264 Adventures in Scenery 



plain to join the waters of the San Gabriel River by which it 

 finds passage to the sea. Streams of other canyons, as San 

 Antonio, Cucamonga, Deer, Day, and San Sevaine have in like 

 fashion built up cones at their mouths which have been broad- 

 ened out as fans till they have coalesced and form a continuous 

 alluvial plain, and this remarkable plain, extending from La 

 Canada on the west to El Cajon Canyon on the east is known 

 as the San Gabriel Valley. This alluvial valley floor extends 

 eastward from the Lytle Creek and El Cajon washes to the great 

 Santa Ana and Mill Creek washes at the east end of the San 

 Bernardino Valley. 



The San Bernardino Basin 



The eastern end of the San Bernardino Valley is called the 

 Basin. The north wall of the Basin is formed by the San 

 Andreas fault. The Ferris faulted block was tilted downward 

 at its northern end. The axis of the Basin is the line of meeting 

 of the slope from the fault with the northward dipping slope 

 of the granitic Ferris Plain. Along this axis passes the Santa 

 Ana River. 



The depth of the Basin to the underlying granite bed-rock 

 is not known. The Basin has been depressed below sea level 

 and filled with detritus borne by the Santa Ana River, Mill 

 Creek, and San Timoteo Creek, and other smaller streams. 

 Wells sunk to a depth of 1,000 feet have not reached the bot- 

 tom, but the floor of the Basin is doubtless granitic rock such 

 as forms the floor of the Ferris Plain to the south. The coarse 

 detritus that has been borne from the mountains by torrential 

 streams furnishes a vast storage reservoir for water. The 

 Bunker Hill Dike acting as dam holds back the water that 

 penetrates the alluvial washes or fans of the debouching streams. 

 The Basin thus becomes filled with water, and under pressure 

 of the head due to the higher level of the washes this rises as 

 springs, and flows over the Bunker Hill Dike. Such a basin, 

 in a semi-arid region, filled with porous detritus and saturated 



