LOWER CRETACEOUS DEPOSITS CALIFORNIA AND OREGON

74 3WJWEH CRETACEOUS DEPOSITS IN CALIFORNIA AND OREGON
become intolerable to them. On the other hand, migrations of southern
origin following the littoral northward would enter the basin at a more
southern gateway and would be carried northward by inflowing tidal
currents, to enter whatever suitable places were open to them.
In this way the areas north and south of the delta could be colonized
during any and every epoch of the period. Such currents, and such
Mollusea as found it congenial, could also enter the Yolla Bolly basin,
and during the time of its marine occupation they evidently did so, although
the freshened condition of its water had a selective influence upon
them. The water discharged from this basin would of course take the
shortest way to the sea and thus follow the west border of the embayment
southward.
All the known facts pertaining to this embayment tend to support the
assumption of circulating currents within it during the whole of Cretaceous
time.
DLKSTRQPHIC HISTORY
The crustal disturbances that have affected the Cretaceous troughs and
basins of the Pacific Coast, although known mainly from local effects,
are not regarded as of only local import. Some of them at least seem to
have been felt in many latitudes along the coast between Alaska and
Patagonia and through many degrees of longitude as well, probably reaching
many parts of the Pacific basin. It may be that the succession of
these disturbances began in pre-Cretaceous time, perhaps at the beginning
of the Knoxville epoch, but a complete record of these events cannot be
attempted here. In the deposits left in the several embayments in Oregon
and California, especially in later Cretaceous time, there are many dbconformities
that indicate that during this period conditions of instability
existed in and about these troughs. It is possible that this condition
pertained essentially to the basins rather than to the areas and structures
within the enclosing mountain areas, although both have participated in
the readjustments. The movements recorded in these deposits may have
been only local effects resulting from widely extended causes, although
the denudation of neighboring land areas and the consequent loading of
basin floors would necessitate repeated readjustments. Whatever the
cause, it is now possible to recognize a suc.ces.sion of such events in the
later Mesozoic deposits of the Pacific Coast, beginning with tbe opening
of Knoxville time. Tbe diastrophic records that pertain to the Cretaceous
period itself began at the close of Knoxville time and may be summarized
as follows:
(1) A poBt-KnGXvitle disturbance, regional in geographic spread, but differencial
in effoets, producing local uplifts, withdrawal of the sea from many arem, and the
shifting of strandliue» in the troughs still occupied by the sea. The Joaquin embay-