Late Quaternary surface fluctuations Journal, this issue). These four drifts are from 10 centimeters to several meters thick. They are composed largely of unconsoli- of Beardmore , dated gravel. Numerous included striated clasts were probably reworked from Sirius drift. Thin boulder-belt com- monly mark drift surfaces and define outer edges of drift sheets. The thin drift sheets overlie well-preserved morphological fea- tures, particularly in Sirius deposits. Figure 3 shows former surfaces of repre- G. H. DENTON sented by the four drift sheets. The upper limit of Plunket drift parallels the present surface of Beardmore Glacier along its Institute for Quaternary Studies entire length. It also fringes the snout of , and which drains the local on the . This drift Department of Geological Sciences configuration shows similar behavior of these two dur- University of Maine Orono, Maine 04469 ing deposition of Plunket drift. The upper limit of Beardmore and Meyer drifts are close to the present surface of Beardmore Glacier near the polar plateau but systematically rise above the B.C. ANDERSEN present surface in the downglacier direction. Further, the areal patterns of Beardmore and Meyer drifts show recession of Department of Geology Rutkowski Glacier concurrent with expansion of Beardmore University of Oslo Glacier. Dominion drift occurs on the northern flank of the Oslo, Noruay Dominion Range, where it reaches high above Beardmore Glacier (Prentice et al., Antarctic Journal, this issue). We draw several inferences from the configuration, physical H.W. CONWAY characteristics, and weathering of these four drift sheets. The first involves their age. From advanced soil development Department of Chemical and Process Engineering (Bockheim et al, Antarctic Journal, this issue), we infer that Do- University of Canterbury, Private Bag minion drift is pre-late Quaternary in age. Beardmore and Christchurch, New Zealand Meyer drifts are judged to be late Quaternary in age on the basis of soil development (Bockheim et al., Antarctic Journal, this issue). Most likely, Beardmore drift correlates with Britannia I and II (Hatherton and Darwin Glaciers) and Ross Sea (McMur- do Sound region) drifts, which are radiocarbon dated to late Wisconsin age (Stuvier et al. 1981; Denton, Prentice, and Bur- ckle in press). Meyer drift most likely corresponds to stage 6 The Beardmore Glacier drains ice from the east antarctic polar drift in the McMurdo Sound area (Denton et al. in press). plateau through the to the Ross Ice Plunket drift shows less soil development than Beardmore drift Shelf (figures 1 and 2). Because of its interior location and (Bockheim et al., Antarctic Journal, this issue) and is probably adjacent ice-free areas, the Beardmore Glacier is particularly Holocene in age. Plunket drift is similar in position, mor- well suited to document antarctic ice-sheet behavior during late phology, and soil development to ice-cored lateral moraines in Quaternary time. This behavior, in turn, can illustrate the role of the ice-free valleys of southern Victoria Land that have max- the antarctic in global ice ages. During the austral field imum radiocarbon dates of 3,100 years (Denton et al. in press). season of 1985-1986, we mapped glacial drift sheets to resolve From their physical characteristics, we infer deposition of conflicting interpretations of late Quaternary surface-level these four drift sheets by polar ice with a frozen bed. They thus changes of Beardmore Glacier. stand in marked contrast to underlying Sirius drift, which was From Beardmore lateral moraines, Mercer (1972) inferred ex- deposited under temperate conditions with woody vegetation tensive grounding of the Ross Ice Shelf accompanied by little or and extensive summer ice melting (Prentice et al., Antarctic no change in interior East Antarctica during the late Quaternary Journal, this issue). These two contrasting styles of glaciation ice ages. In sharp contrast, Mayewski (1975) concluded that the mark a profound climatic change (see also Mercer 1972 and lateral moraines reflect former thickening of east antarctic ice Prentice et al., Antarctic Journal, this issue) that was pre-late accompanied by only minor grounding-line advance along the Quaternary in age. We conclude that Dominion drift represents inland periphery of the Ross Ice Shelf. These two interpreta- extension thickening of polar ice subsequent to this climatic tions of past changes in Beardmore Glacier imply fundamen- change. tally different controls of the antarctic ice sheet during late The longitudinal ice-surface profiles derived from Beardmore Quaternary ice ages. and Meyer drift show thick blocking ice near the Ross Ice Shelf Our research strategy combines geologic mapping with soil and little elevation change in the polar plateau during late studies that document postdepositional weathering of drift Quaternary glaciations (figure 3). In fact, our data permit a sheets. This strategy allowed drift sheets to be differentiated in decrease in the level of the polar plateau inland of Beardmore each ice-free area and then to be correlated on a local and Glacier. This is consistent with a dual control of the antarctic ice regional scale. Differentiation of drift sheets was by mor- sheet by eustatic sea-level lowering (causing widespread phology, cross-cutting geometric relationships, depth of oxida- grounding of the Ross Ice Shelf) and by a decrease in precipita- tion, solum thickness, morphogenetic salt stage, and weather- tion due to colder atmospheric temperatures (resulting in little ing stage (Bockheim, Wilson, and Leide, Antarctic Journal, this change or even slight decline of the polar plateau). This dual issue). Four drift sheets were found to mantle the present control is in accord with the out-of-phase behavior of walls and, in places, rest on Sirius drift (Prentice et al., Antarctic Beardmore Glacier and the Rutkowski outlet of the ice cap on

90 ANTARCTIC JOURNAL Figure 1. Index map of Antarctica.

the Dominion Range. When Beardmore Glacier thickened Mercer (1972) likewise concluded that sea-level lowering and (grounding of the Ross Ice Shelf), Rutkowski Glacier contracted grounding of the Ross Ice Shelf caused thickening of the lower (decreased precipitation). Comparison of profile C-C" in figure reaches of Beardmore Glacier shown by these two drifts. Hence, 3 with profiles A-A" and B-B in figures 2 and 3 of Prentice et al. our Beardmore data and those of Mercer (1972) both support (Antarctic Journal, this issue) shows that late Quaternary fluctua- our conclusion based on field work further north that wide- tions of interior antarctic ice were far less severe than those of spread grounding occurred in the area of the present Ross Ice pre-late Quaternary time. Shelf and Ross Sea during late Quaternary global ice ages Our inferences concerning late quaternary drift sheets are in (Stuiver et al. 1981). substantial agreement with those of Mercer (1972). Mercers This research was supported by National Science Foundation Beardmore III drift is largely equivalent to our Beardmore drift, grant DPP 83-18808. We are very grateful to VXE-6 for helicopter and his Beardmore II drift corresponds with our Meyer drift. support in the Beardmore Glacier area.

1986 REVIEW 91

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Figure 2. Sketch map of the Beardmore Glacier region. C-C" shows position of longitudinal profile in figure 3. A-A" and B-B show profiles in Prentice et at. (Antarctic Journal, this issue).

References C C C" C" C" 3800 0 Ml,,imo,,, Ice eo,tac. of Dominion drill Bockheim, J. G., S.W. Wilson, and J. E. Leide. 1986. Soil development in the Beardmore Glacier region, Antarctica. Antarctic Journal of the U. S., - -d 3000 - 21(5). IC. .o,faC. of PlonImI drift 113 0 - 17T7717lll Present surface of Beardmore Gliol., — Denton, G.H., M. L. Prentice, and L. H. Burckle. In press. Late Cenozoic 2800 • D history of the Antarctic Ice Sheet. In R.J. Tingey (Ed.), The geology of

2000-: Antarctica. Cambridge: Oxford University Press. Mayewski, P.A. 1975. Glacial geology and late Cenozoic history of the Trans- antarctic Mountains, Antarctica. (Institute of Polar Studies, Report No. Lu 56.) Columbus: Ohio State University. Mercer, J.H. 1972. Some observations on the glacial geology of the Beardmore Glacier area. In R.J. Adie (Ed.), Antarctic geology and geophysics. Oslo: Universitetsforlaget. Prentice, ML., G.H. Denton, TV. Lowell, H.C. Hughes and L.E. 0 20 40 80 80 100 120 140 180 180 200 220 240 Distance From Ross Ice Shell (Km) Heusser. 1986, Pre-late Quaternary glaciation of the Beardmore Glacier region, Antarctica. Antarctic Journal of the U.S. 21(5). Figure 3. Present and former longitudinal profiles of the surface of Stuiver, M., G. H. Denton, T.J. Hughes, and J. L. Fastook, 1981. History Beardmore Glacier. The position of C-C" shown in figure 2. of the marine ice sheet in West Antarctica during the last glaciation: A evaluations are projected onto the profile from ice-free areas desig- working hypothesis. InG.H. Denton, andT.J. Hughes, (Eds.), The last nated above the profiles. great ice sheets. New York: Wiley-Interscience.

92 ANTARCTIC JOURNAL