Geology from a Mo for Car 379 



240 miles (El. 998 feet) ; Atascadero, 248 miles (El. 834 feet) ; 

 Paso Robles, 259 miles (El. 740 feet). 



Old Volcanoes; Tunnels through Santa 

 Lucia Mountains 



San Luis Obispo (Bishop of St. Louis) is one of the old 

 Spanish towns of California. The Mission building founded in 

 1772 by Father Junipera Serra is still standing. A most strik- 

 ing geologic feature in the vicinity of San Luis Obispo is a row 

 of eight hills, four northwest and four southeast of the city, 

 which are the cores of volcanoes which broke through the sedi- 

 mentary (Franciscan) rocks which occur in the region. 



The railroad elevation of San Luis Obispo is 240 feet. At 

 the summit of the Santa Lucia Range, where the railroad passes 

 through a tunnel 3,616 feet in length, the elevation is 1,570 feet, 

 a climb of 1,338 feet in a distance in a straight line of six miles, 

 the railroad winding a circuitous way about double the distance 

 through six tunnels. The long tunnel (Cuesta Pass) is through 

 the divide of Santa Lucia Range between San Luis Obispo Creek 

 and Salinas Valley, nearly 600 feet below. 



Upper Salinas Valley a Structural 

 Basin 



Upper Salinas Valley (as distinguished from the lower and 

 main Salinas Valley) is a basin nearly 20 miles in length, a long 

 narrow trough through which Salinas River flows for most of 

 its length, but the valley does not reach the ocean. It is a struc- 

 tural valley, bent down into a syncline, with hard granitic rocks 

 on the northeast side and the Santa Lucia Range on the south- 

 west. Tertiary sediments were deposited in the trough. Dur- 

 ing the long processes of erosion the valley floor was lowered. 

 The Salinas River had become established in a course through 

 the soft friable Tertiary deposits. As the valley floor was low- 

 ered the channel of Salinas River reached the underlying gran- 

 ite. Its course having been established it could not leave its 



