12 CALIFORNIA ACADEMY OF SCIENCES. 



from this bluff. Two and a half miles north of Long Beach is Los Cerritos, or 

 Signal Hill, as it is commonly called, the most prominent point in a series of low hills 

 extending to Dominguez Hill, six miles to the northwest. Los Cerritos is three 

 hundred and sixty-four feet high. Its northern side is smooth and slopes gently into 

 the great Los Angeles plain. Its southern slope is much steeper, and is cut by many 

 deep, narrow ravines, which offer a fine opportunity for studying the geology of the hill. 



2. General Geology. 



The oldest formation exposed in the immediate vicinity of San Pedro is the 

 Miocene, or Monterey series. The shales of this formation are exposed along the sea 

 cliff on the eastern end of San Pedro Hill, and also at Deadman Island. After the 

 deposition of the Miocene the shale beds were raised and contorted and subjected to 

 erosion. During the Pliocene period a submergence took place and a deposit of fine, 

 yellow, clayey sand of unknown thickness was laid down on the surface of the eroded 

 Miocene shales. 



A post-Pliocene uplift laid bare the sandstones, which were worn away com- 

 pletely from some parts of the eastern base of San Pedro Hill. Timra's Point and 

 Deadman Island are the only places at which they are now exposed. 



Again there was a change of conditions. The eroded surface of the Pliocene 

 became sea bottom, and deposits of fine, gray, silicious sand, extremely fossiliferous 

 in places, were laid down unconformably on it. This particular horizon is repre- 

 sented by the gray sand deposit of Deadman Island, by the gray, sandy strata 

 exposed along the bluff southeast of San Pedro and in the lower part of the continua- 

 tion of this bluff just north of the San Pedro valley. The lower series of sandstones 

 and conglomerates of Los Cerritos may be contemporaneous with these last men- 

 tioned beds. On account of its fauna and its unconformable position on the Pliocene 

 this horizon is thought to be of Pleistocene age. It is called in this paper the 

 lower San Pedro series. The maximum thickness of this particular horizon, so far 

 seen, does not exceed fifty feet. 



After the lower San Pedro there was a period of shallow water, lagoon and 

 dune conditions prevalent along this part of the coast, during which the conglomerates 

 of Deadman Island, San Pedro and Los Cerritos were laid down, and the sandy forma- 

 tions in the bluffs one-half mile north of San Pedro and along the Long Beach water 

 front were dejiosited. This period was one of rapidly changing conditions, as is 

 shown by sand-dune deposits and by the nearly horizontal aqueous deposits of both 

 tine sand and gravels in alternating beds. These beds dip gently away from the 

 centers of uplift, and many of the strata are very fossiliferous. This series of strata 

 is called the upper San Pedro series. The maximum thickness of the strata of this 

 horizon is over fifty feet, as is shown by the exposure in the sea-cliff southeast of 

 Long Beach. 



Overlying all these deposits is the alluvial soil, varying in depth from two to 

 ten feet, mostly adobe, and filled in some places, notably along the San Pedro bluffs 

 and Deadman Island, with the shells of edible mollusks. These refuse heaps, or 

 ancient kitchen-middens, are abundant on this part of the coast. 



