CHAPTER IX. 



VALLEY or SAN LUIS OBISPO. 



TorOGRAPBY OF THE VALLEY SLOPE AND DIBECTIOM OF THE 8TRATA. — EfFEOT OF THE DISPOSITIOM OF THE RANQES UPOS THE COAST LIHB 

 AND I'PON THE CLIMATE. — DrAISAGE OF THE VALLEY.— StUCCTURE OF THE COAST HILLS. — NcMBER OF THE BEDS INCLODED DNDER THE 



ASPIIALTE OROUPE. — PoLTTHALAMOUS BEDS FoSSILS OP THE GRODP. — PbOBABLE GEOLOGICAL AOB OF THIS SERIES. — RELATION OF 



POSITION TO THE SANTA MARGARITA VALLEY. — POSITION OF THE GRODP ON SANTA MARIA RIVER. — GEOGRAPHICAL EXTENT. — VARIETY 



IN DIP. — Alteration in beds bueseqcent to depositioh.— Denddation — Character of the valley proper. 



The ralley or plain of San Luis Obispo is separated from that of Santa Margarita by the 

 Santa Lucia mountains; between this range and the Pacific ocean is a plain in which the 

 village of San Luis Obispo lies, and from which the plain derives its name. 



The level of this valley is much below that of Santa Margarita, its altitude not much exceed- 

 ing 150 feet ; it slopes gradually to the ocean, from which it is separated by a range of hills 

 which stretch from the shore to a distance of six miles inward ; these hills do not exceed GOO 

 feet high, and dip variously in opposite ends of the valley ; about the San Luis river, which 

 finds its way to the sea through a break in them, and from thence southward the strata dip 

 toward the shore, while in the northern part of the valley these strata dip toward the valley. This 

 diiferent dip is caused by the serpentine and trappean protrusions, which are the elevating rocks 

 of the valley, passing across the strata in an oblique line from N. W. to S. E., not producing 

 anticlinal axes, but simply lifting the beds to the east at the north end of the valley and to the 

 west on the shore. The consequence of this different dip is evident by inspecting the coast 

 line, north of the bay of San Luis, where the dip is to the east and the lowermost beds of the 

 series are exposed ; these are hard conglomerates of a greenish tint, arising from pebbles of 

 serpentine and trappean rock, and have a dip from 15° to 20° S. 35° E. Here the strata stretch 

 out into the sea and form bold headlands which are washed and torn by the force of the 

 waves ; this character of shore line continuing to Estero bay on the north, where, owing to 

 the occurrence of the soft uppermost rocks on shore, the sea makes an inroad, which is checked 

 further north by the sharp crest of the Santa Lucia range, at Punto Gordo. Between this 

 headland and the south border of the Estero bay the valley of San Luis extends, opening to the 

 N. W., and allowing the winds and moisture to enter and supply it with a much greater rain- 

 fall and heavier dews than are to be found in the valleys east of the range ; owing to this, as 

 well as the lesser altitude, the climate is warmer, and the vegetation approaches an intertropical 

 character. 



The eastern margin of the valley is formed of the sandstone lying against the serpentine axis 

 of the Santa Lucia range ; these sandstones dip toward the valley, and are lost beneath the 

 alluvium. 



Along the east side of the valley, lying close to the foot of the sandstones, (not more than one 

 mile west,) is a series of elevated buttes of serpentine and trappean rock, which can be traced 

 traversing the valley from its northern limit, following the base of the Santa Lucia range, of a 

 triangular form, the serpentine, lying on the west side of the butte, forming its chief mass 



