The Los Angeles Basin 



277 



had been turned as on a pivot clock-wise from left to right. 

 The deep-lying folds or dome-like structures which have been 

 the sources of the productive wells are generally represented by 

 surface elevation. These elevations range from the elongated 

 hills of Dominquez and Long Beach to the low swells of Rich- 

 field and Huntington Beach. The surface domes closely follow 

 the sub-surface structure. 



PLEISTOCENE TERTIARY 



Fernando Group 



Alluvium and asphalt, Sandy-clay, gravel, sand, 

 sand, clay, gravel, rubble. oil-sand. 



Courtesy Los Angeles Museum Publications 



FIG. 77. Cross section showing geologic structure of formations at 

 Rancho La Brea during period of miring of Pleistocene animals and plants. 

 From section in Salt Lake oil field, after Arnold (U. S. Geol. Survey) . (After 

 Stock: Los Angeles Museum Publications, No. 1). 



Oil Accumulated through Joint 

 Cracks 



The accumulation of oil in pools and in porous sand rocks 

 has probably been facilitated by the fracture or cracking of the 

 rock strata into joints. All the rocks in the Los Angeles dis- 

 trict are intersected by numerous joint cracks. In many places 

 a slight displacement by slipping has occurred. These tiny 

 cracks in the rocks are thought to have played an important 

 part in the accumulation of oil. The joints or cracks seem to 



