208 MEMOIRS OF THE UNIVERSITY OF CALIFORNIA. 



Salt Lake Oil Field is nearly continuous with marine beds of upper San Pedro 

 age. As the Rancho La Brea Beds are, so far as known, an alluvial accumu- 

 lation, they are presumably not perfectly continuous with the marine San 

 Pedro, though an oscillating shore line might cause the intercligitation of 

 marine and alluvial lenses, or estuary conditions might give a gradation from 

 fresh-water to marine beds. That the shore line changed its position some- 

 what during the deposition of the San Pedro Series is evidenced by the uncon- 

 formity between the upper and lower members of this series, and by the 

 varying nature of the deposits of the Upper San Pedro. The unconformity 

 between the upper and lower divisions indicates considerable crustal 

 movements. 



A suggestion as to the relative age of the Rancho La Brea and San Pedro 

 beds is perhaps to be obtained by consideration of possible relationship of the 

 crustal movements which have affected the two groups of deposits. The series 

 of strata at Rancho La Brea and in the Salt Lake Oil Field is presumably 

 derived in a large measure from erosion of the Santa Monica Range immedi- 

 ately to the north. The abrupt southern slope of this range can hardly be 

 explained on any hypothesis other than that it represents a fault-scarp. In 

 the Salt Lake Oil Field the Pleistocene alluvial beds consist in general of 

 coarser materials below and finer above, showing a gradual adjustment of 

 accumulation to conditions somewhat similar to those obtaining at the present 

 time. Excepting the Recent alluvium, no series of beds later than those in the 

 Pleistocene section penetrated by the oil wells has been observed in the imme- 

 diate vicinity, and it is to be presumed that these Pleistocene beds represent 

 The accumulation following the last important movement along the faidt bor- 

 dering tbe southern side of the Santa Monica Range. In view of the fact that 

 the topography of this range is considered to indicate the completion of the 

 faulting movement at no very remote date, it is not improbable that one of the 

 last movements occurred in the period of local changes which were taking- 

 place during the deposition of the San Pedro, or at the close of that time. It 

 is true that important movements have occurred in this region of the coast 

 since the deposition of the San Pedro Series, as is evidenced by the prominent 

 coastal terraces which have cut sharply into the San Pedro. Terraces of im- 

 portance crossing a formation apparently not to be distinguished from the 

 Rancho La Brea Beds have also been referred to as occurring in low hills south 

 of the Salt Lake Oil Field. These movements have, however, been largely of 

 an epeirogenic character. They do not appear to have been accompanied by 

 Local faulting, and may not have changed the tectonic relations of the forma- 

 tions at San Pedro and in the vicinity of the Santa Monica Range. 



