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A thin glacial diamicton, informally termed Granite drift, occupies the floor of central Beacon Valley in southern Victoria Land, Antarctica. This drift is 40 Ar/ 39 Ar analyses of presumed in situ ash-fall deposits that occur within Granite drift. At odds with the great age of this ice are high-centered polygons that cut Granite drift. If polygon development has reworked and retransported ash-fall deposits, then they are untenable as chronostratigraphic markers and cannot be used to place a minimum age on the underlying glacier ice. Our results show that the surface of Granite drift is stable at polygon centers and that enclosed ash-fall deposits can be used to define the age of underlying glacier ice. In our model for patterned-ground development, active regions lie only above polygon troughs, where enhanced sublimation of underlying ice outlines high-centered polygons. The rate of sublimation is influenced by the development of porous gravel-and-cobble lag deposits that form above thermal-contraction cracks in the underlying ice. A negative feedback associated with the development of secondary-ice lenses at the base of polygon troughs prevents runaway ice loss. Secondary-ice lenses contrast markedly with glacial ice by lying on a δD versus δ 18 O slope of 5 rather than a precipitation slope of 8 and by possessing a strongly negative deuterium excess. The latter indicates that secondary-ice lenses likely formed by melting, downward percolation, and subsequent refreezing of snow trapped preferentially in deep polygon troughs. The internal stratigraphy of Granite drift is related to the formation of surface polygons and surrounding troughs. The drift is composed of two facies: A nonweathered, matrix-supported diamicton that contains >25% striated clasts in the >16 mm fraction and a weathered, clast-supported diamicton with varnished and wind-faceted gravels and cobbles. The weathered facies is a coarse-grained lag of Granite drift that occurs at the base of polygon troughs and in lenses within the nonweathered facies. The concentration of cosmogenic 3 He in dolerite cobbles from two profiles through the nonweathered drift facies exhibits steadily decreasing values and shows the drift to have formed by sublimation of underlying ice. These profile patterns and the 3 He surface-exposure ages of 1.18 ± 0.08 Ma and 0.18 ± 0.01 Ma atop these profiles indicate that churning of clasts by cryoturbation has not occurred at these sites in at least the past 10 5 and 10 6 yr. Although Granite drift is stable at polygon centers, low-frequency slump events occur at the margin of active polygons. Slumping, together with weathering of surface clasts, creates the large range of cosmogenic-nuclide surface-exposure ages observed for Granite drift. Maximum rates of sublimation near active thermal-contraction cracks, calculated by using the two 3 He depth profiles, range from 5 m/m.y. to 90 m/m.y. Sublimation rates are likely highest immediately following major slump events and decrease thereafter to values well below our maximum estimates. Nevertheless, these rates are orders of magnitude lower than those computed on theoretical grounds. During eruptions of the nearby McMurdo Group volcanic centers, ash-fall debris collects at the surface of Granite drift, either in open thermal-contraction cracks or in deep troughs that lie above contraction cracks; these deposits subsequently lower passively as the underlying glacier ice sublimes. The fact that some regions of Granite drift have escaped modification by patterned ground for at least 8.1 Ma indicates long-term geomorphic stability of individual polygons. Once established, polygon toughs likely persist for as long as 10 5 –10 6 yr. Our model of patterned-ground formation, which applies to the hyperarid, cold-desert, polar climate of Antarctica, may also apply to similar-sized polygons on Mars that occur over buried ice in Utopia Planitia.

A 9.5 m ice core was extracted from beneath the surficial debris cover of a rock glacier at Galena Creek, northwestern Wyoming. The core contains clean, bubble‐rich ice with silty debris layers spa...

Galena Creek rock glacier (GCRG), northwest Wyoming, exhibits most of the classic characteristics of rock glaciers. Clean ice with silty bands was found beneath a c. 1 m thick debris mantle by Potter. He inferred that the ice is glacigenic, originating in the small snowfield in the cirque at the head of GCRG. This view was challenged by Barsch, who asserted that the ice in GCRG is of “permafrost” origin. Since then GCRG has become a lightning rod for opponents and proponents of the glacigenic ice model for rock glaciers. We review evidence for that model here. Movement marks emplaced on GCRG in the 1960s were resurveyed in 1995 for a 30+ year record of movement. Maximum surface velocity is 45 cm/yr on gentle slopes and 80 cm/yr in a steep reach where GCRG spills out of the cirque. The less active, down-valley third of GCRG is moving at a maximum 14 cm/yr, and lobes formed between the more and less active parts have complex movement and are advancing down-valley over adjacent lobes at a maximum of 6.5 cm/yr. New refraction seismic profiles on GCRG were used to determine the thickness of the debris mantle over ice. On the up-valley, active part of GCRG, the debris mantle is a relatively uniform c. 1 m thick. On the down-valley, less active part, the thickness of the debris mantle is much more variable, but it is generally thicker. We cannot tell, on the basis of seismic data alone, whether the frozen material beneath the debris mantle is ice or a debris—ice mixture, but the results are not inconsistent with the glacigenic model for the origin of the ice. Two long-profiles in the cirque may identify bedrock at about 20–25 m depth.

We report the discovery of numerous in situ Miocene and Pliocene airfall volcanic ashes that occur within the hyperarid Dry Valleys region of the Transantarctic Mountains in southern Victoria Land, Antarctica. Ashes that occur above 1000 m elevation rest at the ground surface, covered only by a thin ventifact pavement 1 to 2 cm thick. The ash deposits are loose and unconsolidated and show no signs of chemical weathering. Laser-fusion 40 Ar/ 39 Ar analyses of volcanic crystals and glass shards indicate that the ashes range from 4.33 Ma to 15.15 Ma in age. The Arena Valley ash (4.33 ± 0.07 Ma) rests on the surface of a well-developed desert pavement and ultraxerous soil profile at 1410 m elevation. Lack of geomorphic evidence of liquid water on surficial sediments coeval and older than the Arena Valley ash, together with the pristine condition of volcanic crystals and lack of authigenic clay formation, indicates a cold desert at and since 4.33 Ma. The Beacon Valley ash (10.66 ± 0.29 Ma), the Koenig Valley ash (13.65 ± 0.06 Ma), and the Nibelungen Valley ash (15.15 ± 0.02 Ma) fill the upper half of relict sand-wedge troughs that form only in cold-desert conditions. The lack of authigenic clay-sized minerals in these ash deposits, along with preservation of sharp lateral contacts with surrounding sand-and-gravel deposits, suggests that frozen conditions (without rain or well-developed active layers during summer months) have persisted in Beacon, Koenig, and Nibelungen Valleys since ash deposition. Ash-avalanche deposits that rest on rectilinear slopes contain matrix ash dated to 7.42 ± 0.31 Ma in upper Arena Valley and 11.28 ± 0.05 Ma in lower Arena Valley. Little slope development has occurred since emplacement of these ash-avalanche deposits. Such slope stability is consistent with cold-desert conditions well below 0 °C. Taken together, these ash deposits point to persistent polar conditions similar to the present at elevations above 1000 m in the western Dry Valleys region during at least the last 15.0 m.y. This conclusion contradicts the view that, during part of the Pliocene epoch, East Antarctica was largely free of glacier ice and that scrub vegetation (Nothofagus, Southern Beech) survived along the Transantarctic Mountain front in the Dry Valleys region and to at least lat 86°S (Webb and Harwood, 1993). Instead, it supports marine and geomorphological evidence that calls for a stable Antarctic cryosphere, much the same as today, since middle Miocene time.

ANTARCTIC climate during the Pliocene has been the subject of considerable debate. One view holds that, during part of the Pliocene, East Antarctica was largely free of glacier ice and that vegetation survived on the coastal mountains1a¤-4. An alternative viewpoint argues for the development of a stable polar ice sheet by the middle Miocene, which has persisted since then5a¤-10. Here we report the discovery of buried glacier ice in Beacon valley, East Antarctica, which appears to have survived for at least 8.1 million years. We have dated the ice by 40Ar/39Ar analysis of volcanic ash in the thin, overlying glacial till which, we argue, has undergone little (if any) reworking. Isotope and crystal fabric analyses of the ice show that it was derived from an ice sheet. We suggest that stable polar conditions must have persisted in this region for at least 8.1 million years for this ice to have avoided sublimation.

We surveyed adjacent reaches with differing riparian vegetation to explain why channels with forested banks are wider than channels with nonforested banks. Cross sections and geomorphic mapping demonstrate that erosion occurs at cutbanks in curving reaches, while deposition is localized on active floodplains on the insides of bends. Our data indicate that rates of deposition and lateral migration are both higher in nonforested reaches than in forested reaches. Two dimensionless parameters, α and E , explain our observations. α represents the influence of grassy vegetation on rates of active floodplain deposition; it is 5 times higher in nonforested reaches than in forested reaches. E is proportional to rates of cutbank migration; it is 3 times higher in nonforested reaches than in forested reaches. Differences in width between forested and nonforested reaches are proportional to E/ α. In forested reaches, channels are wide with banks that are difficult to erode. Dense tree roots create a low value of E , and the channel migrates slowly. E/ α is high, however, because α is very low: shade from trees inhibits the growth of grass on active floodplains. In nonforested reaches, channels are narrow with banks that are easy to erode. E is high, and the channel migrates rapidly. E/ α is low, however, due to a very large value of α: grass grows readily on nonforested convex bank floodplains. Thus, differences in width between forested and nonforested reaches are related to a balance between rates of cutbank erosion and rates of deposition on active floodplains, implying that equilibrium widths develop to equalize rates of cutbank erosion and vegetation-mediated rates of deposition on active flood-plains. These results suggest that accurate models of width adjustment should consider the combined effects of bank erodibility and floodplain depositional processes, rather than focusing on these processes in isolation from one another.

Many rock glaciers contain massive ice that may be useful in paleoclimate studies. Interpreting geochemical ice-core records from rock glaciers requires a thorough understanding of rock glacier structure and dynamics. High-precision surface-velocity data were obtained for the Galena Creek rock glacier, Absaroka Mountains, Wyoming. Surface velocities range from 0 to 1.00 m/yr and vary across the rock glacier in a manner similar to true glaciers. We used Glen's flow law to calculate the thickness of the deforming ice layer. The modeled ice thickness ranges from 0 to 50 m, and is confirmed by direct observations. This agreement shows that rock glacier movement can be entirely explained by deformation of massive ice within the rock glacier; neither basal sliding nor deformation of basal debris is necessary. Recovered ice cores (to depths of 25 m) contain thin debris layers associated with summer ablation in the accumulation zone. The ages of four samples of organic material removed from several debris layers inthe southern half of the rock glacier range from 200 ± 40 to 2250 ± 35 14C yr B.P., demonstrating that the rock glacier formed well before the Little Ice Age and may contain ice dating to the middle Holocene or earlier.

Anyone who spends much time above the treeline has probably seen rock glaciers and paused to wonder about them. Their curious and occasionally spectacular forms (Figure 1) occur in alpine and polar regions throughout the world, yet much remains uncertain about how they develop. A core of ice recently recovered from a rock glacier in the Absaroka Mountains of northwestern Wyoming vividly illustrates several important aspects about rock glaciers. At least some rock glaciers are a form of debris-covered glacier, and original isotopic stratigraphy may be preserved within their ice. Perhaps most interesting of all, the core of some rock glaciers is composed of layered ice that can be drilled and recovered, and some of this ice is exceptionally old.

There are two competing scenarios on the behavior of the East Antarctic ice sheet during the late Tertiary. In one scenario, the ice sheet was very dynamic and underwent major drawdown and renewal as late as the Pliocene. In the other, the ice sheet was relatively stable during the late Neogene. The presence of marine diatoms in Sirius Group sedimentary rocks in East Antarctica is at the center of the disagreement. One side regards the diatoms as the major piece of evidence to support the drawdown and renewal hypothesis and infers that they were introduced into the Sirius during renewed glaciation of East Antarctica; others suggest that these diatoms were likely introduced into the Sirius by atmospheric (largely eolian) processes. We propose a simple test of the eolian hypothesis. If diatoms were introduced into the Sirius by eolian processes, then they should also be present in older (Paleozoic and Mesozoic) sedimentary and igneous rocks. Samples from two units of the Beacon Supergroup (Devonian to Jurassic) from Beacon Valley, East Antarctica, were analyzed: the Beacon Heights Orthoquartzite (Devonian) and the Feather Conglomerate (Permian-Triassic). Also examined was sediment found in cracks of Paleozoic and Mesozoic (Devonian to Cretaceous) igneous rocks from Marie Byrd Land, West Antarctica. Largely Pliocene-Pleistocene planktonic marine diatoms were found in all sample sets. Because neither Beacon Supergroup sedimentary rocks nor igneous rocks from Marie Byrd Land are Pliocene-Pleistocene in age, such findings strongly suggest that diatoms were introduced into them by eolian processes. This same scenario can be applied to Sirius Group sedimentary rocks.

The concept that erosion, over a long period of time, would produce an evolutionary progression of landforms culminating in a nearly flat plain, the peneplain, was formulated into a coherent theory by W. M. Davis. Subsequent to early identification of the Fall Zone (oldest), Schooley, Harrisburg, and Somerville (youngest) peneplains, numerous workers pursued identification, correlation, description, folding, formative processes, and dating of Appalachian peneplains for 6 decades. Following a peak of interest and activity in the 1930's, work on Appalachian peneplains declined rapidly. Reasons for the decline include: death of former workers, diversion into other lines of research, and rise of process geomorphology. Phenomena attributed to a dependence on peneplanation include: origin of present drainage, origin of some mineral resources, and cementation of rock units. Attacks on the peneplain idea have been largely unsuccessful except for the dynamic equilibrium concept advocated by John Hack. Controversy exists about whether the disparity between rates of uplift and denundation allow adequate time for peneplanation to occur. The relationship of some surficial deposits to presumed peneplain surfaces is problematical. The peneplain concept is still alive, but new lines of research are required to resolve its controversial position.

Blue Rocks (40°36′ N., 75°55′ W.) is a half-mile-long block field on the south slope of Blue Mountain, 3 miles northeast of Hamburg, Pennsylvania. The block field consists of angular blocks of Tuscarora Quartzite 4 inches to 20 feet long. The blocks were derived from strongly jointed quartzite cliffs on Blue Mountain and have moved downslope over the Martinsburg Shale. On two sides of the field of open blocks are forested rubble deposits that contain boulders equivalent in size to those in the block field, but with fine material in the interstices. The rubble adjacent to the block field occurs as gently sloping, step-like terraces with fronts 10 to 30 feet high and slopes of 18° to 22°. The block field is separated from the source cliffs by about a half mile of rubble covered by mature forest. Tabular blocks in the block field have an imbricate structure and describe a series of lobes in which the blocks dip steeply upslope. The blocks are crudely sorted vertically, with coarser material at the surface. Locally, horizontal sorting on a small scale is also apparent. Blue Rocks and the adjacent rubble deposits closely resemble solifluction sheets or terraces on the Seward Peninsula, Alaska. The lack of fine material in the block field probably resulted in part from the flushing of fines after downslope movement ceased. The block field is older than the mature forest on the rubble between the block field and the source cliffs. It was probably formed by solifluction or creep in the periglacial climate of the Wisconsin glaciation, after which removal of fines occurred.

Galena Creek rock glacier (44°38930″ N., 109°47930″ W., elevation 2,680 to 3,110 m, length 1.6 km) originates in a north-facing cirque. Although this rock glacier morphologically resembles others described elsewhere, its upvalley two-thirds is composed of a continuous layer of debris 1 to 1.5 m thick over relatively clean glacier ice and has a maximum measured surface velocity of 80 cm/yr. The downvalley one-third is mantled by 2 to 3 m of debris (measured by seismic refraction) over ice of unknown debris content; it has a maximum measured velocity of 14 cm/yr. The transition zone between these two regions has several large (6-m-high, 90-m-wide) lobes that override one another at a maximum measured velocity of 6 cm/yr. Accumulation occurs primarily as wind-drifted snow in a narrow lens-shaped area against the cirque headwall. Most of the coarse debris is not incorporated in the ice, but is carried past the steep (13° to 33°) snow accumulation area beneath the cirque headwall by snow avalanche and rockfall to form the debris mantle. The debris mantle is sorted, with coarse fragments dominant at the surface and a zone of fines just above the debris-ice contact. The ice beneath the debris mantle contains a maximum of 10 to 12 percent debris by volume, except in probable longitudinal septa downglacier from large debris concentrations in the source area. Intersecting ridges and furrows on the up-valley portion of the rock glacier probably differ in age, according to lichen sizes and ridge sharpness, and are probably formed by compression below steep reaches of the glacier and by collapse into crevasses. Ice-cored rock glaciers uniquely have a very low ratio of accumulation area to ablation area (1:7 in this case). This is mainly the result of an ablation rate beneath the debris mantle that is estimated to be about two orders of magnitude less than that of clean ice. The slow rate of addition of ice makes the glacier thin and thus slow-moving. Because of the debris cover, rock glaciers are not nearly so sensitive to climate as are clean glaciers. The lag effect between retreat of clean glaciers and deactivation of rock glaciers may be several thousand years, and therefore mountain glacier moraines should be correlated with rock glaciers only with extreme care.