224 Adventures in Scenery 



east and tilted toward the west and south. The body of the 

 great plateau mass is made up of plutonic rocks, probably of 

 late Jurassic age. A vast mass of intrusive rocks was forced up 

 as a batholith underneath a roof of sedimentary rocks of prob- 

 ably Triassic age. These intrusive rocks cut into pieces the 

 once widespread sedimentary formations. Only remnants o'f 

 the sedimentary roof rocks remain, and these appear as meta- 

 morphic schists in scattered areas. Volcanic lavas lie along the 

 western flank of the range, notably in Otay, Jamul, and San 

 Miguel mountains. Bordering the range on the west, and 

 forming the coastal belt, are upper Cretaceous and Tertiary 

 strata, capped by a series of Quaternary marine terraces. 

 Strata of Tertiary age occur in the foothills of the Desert region 

 just east of the main range. 



Viewed in a broad way the whole mountainous region may 

 be looked upon as a vast much eroded Quaternary fault block, 

 with a steep eastern scarp and a general downward tilt to the 

 west and southwest. This plateau or highland region, 30 to 50 

 miles wide, is dissected by many streams. Numerous gorges 

 and canyons, from 500 to 1,500 feet deep, have been cut into 

 the plateau. The plateau is cut and broken by many faults, 

 and some of the more important streams follow fault zones as 

 lines of weakness in the rocks. The plateau surface generally 

 increases in altitude from 1,500 feet or more in its western por- 

 tion to over 4,000 feet in McCain's Plateau and more than 

 6,000 feet in the Laguna Mountains in the eastern portion of 

 the highland. The increase in altitude eastward is not uniform, 

 as the region has been broken by faulting into many large and 

 small blocks, most of which stand out in the form of more or 

 less well defined secondary plateaus, while others, relatively less 

 uplifted and usually smaller, mark sites of valleys or basins. 

 Various small and large parts of the great broken-up plateau 

 surface, which represent a former old age erosional surface, are 

 but little affected by stream dissection. Examples are south of 

 Alpine; south and southwest of Gautay Mountain; west of 



