LOWER CRETACEOUS DEPOSITS CALIFORNIA AND OREGON

EMBAYMENTH OF THE TROUGH 27
port in these isostatic readjustments. The subsidence may be assumed
to have extended with some modification into Joaquin embayment as
well, and the record of its deposits, as here interpreted, shows that it did
so, but not wholly in coincidence with that of the northern embayment.
Limits of Subsidence.—No evidence has been found in the deposits of
the Great Valley, or in their environs, showing that the subsidence had
been so profound, or laterally so extensive, as to submerge the Coast
Ranges, except to a limited extent along their borders. Movements in
other parts of the West Coast, coincident with this subsidence, were undoubtedly
felt, since contemporary deposits are found at many places on
the coastal border, but they were not of the same vertical extent. However,
in both embayments in the Great Valley, as soon as accumulation of
sediment began, it would at once become the cause of further subsidence,
and erosion and removal of materials from the land surfaces would be an
isostatic cause of their elevation. If the Coast Ranges, or the whole of
Klamathonia, continued to "gently rise" they could not at the same time
become submerged, and if submerged at any time during the Cretaceous
period they could not have been tbe source of the sediments described
herein.
These general facts should apply to all the troughs and should prove
that subsidence would probably be confined to them. Such operations
seem to have extended to all such troughs along the coastal border from
Alaska to Mexico, or farther. If, at some points in the Coast Ranges,
Cretaceous deposits are now found at elevations of 4000 feet, or more,
the fact should be attributed to faulting subsequent to deposition rather
than to a regional subsidence during the period.
Sources of Sediment.—An assumption that large land areas had existed
east of the Great Valley trough in Cretaceous time has led some writers
to suppose that the sources of these sediments lay in that direction, and
undoubtedly a minor part of them may be traced to land areas in this
direction. It would be difficult to show, however, that any considerable
part of the sediments now found on tbe western borders of the Great
Valley embayments had entered it from its eastern side. No evidence
has been found that any large stream had entered the trough on its eastern
side during the earlier half of Cretaceous time, and no early Cretaceous
deposits are now found there. Only on its northeastern border, and during
later Cretaceous time, has any evidence of such a stream been recognized—that
is, in the region of Pitt River.
On the contrary, thick deposits, already described, are found on the
western borders of these embayments, and at least one important river
system leading into it must be recognized, as already pointed out, namely
that coming from the interior of Klamathonia. But in addition to this
there were many smaller streams coming from the same direction.