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, be 

 asserted that here they are of a similar age. They constitute lofty hills which have sufiered 

 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 be 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 San 

 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. j\Iessrs. 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.)t 



1. Alluvium, 6 feet. 



2. Blue clay, 30 feet. 



3. Driit gravel, 22 feet. 



4. Arenaceous clay, 16 feet. 



5. Tenaceous blue clay, 160 feet. 



This catalogue only embraces 234 feet ; further enq[uiry 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. Sucli a thickness of deposit might be 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 be 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 



- Kep. 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. 



