128 SOUTHERN CALIFORNIA GEOLOGY 



composed of corresponding parellel folds, but are elon- 

 gated, tilted monoclinical blocks each of which con- 

 sists of a series of segments of cross folds of opposite 

 directions which have been inherited from some pre- 

 ceding physiographic condition that has since been de- 

 faced by the later structures. 



PROVISIONAL CLASSIFICATION OF THE RIFTS 

 OF SOUTHERN CALIFORNIA 

 With full knowledge that our classification is ephe- 

 meral and incomplete, for conveniences of discussion, I 

 am going to arbitrarily and tentatively classify the 

 many fault rifts of Southern California into four direc- 

 tional belts or groups. 



1. Rifts of north-south directions. 



2. Rifts of northwesterly directions (first group). 



3. Rifts of northeasterly directions. 



4. Rifts of east-west directions. 



5. Second group of rifts of northwest southeasterly 

 directions. 



The different groups of belted structures are in most 

 instances of different ages. Although deductions are 

 not final as to this, I will tentatively hold, for example, 

 that the north-south extending ranges and valleys 

 of the Great Basin and Colorado Depression regions are 

 practically reflections of displacements of similar trend 

 which were made at the beginning of, or before, the 

 Miocene epoch as has been set forth by others; that 

 some of the first, north-west group of true Coast Ran- 

 ges of Northern California and probably the Catalina 

 Ranges of the Continental Shelf developed in late mid- 

 dle or post-Miocene inasmuch as early Miocene strata 

 are folded ; that the Transverse trends were formed in 

 Pliocene and early Pleistocene and the second group of 



