LOWER CRETACEOUS DEPOSITS CALIFORNIA AND OREGON

EMBAYMENTS OF THE TROUGH 37
On tbe eastern margin of tbe embayment the sorting effects of currents
are not so clear, partly because tbe deposits are for the most part hidden
by later (in part volcanic) terrains. On this side of the embayment the
effect of currents is more apparent on the molluscon life of the later Cretaceous
epochs, due probably to the absence of freshened water and the more
constant presence of marine water which permitted its more abundant
development. But, as no early Cretaceous deposits appear on the eastern
margin of the embayment, no proof is now available as to the validity of
deductive reasoning concerning it.
CRETACEOUS SEDIMENTARY DIVISIONS
GENERAL STATEMENT
The Cretaceous stratigraphic sequence in the Great Valley embayments,
on the whole, divide themselves naturally into two great sedimentary
series, belonging to earlier and later Cretaceous epochs. The depositional
relationship of the two is one of clear unconformity. Wherever their
contact has been found, and in fact wherever evidence has been sufficiently
sought, the basal beds of the upper series are found to rest discordantly
upon those of the lower series. In many places this discordance is marked
by beds of conglomerate which carry convincing evidence of disturbance,
erosion, and of reworked materials, in part derived from the older series,
and in part brought from more distant points, all lodged in the basal beds
of the later series. Such facts are seen at many points about tho embayments,
at the north on the North fork of the Cottonwood Creek, on
Roaring River, north of the delta, and on the Cold fork of the Cottonwood,
at Elder Creek, and at other points to the south. At other places on the
borders of the Sacramento embayment, and on the western border of the
Joaquin embayment, conditions of overlap of the later scries upon the
pre-Cretaceous formations are well known. Such facts are known also far
beyond the limits of the Great Valley, in both California and Oregon,
and farther north. On the whole the aggregate thickness of the respective
series is somewhat equal, although not in any single section. On tho
western border of the Sacramento Valley (Shasta and Tehama counties),
tbe thickness of the older series far exceeds that of the later, whereas in
the Joaquin embayment the reverse is true. These conditions are believed
to be for the most part due to a difference in depositional history in the
two embayments, although faulting has in some places obscured the
essential facts. The respective divisions in the sedimentary succession of
the Cretaceous deposits in the Great Valley embayments are shown in
Table 1,
SHASTA SERIES
General Statement.—The Shasta series includes that portion of the
California Cretaceous succession that was described by Gabb and Whitney