CHAPTER IX. 
VALLEY OP SAN LUIS OBISPO. 
Topography of the valley slope and direction of the strata.—Effect of the disposition of the ranges upon the coast line 
AND UPON THE CLIMATE.—DRAINAGE OF THE VALLEY.'—STRUCTURE OF THE COAST HILLS.-NUMBER OF THE BEDS INCLUDED UNDER THE 
ASPHALTE GROUPE.—POLYTHALAMOUS BEDS.-FOSSILS OF THE GROUP.-PROBABLE GEOLOGICAL AGE OF THIS SERIES.—RELATION OF 
POSITION TO THE SANTA MARGARITA VALLEY.—POSITION OF THE GROUP ON SANTA MARIA RIVER.—GEOGRAPHICAL EXTENT.—VARIETY 
in dip.—Alteration in beds subsequent to deposition.—Denudation.—Character of the valley proper. 
The valley 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 600 
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 
different 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 Grord. 0 . 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 
