154 
OIL DISTRICTS OF SOUTHERN CALIFORNIA. 
The thickness of the Pleistocene, especially along the southern 
edge of the area mapped, is probably 300 to 500 feet, while in cer¬ 
tain of the terrace deposits, such as those on the hills north of Sher¬ 
man and on the terraces flanking Los Angeles River and Arroyo Seco, 
it may be only from a few inches to, say, 50 feet. 
BREA DEPOSITS. 
In the region immediately northwest of the Baptist College, in the 
territory one-half to three-fourths mile southwest of Colegrove and 
on the south side of the Rancho la Brea the Pleistocene and alluvial 
deposits are impregnated with asphaltum, forming brea. This mate¬ 
rial originates in two ways—heavy oil or asphaltum exudes from the 
surface and becomes filled with drifting dust and sand, or the fluid 
exuding from the underlying hard strata impregnates overlying 
porous gravel and sand. Details concerning the brea deposits are 
quoted under the heading u Previous knowledge of the region,” the 
older geologists having had an exceptional opportunity for studying 
these u tar springs,” which at the time of their visits were almost 
untouched and much more extensive than at present. In the Rancho 
la Brea deposits at the south side of the Salt Lake oil field many bones 
of extinct mammals have been found. (See PI. XXIV, B.) The fol¬ 
lowing notes concerning these remains are quoted from an unpub¬ 
lished manuscript by Dr. J. C. Merriam: 
The beds in which the bones occur extend over many acres. So far as I am aware 
the bottom has not been reached in excavations carried on to the depth of at least 15 
feet in quarrying the asphalt. Bones are scattered through a large part of the deposit, 
but are very unevenly distributed. In some localities they are present in large num¬ 
bers and in fairly defined layers. 
The asphalt has in many cases penetrated even the minute pores of the bone, but 
the original material of the skeleton is itself practically unchanged. 
The remains recognized up to the present include the following: Elephas, Equus , 
Bison , Mylodon (?), Smilodon, Canis indianensis (?), Cams small species and camel 
remains. Numerous bird bones and remains of insects are also found. 
In a considerable number of cases large parts of the skeleton are found together, 
showing that the carcasses were entombed so quickly that there was not sufficient 
time for decomposition to permit separation of the parts. 
Of the remains recognized up to the present time, an extraordinarily large percent¬ 
age represents Carnivora. The number of carnivores is certainly relatively larger than 
the usual percentage in a well-balanced fauna, and this abundance must be attributed 
to peculiar conditions under which the bbnes accumulated. Undoubtedly most of 
the remains are those of animals that have been entrapped or mired in the asphalt at 
times when it formed a sticky deposit around tar springs. The surface of the asphalt 
is very sticky in some places at the present time, and where cuts are opened in it tar 
may ooze out. Such pools have probably existed here interruptedly through a long 
period, and particularly during Quaternary time when the deposit was forming. 
Carnivores are numerous, because they were attracted by birds and mammals caught 
in the asphalt. Perhaps it is not entirely a coincidence that the carnivore remains are 
usually associated with those of birds or mammals, which would be their natural prey. 
The considerable number of young saber-tooth cats present may indicate that the 
younger and less experienced individuals were more easily lured into the tar pools. 
