94 Adventures in Scenery 



lava beds is known, but they are shown on the geologic map 

 as V, that is, volcanic. 



Quaternary deposits of clay, silt, sand, and gravel have been 

 spread by streams from higher lands upon earlier formations. 

 Detritus borne by streams covers an extensive plain in the Great 

 Central Valley of California, forming the Sacramento-San 

 Joaquin basin floor. Much of the vast desert region of south- 

 ern California is valley fill from streams from higher lands, of 

 Quaternary age. Marine Quaternary deposits occur along the 

 southern coast. 



Columnar Sections Show Succession 

 of Events 



In a columnar section for the San Francisco district (Fig. 

 18) the formations from the Jurassic to the present are shown. 

 The older formations of Palaeozoic time are not represented. 

 At the bottom of the column granitic rocks are shown, the age 

 of which is somewhat uncertain. It is thought that land areas 

 extended farther west than the present coast, but our knowledge 

 of these ancient lands is largely lacking. It is thought that 

 sediments eroded from lands farther west than the present coast 

 were deposited in an arm or bay of the Pacific Ocean that ex- 

 tended over what is now the western slope of the Sierra Nevada 

 in pre-Jurassic time. 



In the Gold Belt section (Fig. 19) very old sedimentary 

 formations, probably Carboniferous or older, are represented as 

 intruded by molten granite in late Jurassic time. The section 

 shows a great unconformity between the Jurassic and the Cre- 

 taceous, indicating a long "lost interval." Most of the gold- 

 quartz veins for which California is famous date from time 

 after the Jurassic upheaval, or early Cretaceous time. 



The ancient roof rocks that were uplifted by the batholith 

 in the Coast Range region have been practically all removed by 

 erosion. The ancient roof of the Sierra region has been largely 

 removed. As has been stated, the Calaveras and Mariposa 



