84 QUATERNARY DEPOSITS — BLUE CLAY. 



level of the plain in its neighborhood, and which are the only remains of a series of beds which 

 have been removed from the lower and more exposed parts of the plain. Its average thickness 

 perhaps might be about 200 feet ; the other beds would preserve throughout a pretty uniform 

 thickness ; of these, bed 4, an arenaceous yellow clay is described as containing small marine 

 shells. The brownish loamy clay (bed 1) is exposed by every creek, and in the sections produced 

 by the Los Angeles river several feet of the bluish clay (2) are exposed ; the beds are deposited 

 almost perfectly horizontal, and are therefore unconformable to the soft sandstones of the San 

 Pedro hills and the Sierra Monica, which in the former case have a dip of 20°, and in the latter 

 are in places almost vertical ; they have, therefore, been deposited posterior to the upheaval of 

 these soft tertiary sandstones, and the surfaces have undergone no material alteration of contour 

 since, the only change being that of elevation of the whole region out of the bed of the sea. 



An investigation into the mineral nature of these various deposits shows that the alluvial 

 covering, to the depth of six or seven feet, is aluminous in its finer parts and granitic pebble in 

 its coarser, and has been the result of the degradation of granitic and felspathic rocks. The 

 soil of the plain is rarely quartzose, except when close to some of the low tertiary hills, which 

 alteration may therefore be due to the wash of these latter. 



The blue clay is generally assigned by geologists to a slow deposit of mud produced by the 

 sifting action of the tide in estuaries or gulfs where matter is not transported by current actions ; 

 it is the evidence of a calm condition of the waters during the period of deposit, and a cessation 

 of upheavals of the land contiguous ; the two beds of bluish clay are separated by nearly 40 

 feet of gravel and sand. 



The drift gravel (bed 3) consists not only of rounded granitic pebbles, but also those of syenite, 

 hornblende schists, metamorjjhic brown sandstones, trap and amygdaloid ; and the underlying 

 sandy bed is chiefly quartzose, and probably is the detritus of the sandstones at the base of the 

 Cordilleras. 



There have been no very large stones seen in the drift beds, there are no loose boulders or 

 erratic blocks, nor is there, either on the surface or in the deposits, any stone which cannot be 

 traced to masses of similar mineral constitution in the ranges bordering the plain. The period 

 of general or polar drift, therefore, which was one of the earliest in the Quaternary epoch, passed 

 by without affecting California — and it was during the later periods of drift that the processes 

 of wearing down continents and depositing them in the seas around took place, and were carried 

 out on an immense scale and over an immensely extended period of time. 



Los Angeles plain is not the only one in California where these deposits of clays and gravels 

 are of great depth. The borings which have been made in Sacramento and San Joaquin plains 

 have revealed a similar structure of basin, while that in Santa Clara valley, Santa Clara county, 

 shows that the deposit has not been to so great a depth in that plain. 



Thus, at the Stockton well boring, after passing through red clays, sands, and gravel, the 

 blue clay was met with at the depth of 400 feet. 



On the Sacramento valley, observed exposed between the city and Pit river, the lava clays 

 and sands covered the blue clay to the depth of (Dr. Trask) 358 feet. 



In the Santa Clara valley, the covering of clay and light sand above blue clay was from 80 to 

 115 feet, averaging 90 feet. 



The depth at which the blue clay is reached in Los Angeles plain (234 feet) stands inter- 

 mediate between the Santa Clara valley and the great plain between the Coast Range and the 

 Sierra Nevada. The bottom of the Santa Clara blue clay has been reached, and has been found 



