LOWER CRETACEOUS DEPOSITS CALIFORNIA AND OREGON

EMBAYMENTH OF THE TROUGH 31
tion. The Siskiyou, Trinity, and Yolla Bolly ranges belong to the older
and more stable of these systems. The mountains on the western border
of Klamathonia, constituting the west rim of the Yolla Bolly basin, may
have belonged to the later and less stable of these systems. The breaking
down of this western rim in post-Cretaceous time and the consequent
reversal of the drainage direction into the modem system were probably
due in part to crustal warping, or subsidence at the west, but in part it
was due to the now well-known faulting and uplift on the eastern rim
of the Yolla Bolly basin since Cretaceous time. The exact locus of the
western rim of the basin is not known. Whether it had direct closure
against the western end of the Trinity Range, or whether it closed against
the equally old Siskiyou Range, is yet to be shown. Assuming the latter
and more probable alternative, it will appear that during Cretaceous time,
as during the Knoxville, that portion of tbe Klamath River system lying
within the limits of Klamathonia, at least, was tributary to tbe Yolla
Bolly basin, and its drainage passed into the Sacramento embayment. It
may also be seen that the assumption of a connected drainage area west
of the present coast line, now lost, although justified upon other grounds,
is unnecessary for providing an adequate source for the sediments outcropping
on the western border of the Sacramento Valley, However,
since this basin is clearly the source of the Knoxville, as well as of the
Cretaceous sediment, it may well have had its origin in the Nevadian
revolution, and the view is supported by many geological facts that
cannot be given here. It might also be suggested, ire cxttnao, that the
more shaly character of the Knoxville sediment may be taken as evidence
of lower relief in this region during late Jurassic time, and that the river
then issuing from the basin carried larger percentages of silt and clay
than at a later (Cretaceous) time. It could also be supposed that Cretaceous
deposition began at an epoch of uplift of the mountain areas enclosing
the basin, accompanied by subsidence of its floor and of much of
the floor of the Sacramento embayment. Thus, the early Cretaceous
sediments were laid down in tbe basin upon basement rocks, whereas, in
the greater embayment they were spread upon Knoxville deposits already
consolidated, Tbe distribution of the sediments constituting the two
series and their stratigraphic relationship clearly harmonize with this view.
YOLLA BOLLY DELTA
The Cretaceous deposits on the western border of the embayment occupy
at the north a wide re-entrant area in the coast line of Klamathonia,
with its apex pointing toward the west. The sides of this area impinge upon
ancient crystalline and metamorphic rocks, From this apex the sediments
spread fan-wise toward tbe central parts of the embayment, forming here
a wide expansion, but narrowing gradually toward the south (Figs. 1 and 2).