MIOCENE AND QUATERNARY BEDS—ARTESIAN BORING. 
83 
line to form the Sierra Susanna, and as they have there been recognized as similar to those on 
the Rio San Buanaventura, which are certainly Miocene, it may, perhaps, without error, he 
asserted that here they are of a similar age. They constitute lofty hills which have suffered 
very much from denudation, and which are daily losing by the effluent streams which carry 
down their detritus to the plain. The low ranges which intersect the plain and divide it into 
two are the outliers of the Sierra Monica, which stretch toward Jurupa, and have suffered by 
the fluviatile erosion already alluded to, and belong to a later period of the same group. These 
strata dip north, while the Susanna sandstones dip south on the north side of San Fernando 
plain. The low ranges of San Pedro, which extend along shore, are a repetition of these beds 
further inland. These strata also dip to the north. Thus two basins are formed by the dipping 
of the strata. In the San Fernando and San Bernardino valleys the strata dip toward each 
other, while in the Los Angeles plain it is only the southern ridge of San Pedro whose strata 
dip north. The hollows formed by the slope of the strata is filled up by Quaternary clays and 
gravels, the depth of which can only he estimated approximately; in the Los Angeles valley 
more exact information has been gleaned, owing to the sinkings made for an artesian well a 
little outside and west of the town. The selection of the spot was most unfortunate for success 
and might have been avoided by a little geological knowledge. During the period of visiting 
these plains boring operations had been suspended until the arrival of fresh apparatus from g!an 
Francisco.* The augur had already perforated 540 feet without meeting with water ; a heap of 
bluish plaster clay with yellow gravel of granitic pebbles were the only indications of the sub¬ 
terranean constituents. Messrs. Butt and Wheeler stated that, after passing through 30 feet of 
clay, sand and gravel were the chief beds met with ; solid rock, however, had not been reached 
when the labors were suspended, so that these incoherent Quaternary beds were over 500 feet in 
thickness. 
Dr. Trask, who examined this locality during the period of artesian boring, gives the following 
as the thickness and nature of these beds to the point at which the sinking had then reached, 
(400 feet.)f 
1. Alluvium, 6 feet. 
2. Blue clay, 30 feet. 
3. Drift gravel, 22 feet. 
4. Arenaceous clay, 16 feet. 
5. Tenaceous blue clay, 160 feet. 
This catalogue only embraces 234 feet; further enquiry at the pueblo only showed that 
nothing but blue clays of various degrees of coherence were met with. This would give to the 
lower bed a thickness of above 300 feet. Such a thickness of deposit might he attributable to • 
the local circumstances, namely, a deep trough in the sandstone strata under an elevation 
almost vertical, close by ; yet that these incoherent beds are usually of great depth is evident 
from the smooth surface of the whole plain, which preserves its gradual slope from the Cordil¬ 
leras to the ocean, independent of the dip or upheaval of the strata beneath. Again, when 
looking from the south entrance of the Cajon pass towards San Bernardino, at an altitude of 
2,000 feet, there may he perceived a broad terrace at the base of the mountain, consisting of 
loose conglomerates, gravel and clay beds, lying at an elevation nearly 200 feet above the present 
* Rep. Geol. of Coast Mountains; Doc. 14, sess. 1855 ; Cal. 
t Since his visit a further sinking of 140 feet was made, and then the operation was abandoned from ill success. 
T. A. 
