SCIENTIFIC BACKGROUND 163 



west end of the Colorado Depression, which it inter- 

 cepts near the mouth of Morongo Canyon. Its course 

 is marked by the long, steep, deep and straight valley 

 of Mill Creek, which separates the summit range of 

 the plateau from a narrow ribbon-like range on the 

 south, lying between the Mill Creek and San Andreas 

 Rifts and which may be called the Mill Creek Range. 

 This range lies between the summit of the plateau 

 and the observer when he looks northward from the 

 line of the Southern Pacific Railway through San 

 Gorgonio Pass. The displacement along this fault is 

 some three thousand feet or more to the southwest. 



The eastward continuation of the Mill Creek Fault 

 passes by the little Indian Reservation marked "Hog 

 Ranch" on the southeast corner of the San Bernardino 

 topographic map of the United States Geological Sur- 

 vey. From the latter point eastward the fault follows 

 the south side of the Little San Bernardino Mountain 

 for an indefinite distance. These mountains make the 

 northern boundary of the northwest end of the Colo- 

 rado Depression. 



This fault is one of the great master faults of South- 

 ern California. Its exposure likewise is the highest 

 and northernmost of a series of master faults between 

 the San Gorgonio summit of the San Bernardino 

 Plateau (altitude nearly 11,485 feet) and the floor of 

 San Jacinto Valley (altitude 1600 feet) which branch 

 out eastward from the main San Andreas Rift Line 

 and step-down to the southwest with a collective down- 

 throw of some nine thousand five hundred feet. 

 THE SAN JACINTO RIFT 



Continuing southwestward from San Gorgonio Pass 

 across the several rifts, one again encounters the vari- 



