MERRIAM: THE FAUNA OP RANCHO LA BREA. 207 



Geologists who have made a study of this region are generally in agreement 

 that the asphalt of Rancho La Brea originates in the shale formations which 

 underlie the Pleistocene beds. According to Arnold, Orcutt, and others who 

 have made a special examination of the conditions in the area west of Los 

 Angeles, the Fernando formation, which immediately underlies the Pleistocene 

 beds of the Salt Lake Oil Field, is in this region compressed into a fold, and 

 there are reasons for considering that the crest of this fold is more or less 

 irregularly broken, thus offering an unusual opportunity for the escape of 

 bituminous substances at this place. The seepage of oil and tar has presum- 

 ably been in progress almost continuously during the whole period of formation 

 of the Rancho La Brea Beds. The asphaltum deposit which has formed rep- 

 resents the residuum left after evaporation of the more volatile portions of 

 the oil. In accordance with slightly varying conditions of deposition, the 

 supply of asphalt has at times been relatively abundant enough to form the 

 greater part of the accumulation ; at other times it has sufficed only to make 

 up a small fraction of the material being heaped up in this place. 



In the earlier stages of deposition of the Pleistocene formation the quantity 

 of oil reaching the surface would presumably be larger than in the later periods, 

 when the thick accumulation of beds, consisting in part of asphalt, would tend 

 to prevent the escape of bituminous substances coming up from the lower 

 formation. During the later stages of accumulation, as at the present time, it 

 is evident that in addition to slow seepage through the mass, a part of the oil 

 and gas reached the surface through fairly defined channels or pipes. Through 

 passageways of this nature, which are common at Rancho La Brea, the move- 

 ment of masses of tar may take place spasmodically, the holes being apparently 

 empty at times, and on other occasions discharging volumin6usly. 



The mode of accumulation of the deposit is pretty clearly illustrated by the 

 conditions now obtaining in the vicinity of Rancho La Brea. This region is 

 at the present time a comparatively level country bordered on the northern side 

 by the Santa Monica Range, which rises abruptly to an elevation of about 

 twelve hundred feet above the plain. The lowlands are traversed by a few 

 drainage lines along which detritus is carried from the Santa Monica Range, 

 or from the lower hills, to the east and south. At a number of localities the 

 drainage is so balanced that water easily accumulates to form marshy spots. 

 Tar seepages are common, and lenses of asphalt of irregular shape and thick- 

 ness are continuously forming. 



The relation of the formation containing the mammalian fauna at Rancho 

 La Brea to the marine Pleistocene beds of the San Pedro Series 7 occurring 

 only a few miles to the south is not as yet entirely clear. From Arnold's inves- 

 tigations it would appear that the alluvial or fresh-water Pleistocene of the 



Arnold, R., Mem. Calif. Acad. Sci., vol. 2, 1903. 



