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even thickness and conform to irregularities in the bed
on which they were deposited. These irregularities,
which occur within the lsminatd zones, appear to be
isolated ripples of coarser sand. We interpret the
lamin~e to be draped lamination, 8s described by
Gustavson and others (1975). The tops of sets are
sometimes marked by one or more thin layers that
contain angular granules and pebbles of schist and
granite; some sets contain sand- or pebblyad-filled
channels at their taps. Unit 4, also 6 m thick, consfsts
of laminated to thin+edded sand and silt, and contains
a few zones of sinusoidal ripple marks (Jopling and
Walker, 1968). As in unit 3, the laminae conform to
bed irregulrvities and are here interpreted as draped
Lamination.
Figme 4l.-Quaternary stratipaphic section and
radiocarbon ages at Riverside Bluff.
Units 2 and 5 apparently contain only sediment
transported by traction and are probably fluvial
deposits. However, we believe that unit 4 is lacus-
trine, and unit 3 probably also. The sinusoidal ripple
marks in unit 4 and the draped lamination in units 3
and 4 Indicate deposition from suspension (JopUng and
Walker, 1968; Gustavson md others, 1975), which can
occur in a body of standing water or from overbank
floodwaters. However, draped Iaminae deposited from
floodwaters generally occur in the form of thin sets at
the top of flood-formed sedimentation units that
commonly include seclirnent wMch was transported as
traction load. The thickness and uniformity of unit 4
strongly suggest deposition in a lake. h unit 3, the
occurrence of the draped laminae at the base of indi-
vidual sedimentation units b the reversa of the
sequence to be expected In flood deposits; however,
such an occurrence was reported In sediment deposited
from density currents In glaciolacustrine deltas
(Gustavson and others, 1975). Channels and thln layers
of angular granules and pebbles at the tops of sedimen-
tation units may represent rapid lowering of water
level. We found no organic remains of any kind In units
2 through 5 and believe that these deposits were
formed entirely during a glaclal episode. The lasi
major glacial advance in the Alaska Range probably
began about 25,000 years B.P. (Hamilton, 1982) end
was separated from en early advance of the Wisconsin
Glaciation by a long interstadid interval, the infor-
mally designated Boutelher interval (Hopkins, 19831,
that began at.1-t 49,000 years B.P. Therefore, we
believe that units 2 through 5 are either about 25,000
years old or younger, or older than 49,000 years.
Because of the absence of organic materlds, the
lacustrine beds cannot be directly related to Pernald's
(19654 radiocmbon date of 25,800*00 years, although
he reported that the materlal dated was fine organic
debris collected 14 m below the top of the bluff and 6
rn below the bese of the eolian deposits. An equivalent
position In our section would be in the middle or lower
part of unit 4. Thus, the lacustrine beds may be about
25,000 or 26,000 years old. Although Fernald (1965a)
related this age to interstadia1 conditions, it could as
easily be related to the early phase of the late part of
the Wisconsin Glaciation. Also, deposits of this age
may simply not be present in the part of the bluff that
we measured, and the lacuatdne beds may be early
W isconsinan.
Although we are unsure of the reglonal slgrufi-
cmce of the lecustrine beds in the absence of further
work, the features of the Tanana River vdley down-
river between Dot Lake and the Johnson River suggest
e possible explanation for the occurrence of these
beds. Here, in one of the narrowest parts of the
valley, ice-scoured granitic bedrock is exposed across
the Tanana River valley except for the part occupied
by the modern river. Both sides of this part of the
valley are steep and linear and appear to have been
trimmed by ice. Thus, during at least one glacial
episode, the flow of the Tanana River either was com-
pletely blocked or profoundly impeded by glacial ice.
Such an event could create a large lake in the upper
Tanana Rtver vaUey, and we believe that the lacus-
trhe beds exposed in Rivecside Bluff record one or
more large ice-dam med lakes.
BWERBNC BS CITED
Pernald, A. T., 1965a, Glaciation in the Nabesna River
area, uppel. Tanana River valley, Alaska, Geo-
logical Survey research 1965: U.S. Geological
Survey Professional Paper 525-C, p. C120-Cl23.
-1965b, Recent history of the upper Tanana River
lowlnd, Alaska, ~ Geological Survey research
1965: U.S. Geological Survey Professional Paper
525-C, p. C124-C127.