NATURE AND ORIGIN OF THE RANCHO LA BREA BEDS 



The formation in which the fossil bones occur at Rancho La Brea is essen- 

 tially an alluvial accumulation consisting mainly of heels of clay, sand, and 

 asphalt. In some places bedding planes are quite distinctly shown, especially 

 in the clay or sand strata, At other localities the deposit has been irregular 

 or in pockets, and minor local movements of the asphalt or clay masses have 

 possibly increased the irregularity of the beds. The asphalt may occur as a 

 nearly pure bituminous deposit, though it is usually mixed with sand, clay, or 

 other materials. In some places it appears as fairly definite strata, while at 

 other points it has irregularly impregnated beds of sandy or clayey material, 

 and no definite bedding is shown. 



The thickness of the asphalt deposits containing bones has not been deter- 

 mined, but these beds probably extend considerably deeper than the lowest 

 levels yet reached. A depth of at least thirty feet is recorded for the work 

 carried on by Major Hancock, and nearly the same horizon has been attained 

 in recent excavations. 



The formation in which the asphalt appears has been penetrated by the wells 

 in the Salt Lake Oil Field immediately to the north of the principal brea 

 outcroppings, and seems also to be a part of a series of beds comprising a con- 

 siderable thickness of sand and clay strata exposed in the hills immediately 

 to the south. The well records of the Salt Lake Field, as described by Arnold, 

 indicate the presence of Pleistocene strata from fifty to one hundred feet in 

 thickness overlying the Tertiary formations in which the main oil-bearing beds 

 are found. The Pleistocene section penetrated by the oil wells comprises allu- 

 vium, clay, coarse sand, gravel, and asphalt, the deposits being apparently all 

 of fresh-water or alluvial origin. Asphalt is well represented down to the 

 bottom of the Pleistocene portion of the section. 



The formation exposed in the low, flat ridges immediately to the south of 

 Rancho La Brea is apparently a part of the series represented by the fossil- 

 bearing strata at Rancho La Brea and by the Pleistocene penetrated in the oil 

 wells of the Salt Lake Field. The ridges south of the fossil beds are consid- 

 erably eroded and terraced, and it is to be presumed that the erosion which is 

 indicated occurred in Pleistocene time. It is therefore probable that the fossil- 

 iferous beds now exposed at Rancho La Brea were at one time covered by many 

 feet of strata, which have been removed by erosion inaugurated before the 

 beginning of the Recent epoch. 



o Arnold, K., U. S. G. S. Bull. no. 309, p. 1ST, 1907. 



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