260 Adventures in Scenery 



over the dam to sink again in the gravel and sands below. This 

 furnishes the key to the cienaga which is known as the San 

 Bernardino Basin. 



The series of crustal movements which caused the folding 

 and crumpling of the clays and sands resulted also in the further 

 uplifting of the San Bernardino Mountains so that they stood 

 well above the adjacent valleys, but not to their present height 

 by many hundreds of feet. Mountain streams became active. 

 They cut deep canyons, and the products of erosion were car- 

 ried to the lowlands as they are today. The stream wash was 

 widely distributed over the lowlands south of the mountains. 

 Boulder beds, sand and gravel beds, and clays were laid down in 

 alternating layers in alluvial fans until many hundreds of feet 

 of "valley fill" were laid down. 



San Fernando Formation Marks Close 

 of Tertiary Time 



The Fernando epoch is generally regarded as marking the 

 close of the Tertiary Era. This was a time of great and wide- 

 spread disturbance. The period of deformation is definitely 

 fixed by the fact that Fernando and all older formations are 

 folded, bent, and crumpled, whereas the younger Quaternary 

 formations remain practically undisturbed, and rest horizon- 

 tally upon the eroded and truncated edges of the older forma- 

 tions. As a result of the folding of the rocks the Fernando 

 strata are mostly tilted at considerable angles from their orig- 

 inal horizontal position. Dips of 75 to 85 degrees are common, 

 and in places the strata stand in a vertical position. 



The Fernando formation consists of sandstones, shales, and 

 clays, with some coarser conglomerate. East of Olinda a sec- 

 tion 5,000 to 6,000 feet in thickness is exposed, sandstone and 

 sandy clay, with conglomerate. Farther east, as about River- 

 side, the formation contains comparatively little sand, and forms 

 a generally impervious clayey substratum below the overlying 

 Quaternary riverwash or "valley fill." Farther east beds of 



