LOWER CRETACEOUS DEPOSITS CALIFORNIA AND OREGON

CRETACEOUS EMBAYMENTS 17
110 such outcrops are found along their eastern borders. Whether this lack
of early Cretaceous detritus on the eastern borders of these troughs was
due to limited drainage slopes leading into them from the east or to arid
climatic conditions here, such as now exist, is not known. But it is possible
also that faulting near the central axis of the trough during later
epochs of the period, with elevation of the Sierra Nevada block, and the
denudation of its early Cretaceous sediments, followed by still later
subsidence and marine transgression, may have hidden evidence of their
existence.
In the southern embayment of the trough the suggestion finds some
support in the deep well drilled near Chowchilla, near the axis of the
valley, which passed through only upper Cretaceous sediments and into
basement rocks.
CRETACEOUS DEPOSITS IN THE GREAT VALLEY
From the viewpoint of historical geology, the most important Cretaceous
areas in California, or perhaps in any part of the West Coast, are found
in the Great Valley. Probably the greatest stratigraphic aggregate of
Cretaceous sediment in North America, or perhaps in any country, is
found here. It would appear tbat, as a source of data bearing upon the
beginning and close of Cretaceous time and upon some of its major events,
as recorded in its stratigraphic sequence and its faunas, the equal of these
deposits has not been found in any other West Coast region. The environments
of this trough have already been partially given. As for the deposits
herein contained, the circumstances of their position, their attitude,
character, and relationships, their protection from waste, freedom from
tectonic complications, their succession of rich invertebrate faunas, and
their final exposure, which is exceptional, should make them of unequalled
value in supplying historical criteria bearing upon the early Cretaceous
epochs and events on the West Coast. Their interest is also greatly increased
by their structural and faunal relationships to the underlying late
Jurassic (Knoxville) sequence and the succeeding upper Cretaceous, outcropping
on both sides of the Great Valley.
In a portion of the trough, their older divisions contain a succession of
marine invertebrate faunas ranging from early Valanginian to late Albian
time. In other portions the record is continued almost to the closing
stages of the period. In brief, there is represented in this trough nearly
all the chronologic stages known in the Cretaceous succession of any
country, from Berriasian to Maestrichtian time inclusive. Unfortunately,
however, they are not all found in any single section, and few sections
show a continuous sequence of faunas throughout their column, but in
the several sections within the trough the faunal record is nearly complete,
except in certain parts of its latter half. In the unusual thickness of the