LOWER CRETACEOUS DEPOSITS CALIFORNIA AND OREGON

CaETACEOUfl 8ED1MBNTABT DIVISIONS 63
Tbe top of tbe Horsetown group is indicated generally by the basal
beds of the Cbico series, resting unconformably upon it, and by them it is
more easily followed. The basal conglomerates of the Chico series are
recognizable on most of the streams between Hulen and Elder creeks,
and in many places by distinctive fossils. South of Dry Creek the Cbico
series is often overlapped extensively by the Tehama formation or is exposed
only in the canyons of the larger streams (Cold fork, Redbank, and
Elder creeks).
In tbe Lodoga Hills, conglomerates mark both the top and bottom of
the Horsetown group, of which only the top has been followed far in the
field. These beds seem to indicate epochs of disturbance and of readjustment
of atrandlinea at the beginning and close of Horsetown sedimentation,
but not everywhere of equal effect. More striking evidences of
disturbances, if not of unconformity, are seen in the faunal changes that
appear in tbe sections in passing from Paakenta to Horsetown, and from
the latter to the lower beds of the Chico series. These disturbances appear
to have been of great geographical extant along the Pacific Coast,
reaching from Lower California to Alaska, and probably affected Mexico
and the northern Andes (Colombia and Venezuela).
The timet intervals represented by these unconformities cannot be
measured but, as indicated by the faunal changes effected, they seem to
have been long, although in some places sedimentation seems to have been
nearly continuous from Paskenta to Horsetown times, but not so between
the latter and Chico times. In many places the unconformity of the
Chico series upon the Horsetown, or upon Paskenta beds, ia marked by
evidences of erosion, with the transportation of materials and fossils from
older beds into the basal conglomerates of the Chico series.
DIVIBIONB OF THE HOMETOWN GROUP
General Statement.—For convenience in the discussion which follows,
the Horsetown group is here divided into two major portions, namely, the
Cottonwood beds below and the Hulen beds above, each division being
more typically represented in its outcrops on these streams. In each
division there is a succession of distinctive faunal zones not found in the
other. Together these zones present a faunal sequence comparable to
tbat in corresponding horizons in any other part of the world. In addition
to the criteria supplied by each zone for chronological purposes its contribution
to a general scale is supported and reinforced by those of other
zones above and beneath, throughout the entire group. Neither of these
divisions has been recognized as such beyond the limits of the Great
Valley embayments.
Cottonwood Beds.—In their type district tbe Cottonwood beds form a
belt 2J miles broad, crossing the North and Middle forks of Cottonwood