SCIENTIFIC BACKCiROUNU 125 



San Bernardino, San Jacinto, Morongo and Bullion 

 Master Highlands. The physiographic units resulting 

 from this type of structure may be broadly compared 

 to those of the old-style, brick pavements that still 

 prevail in some parts of the world, in which some of the . 

 blocks have been pushed up above the others and some 

 have been lowered.^ The fault blocks, with but few ex- 

 ceptions, are more oblong or belt-like than equal-sided. 

 Consequently the resulting physiography, is in most 

 instances, of the elongated belted type. The fault 

 blocks have various attitudes anPlspects. Some are 

 upthrust, some are depressed, some are tilted, some 

 folded, as has been described more in detail by others. 

 Not only do the highlands of Southern California 

 consist in part of fault blocks of this type bounded by 

 structural lines, but some of the valley plains also. 

 These latter may be considered down-thrown blocks, 

 partially of faulted origin, and usually bounded on at 

 least one side by linear faults. 



Several of the larger valleys like the Fernando and 

 Downey plain are bordered on one side by strata dip- 

 ping towards the center and on the other by fault dis- 

 placements. 



CUMULATIVE AND RENEWED UPLIFTS ALONG 

 FAULT LINES 

 In instances, the highland and lowland blocks may be 

 alternately rising and sinking. Again it is also prob- 

 able, as shown by cross-sections of the Peninsula 

 Highlands in San Diego County, that the general suc- 

 cession of faults, folds and fault blocks from the inter- 

 ior shoreward has resulted from a series of cumulative 

 uplifts of the continental side away from the sea. 



'R. T. Hill, Bull. Seismological Society of America, 1920. 



