116 GUIDEBOOK OF THE WESTERN UNITED STATES. 



On leaving San Luis Obispo the train crosses San Luis Obispo Creek, 

 which emerges from a canyon in the Santa Lucia Kange, on the right 

 (east). On the left is Cerro San Luis Obispo, one of the old volcanic 

 necks. A mile and a half beyond the station, on the right, is the Cali- 

 fornia Polytechnic School and in the foreground a ledge of rocks char- 

 acteristic of the Franciscan, 



Tlic railroad curve kno^^^I as "the horseshoe" begins at Goldtree, 4 



miles from San Luis Obispo, with a grade of 2.2 per cent. The ascent 

 continues thi'ough shallow cuts in sandstone belonging to the Fran- 

 ciscan group, but at the end of the horseshoe, above Goldtree, the road 

 swings to the left and crosses a belt of serpentine to the open hilly 

 country of the overlying Monterey shale. Near Serrano (elevation 

 941 feet) Franciscan rocks again appear in complex association with 

 serpentine and dark intrusive rocks. The railroad continues on these 

 rocks principally for 4 miles, passing by six tunnels through ridges of 

 diabase. The longest tunnel, 3,616 feet long, is farthest north and 

 pierces the divide into Salinas Valley, nearly 600 feet beneath the 

 summit. 



The black earthy shales seen between the northernmost tunnel and 

 Cuesta (239 miles from San Francisco) are of Lower Cretaceous (Knox- 

 viUe) age. A little of the same shale was crossed between the first 

 and second tuiuiels. The shale overhes the Franciscan rocks, but the 

 two formations were not deposited continuously. The Franciscan 

 rocks were raised above the sea and eroded, then submerged again and 

 covered by the muds now consohdated as the Knoxville shale. The 

 geologist expresses these facts briefly by saying that the Knoxville Hes 

 unconformably on the Franciscan. On the right (east) at Cuesta is 



an area of dark intrusive rock (olivine diabase). Beyond this the 

 railroad passes across a small area of light-colored shale (Monterey), 

 traverses another area of dark Knoxville shale, and enters upon a belt 

 of sandstone, which with some conglomerate and shale composes the 

 characteristic Chico (Upper Cretaceous) formation. The sandstones 

 of the Chico weather into large blocks. Near Santa Margarita the 

 sandstone is covered by recent alluvium. 



The general trend or strike of the formations here, as elsewhere in 

 this part of California, is northwest, in accord with the general align- 

 ment of ridges and valleys. 



Santa Margarita is on the southwest side of a nortliwest-south- 

 east depression which is followed for most of its length hy Sali- 

 nas (sa-lee'nas) River. The valley extends for 8 or 

 Santa Margarita, g miles southeast of the town and is followed by 

 Elevation 995 feet. the raiboad for ahout 10 miles to the northwest. It 



Popukition 918. 'it *t i t - ■■ j i 



Los Angeles 239 miles. ^^^ ^^s lorm partly to geologic structure and partly 



to erosion, as it is occupied by comparatively soft 

 Tertiary formations, while the hills on both sides are composed 

 mauily of harder, older rocks. Along the southwest side of the 



