LOWER CRETACEOUS DEPOSITS CALIFORNIA AND OREGON

CRETACEOUS SEDIMENTARY DIVISIONS 43
beds diminish in volume or are lost in tbe axial areas of the delta. Toward
the south, on the Cold fork of the Cottonwood, are thick beds of conglomerate
which seem to represent the lower part of the Horsetown group, as
evidenced by fossils found nearby. About 4500 feet lower in the section
the basal beds of the Paskenta group (and Shasta series) is marked by a
heavy bed of conglomerate (near the Stephenson place, now abandoned)
containing many broken and beach worn shells, some of which have
been described by Stanton (1895, p. 14, et seq.).
Between Elder Creek and Thomes Creek the line of division is not
marked by conglomerates, and the basis of separation is almost wholly
paleontologies], although there are lithologiesal differences. The Paskenta
group here contains a greater percentage of sandy sediment than the
Horsetown, which is composed largely of shales. The same (ithological
differences are conspicuous in the Shasta series southward as far as Berryessa
Valley, Napa County, and farther. In some places the lower part of
tbe Paskenta group contains limestones, as near Wilbur Springs, Morgan
Valley, Pope Valley, and near MonticeUo. Such limestones have also been
described near Dillard, Douglas County, Oregon, and in some of the
Alaskan sections ("Nelchina limestone") (Martin, 1926, p. 313-315).
For the most part these limestones have their own facies of moliuscan
fossils, but in all cases they show a low position in the Cretaceous sequence,
as is indicated in the descriptions of many of them given by Stanton.
The relative stratigraphic thickness of the Paskenta and Horsetown
groups approximates generally that in the McCarthy Creek section, in
which they are represented by the following figures:
Horaotown group
Paskenta group
Total thkkccsa,,
5,340
13.,000
Faunal Zones.—In both the Paskenta and Horsetown groups various
faunal stones have been found which are recognizable as such in the several
districts where work has been done north of the Berryessa Valley. Many
of these zones are rich in fossils and con be traced for considerable distances
along their strikes. As most of the larger streams on the west border of
the Sacramento Valley cross tbe Cretaceous zones nearly at right angles
to tbe strike and give good exposures of the beds, these fossil zones may
be recognized in the successive streams, and the sequence of these zones
can be determined. In this way the faunas of the several zones become
enriched by additions obtained from the successive streams.
In addition to the fossil zones that are recognizable there are some
interspersed fossil localities whose stratigraphic positions have been deter*
mined by traverses. In the Paskenta group these zones are more general,