Ice sheet overriding of the Transantarctic Mountains
boulders beyond the Ross I and Alpine I drift limits but within the Taylor III drift limit of Denton et al. (1970, figure 3) are grussified, cavernously weathered, and wind faceted. Stones on moraines above (and thus older than) low moraines farther south in the Transantarctic Mountains are also highly weathered (Elliot et al. 1974; Mayewski 1975). The sparse weathering of erratics on nunataks in the Lassiter Coast therefore suggests that the thick-ice stage evidenced in the southern Antarctic Peninsula is late-Wisconsin age. Field work was done during austral summer 1972-1973 incidental to bedrock mapping by the U.S. Geological Survey. The field project was financed by National Science Foundation grant AG-187 and logistically supported by U.S. Navy Operation Deep Freeze.
References Barrett, P. J . , and D. H. Elliot. 1973. Reconnaissance geologic map of the
Buckley Island quadrangle, Transantarctic Mountains, Antarctica. (U.S.
Geological Survey Antarctic Geologic Map, A-3.) Washington, D.C.: U.S. Government Printing Office. Carrara, P. 1981. Evidence for a former large ice sheet in the Orville Coast-Ronne Ice Shelf area, Antarctica. Journal of Glaciology 27, 487-491. Denton, G. H., R. L. Armstrong, and M. Stuiver. 1970. Late Cenozoic glaciation in Antarctica-The record in the McMurdo Sound region. Antarctic Journal of the U.S., 5, 15-21. Denton, G. H., R. L. Armstrong, and M. Stuiver. 1971. The late Cenozoic glacial history of Antarctica. In K. K Turekian (Ed.), The Late Cenozoic Glacial Ages. New Haven: Yale University Press. Denton, G. H., and T. J . Hughes. 1981. The Last Great Ice Sheets, New York: Wiley-Interscience. Elliot, D. H., P. J . Barrett, and P. A. Mayewski. 1974. Reconnaissance geologic map of the Plunket Point quadrangle, Transantarctic Mountains,
Ice sheet overriding of the Transantarctic Mountains G. H. DENTON, D. E. KELLOGG,
and
T. B. KELLOGG
Department of Geological Sciences and Institute for Quaternary Studies University of Maine Orono, Maine 04469
M. L. PRENTICE Department of Geological Sciences Brown University Providence, Rhode Island 02912
During the past three austral field seasons, we investigated late Tertiary overridings of the Transantarctic Mountains by an expanded antarctic ice sheet considerably larger than those of late Quaternary ice ages. Geologic and glaciologic studies both indicate that eustatic sea-level changes caused by Northern Hemisphere ice sheets drove late Quaternary fluctuations of the
1983 REVIEW
Antarctica. (U.S. Geological Survey Antarctic Geologic Map, A-4.)
Washington, D.C.: U.S. Government Printing Office. Hollin, J . T. 1962. On the glacial history of Antarctica. Journal of Glaciology, 4, 173-195. Mayewski, P. A. 1975. Glacial geology and late Cenozoic history of the Transantarctic Mountains, Antarctica. The Ohio State University, Institute of Polar Studies, Research Report, 56. Paterson, W. S. B. 1980. Ice sheets and ice shelves. In S. C. Colbeck (Ed.), Dynamics of snow and ice. New York: Academic Press. Paterson, W. S. B. 1981. The physics of glaciers. New York: Pergamon Press. Rowley, P. D. and P. L. Williams. 1982. Geology of the northern Lassiter Coast and southern Black Coast, Antarctic Peninsula. In C. Craddock (Ed.), Antarctic Geoscience. Madison: University of Wisconsin Press. Rowley, P. D., D. L. Schmidt, and P. L. Williams. 1982. Mount Poster Formation, southern Antarctic Peninsula and western Ellsworth Land. Antarctic Journal of the U.S., 17(5), 38-39. Rowley, P. D., K. S. Kellogg, W. R. Vennum, R. B. Waitt, Jr., and S. J. Boyer. In press. Geology of the southern Black Coast, Antarctic Peninsula. (U.S. Geological Survey Professional Paper 1170-A.) Washington, D.C.: U.S. Government Printing Office. Stuiver, M., G. H. Denton, T. J . Hughes, and J. L. Fastook. 1981. History of the marine ice sheet in West Antarctica during the last glaciation-A working hypothesis. In G. H. Denton and T. J . Hughes (Eds.), The last great ice sheets. New York: Wiley-Interscience. Thomas, R. H. 1979. The dynamics of marine ice sheets. Journal of Glaciology, 24, 167-177. Thomas, R. H., and C. R. Bentley. 1978. A model for Holocene retreat of the West Antarctic ice sheet. Quaternary Research, 10, 150-170. Waitt, R. B., Jr. 1972. Geomorphology and glacial geology of the Methow drainage basin, eastern Cascade Range, Washington. Unpublished doc-
toral dissertation, University of Washington. Waitt, R. B., Jr. 1981. Radial outflow and unsteady retreat of Late Wisconsin to early Holocene icecap in the northern Long Range upland, Newfoundland. Geological Society of America Bulletin, Part I,
92, 834-838.
antarctic ice sheet, when it repeatedly grounded on peripheral continental shelves but did not thicken sufficiently to inundate the Transantarctic Mountains (Hollin 1962; Stuiver and others 1981). We suggest that eustatic sea level also drove ice-sheet variations during at least the youngest Tertiary overriding episode, when the mountains were inundated, because evidence of climatic change at that time is absent in the Transantarctic Mountains. If we are correct, the latest overriding episode (and perhaps earlier episodes) reflect large concurrent Northern Hemisphere ice sheets. These large ice sheets could have shaped major fjords in eastern Canada and Greenland that are othewise difficult to explain by limited late Quaternary ice extent in these regions (Andrews and Miller 1976; Funder and Hjort 1973). We outline here evidence for overriding. The dry valleys region in the Transantarctic Mountains shows two major imprints of glacial erosion. The older imprint includes major valley systems that occur on the eastern mountain flank where they form the dry valleys, as well as on the western mountain flank where they lie buried beneath the ice sheet (Drewry 1982). Mountain ranges between dry valleys exhibit glacial erosional features, which complete the older imprint. The major valleys, as well as mountain alpine erosional forms, were cut primarily by a local ice cover rather than by a continental ice sheet, because on the western flank of the Transantarctic 93
Mountains the valleys descend inland beneath the present ice sheet (Drewry 1982). The younger imprint of glacial erosion records angular overriding of the preexisting mountain-and-valley topography by northeastward-flowing ice. We infer that only an extensive ice sheet would be thick enough to cover the mountains and flow diagonally across deep valleys. A local ice cap would simply flow along preexisting valleys. We have field evidence for similar angular overriding between Byrd Glacier and the Convoy Range, which further implies the existence of an ice sheet rather than a local ice cap. The overriding ice sheet modified preexisting alpine topography in the Asgard and Olympus Ranges, Quartermain Mountains, and Kukri Hills, producing an array of subglacial deposits and erosional features. Mountains, cirques, and ridges in the resistant granite and metamorphic bedrock of the eastern dry valleys were molded and smoothed. Here bedrock surfaces with dike swarms show subglacial corrugation. Further west, mountains, cirques, and ridges of less resistant Beacon Supergroup sedimentary rocks and Ferrar Dolerite sills were more severely modified. Alpine cirques and ridge slopes facing upglacier were abraded into smooth stoss slopes; similar features in the lee of ice flow were eroded headward by subglacial meltwater generated during abrasion of adjacent stoss slopes (figure
br Glaciers. Ferrar Dolerite capping several mountain peaks forms tips of boulder trains that extend down lee troughs for 1 t 3 kilometers. Lee troughs commonly exhibit giant potholes, plungepools, and large meltwater channels; all were cut by subglacial meltwater and are commonly adjacent to till patches and glacially corrugated bedrock. Numerous fields of rippled gravel record subglacial sheet flow of meltwater (figure 2).
4?.
Figure 2. Ripple-like features composed of gravel and deposited by basal meltwater beneath overriding ice In a lee trough in the Asgard Range. Individual ripple-like features are up to 100 meters long and 1.0 meter high.
Figure 1. Fossil alpine horn (2,000 meters) In foreground composed of Beacon Supergroup sedimentary rocks with a Ferrar Dolerite cap in the Asgard Range between Taylor and Wright Valleys. An overriding ice sheet produced a smooth, abraded stoss slope on the left and a plucked lee slope on the right.
1). Such headward erosion produced lee troughs, breached divides between opposing cirques, and isolated remnant alpine ridges and horns. Advanced subglacial erosion in the central Olympus Range has left only isolated ridges and peaks separated by remnant trough floors. The overriding ice sheet deposited basal sediments on the glacially eroded landscape. On lee slopes and in lee troughs, systematically arranged patches of till and glacially corrugated bedrock extend in a pattern unbroken by moraines or weathering changes from high mountain peaks down to low Late Pliocene and Pleistocene moraines near Wright Upper and Tay94
Stoss slopes show smoothly abraded bedrock with a sparse surface litter of erratics and irregular till patches. They lack ripple-like features, corrugated bedrock, subglacial channels, and potholes. Northeastward flow of overriding ice is deduced from widespread features in the mountains (boulder trains, high mountains with stoss-and-lee shapes, roche mountonnees, fields of subglacial ripple-like forms, and striations). Even the highest peaks were overrun, including Mount Feather (2,985 meters). Maximum and minimum ages for the youngest overriding episode can be obtained in central Wright Valley. Here a widespread sheet of basal till represents the youngest overriding episode. Near Lake Vanda this till abruptly overlies glacimarine diamicton, and at Prospect Mesa near Bull Pass it rests on a gravel unit that contains numerous valves of an extinct marine mollusk (Turner 1967). Co-occurrence of marine diatoms Denticulopsis lauta and D. hustedtii in the glacimarine diamicton suggests deposition at some time between the middle of the Middle Miocene, approximately 15 million years ago, and the middle of magnetostratigraphic Chron 9, approximately 9 million years ago, in early late Miocene time (Ciesielski in press). On the basis of incomplete stratigraphic ranges of enclosed benthic foraminifera, Webb (1974) estimated a middle Pliocene and Brady (1979, 1982) an Early Pliocene age for the fossiliferous gravel at Prospect Mesa. Near Bartley Glacier, also in central Wright Valley, the basal till unit underlies alpine moraines emplaced between 2.0 and 3.38 million years ago, according to our correlation by field mapping and soil stratigraphy of these morANTARCTIC JOURNAL
ames with glacial deposits potassium/argon dated in Taylor Valley (Denton, Armstrong, and Stuiver 1971; Armstrong 1978). Thus the basal till has a maximum age between 9 and 15 million years and may be younger than Middle to Early Pliocene if the age estimates of the fossiliferous gravel at Prospect Mesa are correct. Further, the basal till has a minimum age between 2 and 3.38 million years. For reasons given in Denton and others (in preparation), we think that the overriding was multiple and that one episode occurred more than 9 to 15 million years ago. We obtained an approximate ice-surface elevation of 3000 meters for the dry valleys region for the latest overriding episode by adjusting mountain elevations for 300 meters of tectonic uplift and then estimating that the highest mountains were buried by 500 meters of ice, the minimum thickness to account for observed subglacial features (Denton et al. in preparation). Why were the Transantarctic Mountains overridden? Diagonal northeastward ice flow across major valleys and intervalley mountain ranges between Byrd Glacier and the Convoy Range could only occur if extensive blocking ice in the Ross Sea (and thus West Antarctica) induced burial of the Transantarctic Mountains and thickening of the adjacent east antarctic ice sheet. Burial of the Transantarctic Mountains would promote a unified antarctic ice sheet with a central dome, because exposed mountains would no longer separate the east and west antarctic ice sheets. Thus we conclude that a greatly expanded antarctic ice sheet with a high, central dome was responsible for overriding of the Transantractic Mountains (Denton et al. in preparation). This work was supported by National Science Foundation grant DPP 80-23714. References Andrews, J . T. and C. H. Miller. 1976. Quaternary glacial chronology of the eastern Canadian Arctic: A review and contribution on amino acide dating of Quaternary molluscs from the Clyde Cliffs. In W. Ma-
1983 REVIEW
haney (Ed.), Quaternary stratigraphy of North America, Stroudsburg, Pa. Dowden, Hutchinson, and Ross. Armstrong, R. L. 1978. K-Ar dating: Late Cenozoic McMurdo Volcanic Group and dry valley glacial history, Victoria Land, Antarctica. New Zealand Journal of Geology and Geophysics, 21,, 685-698. Brady, H. T. 1979. A diatom report on DVD1' cores 3, 4a, 12, 14, 15 and other related surface sections. In T. Nagata (Ed.), Proceedings of the Seminar III on Dry Valley Drilling Project, 1978, Special Memoir 13 Tokyo: National Institute of Polar Research. Brady, H. T. 1982. Late Cenozoic history of Taylor and Wright Valleys and McMurdo Sound inferred from diatoms in Dry Valley Drilling Project cores. In C. Craddock (Ed.) Antarctic geoscience. Madison: University of Wisconsin Press. Ciesielski, P. F. In press. The Neogene diatom biostratigraphy of DSDP Leg 71, Subantarctic sediments. In W. J . Ludwig, V. Drashinnikov et al. (Eds.), Initial reports of the Deep Sea Drilling project, leg 71. Washington, D.C.: U.S. Government Printing Office. Denton, G. H., R. L. Armstrong, and M. Stuiver. 1971. The late Cenozoic glacial history of Antarctica. In K K. Turekian, (Ed.), The Late Cenozoic glacial ages. New Haven: Yale University Press. Denton, G. H., M. L. Prentice, D. E. Kellogg, and T. B. Kellogg. In preparation. Tertiary history of the Antarctic Ice Sheet: Evidence from the dry valleys. Geology. Drewry, D. J . 1982. Ice flow, bedrock, and geothermal studies from radio-echo sounding inland of McMurdo Sound, Antarctica. In C. Craddock (Ed.), Antarctic geoscience. Madison: University of Wisconsin Press. Funder, S., and C. Hjort. 1973. Aspects of the Weichselian chronology in central East Greenland. Boreas, 2, 69-84. Hollin, J . T. 1962. On the glacial history of Antarctica. Journal of Glaciology, 4, 173-195. Stuiver, M., G. H. Denton, T. J . Hughes, and J. L. Fastook. 1981. History of the marine ice sheet in West Antarctica during the last glaciation: A working hypothesis. In G. H. Denton and T. J . Hughes (Eds.), The last great ice sheets. New York: Wiley-Interscience. Turner, R. D. 1967. A new species of fossil Chlamys from Wright Valley, McMurdo Sound, Antarctica. New Zealand Journal of Geology and Geophysics, 10, 446-455.
Webb, P. N. 1974. Micro palaeontology, palaeoecology and correlations of the Pecten gravels, Wright Valley, Antarctica, and description of Trocheolphidiella onyxi, n. gen., n. sp. Journal of Foraminiferal Research, 4, 184-199.
95
References Barrett, P. J . , and D. H. Elliot. 1973. Reconnaissance geologic map of the
Buckley Island quadrangle, Transantarctic Mountains, Antarctica. (U.S.
Geological Survey Antarctic Geologic Map, A-3.) Washington, D.C.: U.S. Government Printing Office. Carrara, P. 1981. Evidence for a former large ice sheet in the Orville Coast-Ronne Ice Shelf area, Antarctica. Journal of Glaciology 27, 487-491. Denton, G. H., R. L. Armstrong, and M. Stuiver. 1970. Late Cenozoic glaciation in Antarctica-The record in the McMurdo Sound region. Antarctic Journal of the U.S., 5, 15-21. Denton, G. H., R. L. Armstrong, and M. Stuiver. 1971. The late Cenozoic glacial history of Antarctica. In K. K Turekian (Ed.), The Late Cenozoic Glacial Ages. New Haven: Yale University Press. Denton, G. H., and T. J . Hughes. 1981. The Last Great Ice Sheets, New York: Wiley-Interscience. Elliot, D. H., P. J . Barrett, and P. A. Mayewski. 1974. Reconnaissance geologic map of the Plunket Point quadrangle, Transantarctic Mountains,
Ice sheet overriding of the Transantarctic Mountains G. H. DENTON, D. E. KELLOGG,
and
T. B. KELLOGG
Department of Geological Sciences and Institute for Quaternary Studies University of Maine Orono, Maine 04469
M. L. PRENTICE Department of Geological Sciences Brown University Providence, Rhode Island 02912
During the past three austral field seasons, we investigated late Tertiary overridings of the Transantarctic Mountains by an expanded antarctic ice sheet considerably larger than those of late Quaternary ice ages. Geologic and glaciologic studies both indicate that eustatic sea-level changes caused by Northern Hemisphere ice sheets drove late Quaternary fluctuations of the
1983 REVIEW
Antarctica. (U.S. Geological Survey Antarctic Geologic Map, A-4.)
Washington, D.C.: U.S. Government Printing Office. Hollin, J . T. 1962. On the glacial history of Antarctica. Journal of Glaciology, 4, 173-195. Mayewski, P. A. 1975. Glacial geology and late Cenozoic history of the Transantarctic Mountains, Antarctica. The Ohio State University, Institute of Polar Studies, Research Report, 56. Paterson, W. S. B. 1980. Ice sheets and ice shelves. In S. C. Colbeck (Ed.), Dynamics of snow and ice. New York: Academic Press. Paterson, W. S. B. 1981. The physics of glaciers. New York: Pergamon Press. Rowley, P. D. and P. L. Williams. 1982. Geology of the northern Lassiter Coast and southern Black Coast, Antarctic Peninsula. In C. Craddock (Ed.), Antarctic Geoscience. Madison: University of Wisconsin Press. Rowley, P. D., D. L. Schmidt, and P. L. Williams. 1982. Mount Poster Formation, southern Antarctic Peninsula and western Ellsworth Land. Antarctic Journal of the U.S., 17(5), 38-39. Rowley, P. D., K. S. Kellogg, W. R. Vennum, R. B. Waitt, Jr., and S. J. Boyer. In press. Geology of the southern Black Coast, Antarctic Peninsula. (U.S. Geological Survey Professional Paper 1170-A.) Washington, D.C.: U.S. Government Printing Office. Stuiver, M., G. H. Denton, T. J . Hughes, and J. L. Fastook. 1981. History of the marine ice sheet in West Antarctica during the last glaciation-A working hypothesis. In G. H. Denton and T. J . Hughes (Eds.), The last great ice sheets. New York: Wiley-Interscience. Thomas, R. H. 1979. The dynamics of marine ice sheets. Journal of Glaciology, 24, 167-177. Thomas, R. H., and C. R. Bentley. 1978. A model for Holocene retreat of the West Antarctic ice sheet. Quaternary Research, 10, 150-170. Waitt, R. B., Jr. 1972. Geomorphology and glacial geology of the Methow drainage basin, eastern Cascade Range, Washington. Unpublished doc-
toral dissertation, University of Washington. Waitt, R. B., Jr. 1981. Radial outflow and unsteady retreat of Late Wisconsin to early Holocene icecap in the northern Long Range upland, Newfoundland. Geological Society of America Bulletin, Part I,
92, 834-838.
antarctic ice sheet, when it repeatedly grounded on peripheral continental shelves but did not thicken sufficiently to inundate the Transantarctic Mountains (Hollin 1962; Stuiver and others 1981). We suggest that eustatic sea level also drove ice-sheet variations during at least the youngest Tertiary overriding episode, when the mountains were inundated, because evidence of climatic change at that time is absent in the Transantarctic Mountains. If we are correct, the latest overriding episode (and perhaps earlier episodes) reflect large concurrent Northern Hemisphere ice sheets. These large ice sheets could have shaped major fjords in eastern Canada and Greenland that are othewise difficult to explain by limited late Quaternary ice extent in these regions (Andrews and Miller 1976; Funder and Hjort 1973). We outline here evidence for overriding. The dry valleys region in the Transantarctic Mountains shows two major imprints of glacial erosion. The older imprint includes major valley systems that occur on the eastern mountain flank where they form the dry valleys, as well as on the western mountain flank where they lie buried beneath the ice sheet (Drewry 1982). Mountain ranges between dry valleys exhibit glacial erosional features, which complete the older imprint. The major valleys, as well as mountain alpine erosional forms, were cut primarily by a local ice cover rather than by a continental ice sheet, because on the western flank of the Transantarctic 93
Mountains the valleys descend inland beneath the present ice sheet (Drewry 1982). The younger imprint of glacial erosion records angular overriding of the preexisting mountain-and-valley topography by northeastward-flowing ice. We infer that only an extensive ice sheet would be thick enough to cover the mountains and flow diagonally across deep valleys. A local ice cap would simply flow along preexisting valleys. We have field evidence for similar angular overriding between Byrd Glacier and the Convoy Range, which further implies the existence of an ice sheet rather than a local ice cap. The overriding ice sheet modified preexisting alpine topography in the Asgard and Olympus Ranges, Quartermain Mountains, and Kukri Hills, producing an array of subglacial deposits and erosional features. Mountains, cirques, and ridges in the resistant granite and metamorphic bedrock of the eastern dry valleys were molded and smoothed. Here bedrock surfaces with dike swarms show subglacial corrugation. Further west, mountains, cirques, and ridges of less resistant Beacon Supergroup sedimentary rocks and Ferrar Dolerite sills were more severely modified. Alpine cirques and ridge slopes facing upglacier were abraded into smooth stoss slopes; similar features in the lee of ice flow were eroded headward by subglacial meltwater generated during abrasion of adjacent stoss slopes (figure
br Glaciers. Ferrar Dolerite capping several mountain peaks forms tips of boulder trains that extend down lee troughs for 1 t 3 kilometers. Lee troughs commonly exhibit giant potholes, plungepools, and large meltwater channels; all were cut by subglacial meltwater and are commonly adjacent to till patches and glacially corrugated bedrock. Numerous fields of rippled gravel record subglacial sheet flow of meltwater (figure 2).
4?.
Figure 2. Ripple-like features composed of gravel and deposited by basal meltwater beneath overriding ice In a lee trough in the Asgard Range. Individual ripple-like features are up to 100 meters long and 1.0 meter high.
Figure 1. Fossil alpine horn (2,000 meters) In foreground composed of Beacon Supergroup sedimentary rocks with a Ferrar Dolerite cap in the Asgard Range between Taylor and Wright Valleys. An overriding ice sheet produced a smooth, abraded stoss slope on the left and a plucked lee slope on the right.
1). Such headward erosion produced lee troughs, breached divides between opposing cirques, and isolated remnant alpine ridges and horns. Advanced subglacial erosion in the central Olympus Range has left only isolated ridges and peaks separated by remnant trough floors. The overriding ice sheet deposited basal sediments on the glacially eroded landscape. On lee slopes and in lee troughs, systematically arranged patches of till and glacially corrugated bedrock extend in a pattern unbroken by moraines or weathering changes from high mountain peaks down to low Late Pliocene and Pleistocene moraines near Wright Upper and Tay94
Stoss slopes show smoothly abraded bedrock with a sparse surface litter of erratics and irregular till patches. They lack ripple-like features, corrugated bedrock, subglacial channels, and potholes. Northeastward flow of overriding ice is deduced from widespread features in the mountains (boulder trains, high mountains with stoss-and-lee shapes, roche mountonnees, fields of subglacial ripple-like forms, and striations). Even the highest peaks were overrun, including Mount Feather (2,985 meters). Maximum and minimum ages for the youngest overriding episode can be obtained in central Wright Valley. Here a widespread sheet of basal till represents the youngest overriding episode. Near Lake Vanda this till abruptly overlies glacimarine diamicton, and at Prospect Mesa near Bull Pass it rests on a gravel unit that contains numerous valves of an extinct marine mollusk (Turner 1967). Co-occurrence of marine diatoms Denticulopsis lauta and D. hustedtii in the glacimarine diamicton suggests deposition at some time between the middle of the Middle Miocene, approximately 15 million years ago, and the middle of magnetostratigraphic Chron 9, approximately 9 million years ago, in early late Miocene time (Ciesielski in press). On the basis of incomplete stratigraphic ranges of enclosed benthic foraminifera, Webb (1974) estimated a middle Pliocene and Brady (1979, 1982) an Early Pliocene age for the fossiliferous gravel at Prospect Mesa. Near Bartley Glacier, also in central Wright Valley, the basal till unit underlies alpine moraines emplaced between 2.0 and 3.38 million years ago, according to our correlation by field mapping and soil stratigraphy of these morANTARCTIC JOURNAL
ames with glacial deposits potassium/argon dated in Taylor Valley (Denton, Armstrong, and Stuiver 1971; Armstrong 1978). Thus the basal till has a maximum age between 9 and 15 million years and may be younger than Middle to Early Pliocene if the age estimates of the fossiliferous gravel at Prospect Mesa are correct. Further, the basal till has a minimum age between 2 and 3.38 million years. For reasons given in Denton and others (in preparation), we think that the overriding was multiple and that one episode occurred more than 9 to 15 million years ago. We obtained an approximate ice-surface elevation of 3000 meters for the dry valleys region for the latest overriding episode by adjusting mountain elevations for 300 meters of tectonic uplift and then estimating that the highest mountains were buried by 500 meters of ice, the minimum thickness to account for observed subglacial features (Denton et al. in preparation). Why were the Transantarctic Mountains overridden? Diagonal northeastward ice flow across major valleys and intervalley mountain ranges between Byrd Glacier and the Convoy Range could only occur if extensive blocking ice in the Ross Sea (and thus West Antarctica) induced burial of the Transantarctic Mountains and thickening of the adjacent east antarctic ice sheet. Burial of the Transantarctic Mountains would promote a unified antarctic ice sheet with a central dome, because exposed mountains would no longer separate the east and west antarctic ice sheets. Thus we conclude that a greatly expanded antarctic ice sheet with a high, central dome was responsible for overriding of the Transantractic Mountains (Denton et al. in preparation). This work was supported by National Science Foundation grant DPP 80-23714. References Andrews, J . T. and C. H. Miller. 1976. Quaternary glacial chronology of the eastern Canadian Arctic: A review and contribution on amino acide dating of Quaternary molluscs from the Clyde Cliffs. In W. Ma-
1983 REVIEW
haney (Ed.), Quaternary stratigraphy of North America, Stroudsburg, Pa. Dowden, Hutchinson, and Ross. Armstrong, R. L. 1978. K-Ar dating: Late Cenozoic McMurdo Volcanic Group and dry valley glacial history, Victoria Land, Antarctica. New Zealand Journal of Geology and Geophysics, 21,, 685-698. Brady, H. T. 1979. A diatom report on DVD1' cores 3, 4a, 12, 14, 15 and other related surface sections. In T. Nagata (Ed.), Proceedings of the Seminar III on Dry Valley Drilling Project, 1978, Special Memoir 13 Tokyo: National Institute of Polar Research. Brady, H. T. 1982. Late Cenozoic history of Taylor and Wright Valleys and McMurdo Sound inferred from diatoms in Dry Valley Drilling Project cores. In C. Craddock (Ed.) Antarctic geoscience. Madison: University of Wisconsin Press. Ciesielski, P. F. In press. The Neogene diatom biostratigraphy of DSDP Leg 71, Subantarctic sediments. In W. J . Ludwig, V. Drashinnikov et al. (Eds.), Initial reports of the Deep Sea Drilling project, leg 71. Washington, D.C.: U.S. Government Printing Office. Denton, G. H., R. L. Armstrong, and M. Stuiver. 1971. The late Cenozoic glacial history of Antarctica. In K K. Turekian, (Ed.), The Late Cenozoic glacial ages. New Haven: Yale University Press. Denton, G. H., M. L. Prentice, D. E. Kellogg, and T. B. Kellogg. In preparation. Tertiary history of the Antarctic Ice Sheet: Evidence from the dry valleys. Geology. Drewry, D. J . 1982. Ice flow, bedrock, and geothermal studies from radio-echo sounding inland of McMurdo Sound, Antarctica. In C. Craddock (Ed.), Antarctic geoscience. Madison: University of Wisconsin Press. Funder, S., and C. Hjort. 1973. Aspects of the Weichselian chronology in central East Greenland. Boreas, 2, 69-84. Hollin, J . T. 1962. On the glacial history of Antarctica. Journal of Glaciology, 4, 173-195. Stuiver, M., G. H. Denton, T. J . Hughes, and J. L. Fastook. 1981. History of the marine ice sheet in West Antarctica during the last glaciation: A working hypothesis. In G. H. Denton and T. J . Hughes (Eds.), The last great ice sheets. New York: Wiley-Interscience. Turner, R. D. 1967. A new species of fossil Chlamys from Wright Valley, McMurdo Sound, Antarctica. New Zealand Journal of Geology and Geophysics, 10, 446-455.
Webb, P. N. 1974. Micro palaeontology, palaeoecology and correlations of the Pecten gravels, Wright Valley, Antarctica, and description of Trocheolphidiella onyxi, n. gen., n. sp. Journal of Foraminiferal Research, 4, 184-199.
95
