CHAPTER XI. 



GEOLOGY OF IHE SIERRAS SANTA SUSANNA AND MONICA. 



Geographical position of these hanges External form op the hills. — The two chains separated by conejo pass and 



SEMEE CREEK. — Analogy of the strata of the sierra susaxna with those on the san buenatestuka at matiliiiaii. — Con- 

 nexion OF THE RANGE WITH THE CORDILLERAS. — SlERRA MO.MCA. — TrAPPEAN ROCKS.— SlOW DISINTEGRATION. — EFFECTS OF 



Absence of timber.- Contrast of semee plain with the valleys of encima and triumpho. — Trachytic upheaves. — 

 Traces of two different volcanic forces. — Trachytic kocks and augitic trap. — Enumeration and order of the sedi- 

 mentary REDS. — Fossiliferocs sandstones and limestones. — List of fossils. — Similarity of the susanna and monica 

 banges. — Occurrence of the asphaltic group in the latter range. — Extension into los angeles valley. — Thickness 

 of the beds of that group. — Resume op the structure of these beds. — Pkobable thickness of the sedimentary strata. 



These hills occupy the triangular space comprised between the Santa Clara river, with its 

 valley on the north, the ocean on the south, and the plains of Los Angeles and San Fernando 

 to the east, embracing a district 40 miles long from west to east, and 26 miles from north to 

 south. They constitute distinct ranges of hills, running in a direction nearly east and west, 

 having a slight northeast trend. 



They present in outline a series of rugged angular crested hills, which run parallel and rise 

 to an altitude of two to three thousand feet. As many as four of these ridges may be counted 

 in the Sierra Susanna, which lie most northerly, and are first met in passing up the Santa Clara, 

 against which river valley they abut their western edges. Each chain to the north lies a little 

 east of its neighbor, and thus the Santa Clara valley runs northeast until the river finally winds 

 behind the most northern ridge, and separates it from the Sierra Madre or Cordilleras, leaving 

 a communication by a pass between the Santa Clara valley and the plains of San Fernando. 



The two sierras are separated from each other by the Conejo pass, a considerably elevated 

 plain rising from the Santa Clara valley and passing in a line N. 30° E., enters the San Fernando 

 valley at its northwest corner ; as the pass narrows it loses its plane character, and is cut 

 across in places by ridges of plutonic rock. Down the slope of this pass rolls the Semee creek, 

 a small stream which fertilizes the pass, and finds its way into the ocean at the eastern end of 

 the Santa Clara valley. The ranch Las Posas lies upon this creek, at the point where it leaves 

 the pass to flow through the valley below. To the north of the pass lie the Sierra Susanna ; 

 to the south, the Sierra Monica. 



The Sierra Susanna, where observed, was made up of the red and yellow sandstones, similar 

 to those observed upon the Buenaventura river, crossing it 12 miles above the Mission. In the 

 most southerly of the ranges the strata dip N.E. about 18° to 20°. No primary rock is observed 

 in this sierra, the intrusive masses being amygdaloidal trachyte and felspathic porphyry rock. 

 Only the southern slopes of these ranges were observed, those upon the north side not having 

 been traversed. This range is connected at its southeast extremity with the Cordilleras by an 

 upheaval of trappean rock, which, starting from these hills, runs northwest toward the shore, 

 carrying upon its flanks the same sandstones, which may be traced from the Cordilleras to San 

 Buenaventura. 



