Page 11 - 1915, Springs of CA.

PHYSICAL FEATURES OF CALIFORNIA. 11
highly altered metamorphic rocks, although limestones and quartz-
ites, only slightly altered, occur in considerable masses in the San
Bernardino Mountains. The rocks in the Coast Range types of
mountains, on the other hand, are usually slates, shales, sandstones,
and conglomerates, ranging in age from Triassic to late Tertiary or
Pleistocene. A fringe of these later sediments lies along the Pacific
border of the Peninsula Range in San Diego and Orange counties,
and a similar belt encircles and probably underlies the Colorado
Desert at the eastern base of this range.
GREAT CENTRAL VALLEY.
The Great Central Valley of California, about 16,000 square miles in
area, includes the Sacramento and San Joaquin valleys, and as its
name indicates, it occupies a central position in the State. The north
end of Sacramento Valley is near Redding, where, at an elevation of
about 600 feet, the lowland along Sacramento River is about 10 miles
wide. The valley increases in width southward to about 20 miles at
Red Bluff and 40 miles at Willows, and it continues at approximately
the latter width to Suisun Bay, into which the Sacramento discharges.
The broad lowland continues southeastward from Suisun Bay as San
Joaquin Valley. Its width is only about 30 miles in the northern
portion, in the vicinity of Stockton, but increases to about 60 miles
at Hanford. Thence it narrows again t9 its south end at the base of
the Tehachapi Mountains, where the valley floor is about 1,200 feet
above sea level.
The Sacramento Valley drains through Sacramento River and its
tributaries into Suisun Bay and thence to the Pacific. The northern
part of the San Joaquin Valley also drains out through San Joaquin
River, but the more arid southern section is an inclosed basin. A low
alluvial divide that is formed by the delta of Kings River separates the
drainage of the southern half from that of ttie northern. The Tulare
basin, whose lowest point is occupied by Tulare Lake, receives a part
of the drainage from Kings River and all excess waters from the valley
tributaries south of it. This fluctuating lake, which occasionally dries
up entirely, is merely a great evaporating pan, from whose surface
the excess waters from Kings, Kaweah, Tule, Kern, .and other rivers
pass into the atmosphere.
The only notable interruption in the surface continuity of the Great
Central Valley is formed by Marysville Buttes, a volcanic mass in the
central part of Sacramento Valley. The valley surface is almost en-
tirely covered by alluvium, but unaltered fresh-water and marine
sediments form most of its bordering slopes and outlying hills, although
east of the middle portions of both the Sacramento and San Joaquin
valleys the alluvium of the valley directly overlies the granitic and
metamorphic rocks of the Sierra. Along the northeastern border of