CHAPTER VII 

 THE GEOLOGIC MAP* 



The Geologic Map 



The State Geologic Map of California shows the areal geol- 

 ogy, that is, the character of the land, the rock formations, as 

 these occur at the surface. It really goes much below the sur- 

 face. It shows what particular formation occurs at the surface 

 at any given place, but to be understood the map should be 

 studied in connection with a geologic cross section. A cross 

 section shows the formations in the order of their occurrence, 

 the younger above the older. The geologic map may be more 

 clearly understood by reference to columnar sections of the 

 formations (Figs. 18 and 19), and cross sections (Figs. 25 and 

 26) . The geologic section shows the order of succession of 

 rock formations, the oldest known rocks being shown at the 

 bottom and later formations represented in the order of their 

 age. On the geologic map the formations are represented as 

 they occur today at the surface. A geologic section at any 

 locality shows at the top the formation that occurs at the sur- 

 face in that locality. From the top down whatever formations 

 are known to occur beneath the surface are shown. 



Geologic sections in different localities of California differ. 

 The succession of "ages" is the same, but formations that ap- 

 pear in one may not appear in another. The ideal section shows 

 the geologic time scale, the historical succession of geologic 

 formations throughout the world. In the actual section gaps 

 or "unconformities" may occur, and formations that appear in 

 one section may be absent in another. (Compare the San 

 Francisco and Gold Belt sections. Figs. 18 and 19.) The 

 absence of a formation in the geologic column or section, or 



* Geologic Map of California, by Olaf P. Jenkins. 



