activity will continue to decline 

 for fifty thousand years or more until an up-turn of 

 the climatic curve sets in and a new glacial period 

 begins. 



It is the general geologic opinion that we are 

 living in the early portion of an interglacial epoch 

 during which the northern ice loads are melting 

 and the earth stresses and strains are lessening. From 

 this fact and others associated with the history of the 

 glacial eras we maintain that Southern California is 

 passing through a period of decline in seismic activity. 



