PUENTE HILLS*. PUENTE FORMATION. 
103 
Llie Puente Hills are the northwestern extension of the Santa Ana 
Mountains, from which, however, they are now separated by the deep 
canyon of Santa Ana River, with a fault, perhaps, as an interruption 
to the present continuity of structure. The general trend of the hills 
is west-northwest. Their highest point, 1,780 feet, is San Juan Hill, 
on the boundary line between San Bernardino and Orange counties. 
In the eastern half of the hills numerous peaks reach elevations between 
1,200 and 1,400 feet, but in the western half this altitude is excep¬ 
tional. The base of the hills lies about 400 feet above sea level. For 
3 or 4 miles to the south an altitude of 200 or 300 feet is maintained, 
the southern edge of this area being defined by a low ridge that lies 
parallel with the hills from the region of Placentia to a point opposite 
Whittier. South of this the elevation drops to less than 150 feet, 
decreasing gradually to the sea. 
The region of the Puente Hills is dry, all the streams being inter¬ 
mittent. The canyons are deeply cut and in places present consid¬ 
erable ruggedness of aspect. Nearly all are the result of erosion, 
although the loci of some of the erosion valleys were doubtless deter¬ 
mined by the folding to which the strata had been subjected. Nar¬ 
row, perpendicular-sided channels characterize the bottoms of many 
of the canyons, especially those cutting through shale. In many 
places, however, the higher slopes are gentle and the hill summits 
rounded and grassed. 
GEOLOGY. 
FORM ATI ON S.a 
The formations involved in the geology of the Puente Hills include 
the Puente formation, largely sandstone and shale, of Miocene age 
and the equivalent of at least a part of the Modelo formation, and 
possibly including some of the Vaqueros; some diabase post-Puente, 
probably contemporaneous with "similar rocks found throughout the 
Coast Range as far north as San Francisco; clay, sandstone, and 
conglomerate of the Fernando formation, largely Pliocene in age; and 
superficial Pleistocene deposits of sand and gravel. (See PI. X.) 
The Puente formation has been divided on lithologic grounds into 
a lower shale, a sandstone, and an upper shale. 
LOWER PUENTE SHALE .b 
The lowest rocks exposed in the Puente Hills, which will be called 
the lower Puente shale or simply lower shale, embrace at least 2,000 
feet of shale, in the main earthy, but with minor members of a sili- 
a A table giving the formations of the Santa Clara Valley, Los Angeles, and Puente Hills districts ; 
together with their probable correlatives of the standard Tertiary formations of California, is given 
on p. 143. 
b It was not the intention of the authors to apply specific names to the divisions of the Puente, 
but to treat them as unnamed parts of that formation. 
