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Unparalleled data availability in the European Alps has led to an ongoing debate about the driving mechanism behind the concurrent patterns of surface denudation and modern rock uplift. Analysis of stream channels reveals that oversteepened stream segments are primarily located in landscapes with strong glacial inheritance. This leads to a transient signal in the landscape, with the result that erosion is spatially focused by a combination of glacial conditioning and lithologic controls. We postulate that the effect of glacial forcing is a positive feedback cycle between erosion and rock uplift, driving rapid rates of both in the Alpine landscape. This mechanism may explain the observed increases in sediment flux since the late Pliocene. doi: 10.1130/G31102.1

The importance of rock type and tectonic history on rates of glacial erosion, and the relative roles of glacial quarrying an abrasion, are poorly understood. We use concentrations of cosmogenic [sup.10]Be in glacial polish and measurements of bedrock fracture spacing to explore the relationship between erosion rates and rock fracturing at 23 sites along Tuolumne River valley and five sites i a Tenaya Canyon in Yosemite National Park, California, USA. Most sites yield [sup.10]Be concentrations that can be best explained as reflecting solely postglacial nuclide accumulation. Six sites, however, display anomalously high concentrations, implying incomplete removal of the pre-glacial nuclide inventory during the last glaciation; these require that erosion in the last glacial cycle was doi: 10.1130/G30576.1

What do polar bears, Pacific walrus, spectacled eiders, and Kittlitz's murrelets have in common? In a word--ice! Although the effects of climate change can now be observed almost anywhere in [...]

[1] The > 5-km-high Rwenzori Mountains in the East African Rift form a promontory on the rift shoulder and are an extreme expression of rift-mountain uplift. This study reports the pattern and the kinematics of major faults in the Rwenzori region. The fault pattern is characterized by a first set of N/NNE-striking normal faults that are offset by mainly NE- to E-striking faults. Fault-slip data indicate a change in the extension direction from ENE/E to SE. The latter direction is in accord with earthquake focal data and borehole breakouts. Major faults with displacements > 6 km occur at the boundaries of the Rwenzori Mountains; within the range minor faults with offsets [less than or equal to] 2 km have been mapped. Flexural isostatic rebound of the footwall of large-displacement back-to-back normal faults bounding the Rwenzori horst and additional footwall uplift of a young crosscutting normal fault probably caused cumulative (surface) uplift of ~3-4 km. I propose that the change in the kinematics of the rift faults and the largely contemporaneous onset of glaciation in the Rwenzori Mountains in the Middle Pleistocene were important for enhanced rock uplift. Glacial erosion rates were 1.5-4 mm/a and eroded 1-2 km of material off the top of the Rwenzoris. Glacial erosion and the retreat of the glaciers during interglacial periods caused removal of loads leading to isostatic rebound, which reduced horizontal stresses promoting normal faulting and enhanced rock uplift, thereby elevating the remaining terrain, transforming it into one with high peaks and deeply incised valleys.

Subglacially-erupted volcanic sequences provide proxies for a unique range of palaeo-ice parameters and they are potentially highly useful archives of palaeoenvironmental information, particularly for pre-Quaternary periods. They can thus be incorporated by climate and ice sheet modellers in the same way as other environmental proxies, yet they remain largely under-utilised. Basaltic volcanic sequences erupted subglacially consist empirically of two major types, corresponding to eruptions under 'thick' and 'thin' ice, respectively. The latter are called subglacial sheet-like sequences and only one genetic type of sequence has been described so far. However, there is now evidence that there are at least two genetic types, with significantly different implications for interpretations of associated palaeo-ice sheet thicknesses. One type, which is relatively well described, is believed to be a diagnostic product of eruptions associated with a relatively thin glacial cover ( 1000 m). Eruptions that form the Dalsheidi-type of sequence commence with the injection and inflation of a sill along the ice: bedrock interface. Such 'interface sills' were predicted theoretically but had no known geological example, until now. Subsequent evolution commonly involves floating of the ice cover, catastrophic meltwater drainage and emplacement of widespread sheets of hyaloclastite, as cohesionless mass flows and hyperconcentrated flows. The water-saturated hyaloclastite is characteristically intruded by apophyses sourced in the underlying 'interface sill'. Eruptions are commonly not explosive until their later stages. Dalsheidi-type deposits are outflow sequences probably linked to subglacial pillow volcanoes, which in Iceland were erupted along fissures. They only provide an indication of minimum thicknesses of the associated overlying ice, although theoretical considerations suggest substantial ice thicknesses in excess of 1000 m. However, they are likely to be characteristic products of eruptions under the thick West Antarctic Ice Sheet, but are currently inaccessible. Such eruptions may be capable of destabilising that ice sheet. Keywords: subglacial eruptions; glaciovolcanic, basaltic; sheet-like; Dalsheidi; Pinafore; ice sheet; palaeoenvironments; West Antarctic Ice Sheet

Seismic-reflection data from the easternmost Ross Sea image buried scour-and-fill troughs and flat-topped ridges interpreted as having formed by glacial erosion and deposition during the Oligocene. The NNW-SSE orientation of the troughs and lack of similar Oligocene glacial features within the central Ross Sea suggests that the ice issued from the highlands of Marie Byrd Land located 100 km away and that portions of the West Antarctic Ice Sheet formed earlier than previously accepted. Existing global climate models (GCMs) do not produce West Antarctic ice caps for the Oligocene, in part due to low elevations modeled for that time. Evidence for Oligocene ice beyond the paleocoast suggests a higher elevation for the early Cenozoic Marie Byrd Land and Ross Embayment than at present. Keywords: Antarctic Ice Sheet, Marie Byrd Land, West Antarctica, seismic stratigraphy, Ross Sea, Oligocene, basins.

Theories that propose feedbacks among climate, tectonics, and surface processes commonly assume that erosion is enhanced by glacial activity. Indeed, studies have shown that glaciers appear to limit the elevation of mountain ranges; however, comparisons between rates of glacial and nonglacial erosion are difficult to make. Ideally, such comparisons must hold precipitation and lithology constant, while only varying the erosional regime. Located in a climatic transition zone during the Pleistocene, the east-west-trending valleys of the Bitterroot Range present an opportunity for a quantitative analysis of glacial and nonglacial erosion because the north-facing sides of the valleys were glaciated, whereas the south-facing slopes were not. The different erosional regimes operating on either side of the valleys created strongly asymmetric ridges. Ridgelines separating the east-west-trending valleys have been pushed southward by glacial headwall retreat such that ridge-to-valley distances are ~50% greater on the north-facing slopes than on the south-facing slopes. In addition, mean hillslope angles are 6[degrees] lower on the glaciated slopes than on the unglaciated slopes, and calculations of geophysical relief suggest that, on average, glaciers have removed nearly twice as much rock as nonglacial processes. Finally, we conclude that, although rates of vertical incision by glacial processes in the Bitterroot Range were more rapid than nonglacial processes, the dominant geomorphological impact of glaciers was lateral erosion by headwall retreat. Keywords: glacial erosion, cirque headwall retreat, divide migration, relief, Bitterroot Range.

What happens beneath a glacier affects the way it flows and the landforms left behind when it retreats. Direct observations from beneath glaciers are, however, rare and the subglacial environment remains poorly understood. We present new, repeat observations from West Antarctica that show active processes beneath a modern glacier which can normally only be postulated from the geological record. We interpret erosion at a rate of 1 m [a.sup.-1] beneath a fast-flowing ice stream, followed by cessation of erosion and the formation of a drumlin from mobilized sediment. We also interpret both mobilization and increased compaction of basal sediment with associated hydrological changes within the glacier bed. All these changes occurred on time scales of a few years or less. This variability suggests that an ice stream can reorganize its bed rapidly, and that present models of ice dynamics may not simulate all the relevant subglacial processes. Keywords: subglacial environment, ice streams, drumlins, erosion.

Sediment accumulation rates of well-dated, primarily terrigenous marine sediments collected along the Chilean continental margin were used to infer erosion rates of the southern Andes and the adjacent Coastal Cordillera. Compared to the Holocene, sediment supply to the margin and thus erosion were substantially enhanced during the last glacial, when continental rainfall in the region was higher. Major changes in precipitation forcing to the southern Andes on such glacial-interglacial time scales appear to be transferred to offshore sedimentation rates by a fixed relation. These data show that marine archives can make it possible to trace variability in erosion at adjacent continents on time scales to [10.sup.3] yr, a much better resolution than provided by commonly used methods such as thermochronometry or seismic studies. Keywords: marine sediments, erosion, climate change, Andes, southeast Pacific, precipitation.

Glacial erosion dramatically alters mountain landscapes, but the pace at which glaciers carve a previously fluvial landscape remains poorly defined because long-term valley incision rates are difficult to measure. Here we reconstruct the lowering history of the Aare Valley, Switzerland, over the past 4 m.y. by dating cave sediments with cosmogenic [sup.26]Al and [sup.10]Be. Incision accelerated from ~120 m/m.y. to -1200 m/m.y, at 0.8-1.0 Ma, at least 1 m.y. after the onset of local glaciation. Rapid incision may have been triggered by lowering of the equilibrium line altitude at the mid-Pleistocene climate transition. Keywords: glacial erosion rate, cave, Switzerland, speleogenesis, cosmogenic, burial age, glacial valley.

We have measured erosion rates using [sup.10]Be from bare-bedrock surfaces exposed at high elevations at Dolly Sods, West Virginia, a classic Appalachian paleoperiglacial plateau. The mean erosion rate from nine samples is 5.7 m/m.y., significantly lower than previously estimated periglacial erosion rates in this region. Measured bare-bedrock erosion rates likely represent the rate at which the highest portions of this broad upland are being lowered. Fluvial incision rates measured in the region over similar time scales are [greater than or equal to] 2 times faster, suggesting relief is increasing in this portion of the Appalachians. This observation of increasing relief is inconsistent with prior work suggesting that the central Appalachian landscape is in dynamic equilibrium or currently decreasing in relief. We hypothesize that late Cenozoic climate change has accelerated fluvial incision rates, creating a disequilibrium landscape with growing relief with hillslopes undergoing adjustment to increased fluvial incision rates. Keywords: erosion, relief, cosmogenic nuclides, landscape evolution, Appalachians.

We review the major subglacial till forming processes as presently understood by glacial researchers and define the parameters within which tills are produced and reconcile them with sedimentary end members. Processes of deformation, flow, sliding, lodgement and ploughing coexist at the base of temperate glacier ice and act to mobilize and transport sediment and deposit it as various end members, ranging from glacitectonically folded and faulted stratified material to texturally homogeneous diamicton. The dominance of any one subglacial process varies both spatially and temporally, giving rise to the possibility that a till or complex till sequence contains a superimposed signature of former transportation/deposition at the ice-bed interface. We recommend that, while glacial geologists and geomorphologists should be able to recognize the sedimentary imprints of various subglacial processes, genetic fingerprinting of subglacial tills should be less process-specific and till classification must reflect the range of products encompassed by the subglacial till production continuum. Glacial geologists can presently unequivocally identify: a) glacitectonite (rock or sediment that has been deformed by subglacial shearing/deformation but retains some of the structural characteristics of the parent material); b) subglacial traction till (sediment deposited by a glacier sole either sliding over and/or deforming its bed, the sediment having been released directly from the ice by pressure melting and/or liberated from the substrate and then disaggregated and completely or largely homogenised by shearing); and support the theoretical case for c) melt-out till (sediment released by the melting of stagnant or slowly moving debris-rich glacier ice, and directly deposited without subsequent transport or deformation). Because observations on contemporary glaciers reveal that their beds are most likely to be mosaics of deformation and sliding and warm based and cold based conditions, the patterns of which change temporally and spatially, it is extremely unlikely that subglacial till end members in the geological record will be anything but hybrids produced by the range of processes operative in the subglacial traction zone. Keywords: till; subglacial processes; subglacial deformation; glacitectonite; subglacial traction till

Alpine glaciers are efficient agents of erosion and capable of significantly modifying topography. Despite recent advances in theoretical and field studies that quantify glacial erosion processes, few studies have documented glacial erosion rates over long (>[10.sup.6] yr) or large (more than tens of kilometers) scales. We use apatite (U-Th)/He (AHe) and apatite fission track (AFT) cooling ages to address the late Miocene to Holocene erosion history across two 60-km-long transects of the heavily glaciated southern Coast Mountains, British Columbia. Observed AHe cooling ages from equal elevation samples range between 1.5 and 8 Ma and suggest that thick alpine glaciers resulted in a 16 km shift of the highest point in the topography in the past 1.5-4.0 m.y. We evaluated temporal and spatial variations in erosion rates using a three-dimensional thermal-kinematic model that predicted AHe and AFT ages at the surface for different erosion histories. Comparison of model predicted and observed cooling ages suggests an increase in erosion rates of as much as 300% over the past 1.5-7 m.y., coincident with the onset of glaciation of this range. Keywords: glacial erosion, apatite (U-Th)/He, numerical modeling, Coast Mountains, paleotopography.

Ku-band dual-polarization radar backscatter measurements from the SeaWinds-on-QuikSCAT scatterometer are used to determine periods of surface freeze and melt in the Antarctic ice shelves. The normalized horizontal-polarization radar backscatter ([[sigma].sup.o]) and backscatter polarization ratio are used in maximum-likelihood estimation of the ice state. This method is used to infer the daily ice-surface conditions for 25 study locations located on the Ronne, Ross, Larsen, Amery, Shackleton, and other ice shelves. The temporal and spatial variations of the radar response are observed for various neighborhood sizes surrounding each given location during the study period. Criteria for determining the dates of melt onset and freeze-up for each Austral summer are presented. Validation of the ice-state and melt-onset date estimates is performed by analyzing the corresponding brightness temperature ([T.sub.b]) measurements from Special Sensor Microwave/Imager (SSM/I) radiometers. QuikSCAT [[sigma].sub.o] measurements from 1999 to 2003 are analyzed and found to be effective in determining periods of melt in Antarctic ice sheets at high temporal and spatial resolutions. These estimates can be used in studies of the climatic effects of the seasonal and interannual melting of the Antarctic ice sheets. Index Terms--Antarctic, ice, ice shelves, melt onset, QuikSCAT, refreeze, SeaWinds, Special Sensor Microwave/Imager (SSM/I).

High-resolution Chirp sub-bottom profiler and side-scan sonar data record the final phases of ice margin activity in the Lower Lough Erne basin at the end of the last cycle of Quaternary glaciations in Ireland. Relative to the terrestrial glacial landforms the features in the Lough are smaller in scale and are considered to represent local ice mass dynamics that followed regional-scale events. Four phases are identified (11 After the last ice sheet-wide readvance associated with the Killard Point Stadial (between 15.0) and 14.1 [sup.14]C ka bp), stagnation zone retreat resulted in isolation of a residual ice block in the Lower Lough Erne basin. (2) Proglacial waters developed coeval with retreat of the western margin of the ice block. Drawdown induced localized surging and the generation of push features and lineations. (3) Squeeze-up features, reflecting a heavily crevassed ice margin, mark the quiescent phase of the local surge cycle. (4) Iceberg grounding pits and keel marks record calving and rapid disintegration of the Lough Erne ice margin. The well-preserved glacigenic features observed in the lake basin suggest formation by a succession of mechanisms that were too short hived to obliterate the underlying evidence of ice margin dynamics.

Loesses form wide belts in front of previously glaciated areas. Their thicknesses may be considerable, changing in Eurasia from maximally a few metres in the west to a hundred metres or more in the east. The Eastern (particularly Chinese) loesses are mostly unrelated to glaciations. The periglacial loesses from China and elsewhere predominantly date from the last Pleistocene glaciation: relatively few comparable occurrences are known from earlier Quaternary glaciations. As it is difficult to imagine that the conditions in front of the land-ice masses during the earlier glaciations differed fundamentally from those of the last one, considerable quantities of loess must have disappeared. This disappearance, which is commonly ascribed to fluvial and eolian erosion, is not easily explained as equivalent deposits that may have the older loesses as a source, are practically absent. A possible explanation might be that loess is recycled during successive glaciations. Some loess disappears during interglacials by erosion, but this quantity is more than compensated by the formation of new silt particles. The implication would be that the loess deposits increase in volume for each new glaciation. Keywords: silt; periglacial processes; sediment recycling; sorting; eolian abrasion; glacial grinding

In temperate climates, ~25% of peak elevations in mountain ranges can be created by isostatic rebound as a response to erosional incision. Significantly more relief generation and peak uplift are, however, possible for glacial erosion in a polar climate. We incorporate regional isostasy using flexure of an elastic plate to show that isostatic rebound as a response to glacial incision can account for as much as 2000 m or 50% of peak elevation in the central Transantarctic Mountains. Differences in relief of at least 5500 m over lateral distances of just 40 km are evident within the central part of the 3000-km-long mountain range. Such strong relief is possible because a polar climate since the middle Miocene has resulted in freezing conditions at high elevations, which acted to preserve the peaks, whereas wet-based glaciers at low elevations have produced optimal conditions for enhanced glacial incision. Because isostatic rebound results in permanent peak uplift, this mechanism provides an explanation of why the Transantarctic Mountains are one of the higher and more long-lived continental rift margins on Earth. Keywords: isostasy, flexure, Transantarctic Mountains, glacial erosion, isostatic rebound.

Many assessments of glacial sediment yield rely solely on measurements of fine-grained suspended sediment. We show that suspended sediment contributes Keywords: glacial erosion, glacial lake, shorelines, sedimentation, Alaska, varves.

We measured the abundance of cosmogenic [sup.10]Be and [sup.26]Al in 22 samples collected from five striated granite, metarhyolite, and quartzite outcrops in south-central Wisconsin that were covered by the late Wisconsin Laurentide Ice Sheet. In two outcrops, measured nuclide abundances are consistent with the existing radiocarbon chronology of ice retreat. In three outcrops, nuclide abundances were up to eight times higher than predicted by the radiocarbon chronology. At these three sites, several thousand years of ice flow eroded only centimeters to decimeters of rock, allowing a significant quantity of nuclides ([10.sup.5]-[10.sup.6] atoms of [sup.10]Be and [sup.26]Al per gram of quartz), produced during prior periods of exposure, to remain. We calculate minimum-limiting glacial erosion rates of 0.01-0.25 mm.[yr.sup.-1] for these rocks. Rock properties, sample location on outcrops, and outcrop proximity to the former ice margin control the magnitude of cosmogenic nuclides inherited from periods of prior exposure. Four of five samples from very hard metarhyolite outcrops with widely spaced joints contain inherited nuclides; two samples carry the equivalent of >150,000 yr of surface exposure, even though they were covered by ice during the last-glacial-maximum advance. Samples highest on the landscape or in plucked areas have less inheritance than those from the lee sides of large hills or lower in the landscape. Three quartzite samples collected ~10 km up-ice from the margin contain three to four times the expected nuclide abundance ([10.sup.5] to [10.sup.6] atoms per gram of quartz). In contrast, eight other quartzite and granite samples from two outcrops >50 km up-ice from the former margin contain only [10.sup.5] atoms of [sup.10]Be per gram of quartz, consistent with late Pleistocene exposure and little, if any, nuclide inheritance. This relationship between glacial erosion and distance from the former terminus is consistent with a marginal zone of minimal subglacial erosion; the ice was either frozen to its bed there, or the ice thickness and duration of ice cover were less near the terminus. These data, together with simple modeling of nuclide production by deeply penetrating muons, suggest that many meters of rock must be removed to reduce inheritance to negligible levels ( Keywords: cosmogenic isotopes, dating, exposure age, glacial erosion.

Here we report previously undescribed features of erosion and deposition by a cold (polar) glacier. A recent study challenged the assumption that cold glaciers neither slide nor abrade their beds, but no geological evidence was offered. The features we describe include abrasion marks, subglacial deposits, glaciotectonically deformed substrate, isolated blocks, ice-cored debris mounds, and boulder trains, all products of a recent cold ice advance and retreat Mapping these features elsewhere in Antarctica will document recent shifts in the East Antarctic Ice Sheet margin, providing new insight on regional mass-balance changes. Keywords: cold-based glaciers, abrasion, Allan Hills, Antarctic ice sheet, glacial deposition.

Swath-bathymetry data and high-resolution seismic reflection profiles allow us to portray a subglacial sedimentary system off the northern tip of the Antarctic Peninsula, in the Central Bransfield Basin, during the Last Glacial Maximum with unprecedented detail. Postglacial reworking and sedimentation are weak enough for the subglacial morphology of the Last Glacial Maximum to be preserved on the present seafloor. The studied sedimentary system extends 250 km, from ~1000 m above sea level to ~2000 m water depth. The data set supports a model for subglacial sedimentary systems that consists of: (1) an upper ice catchment or erosional zone on the innermost continental shelf, extending onshore; (2) a transitional erosional-depositional zone on the inner shelf with drumlinized seafloor; (3) a depositional outer shelf zone with mega-scale bundle glacial lineations; and (4) a debris apron on the continental slope and base of slope formed under floating ice shelves with debris delivery linked to grounding lines along the shelf break. Keywords: subglacial sedimentation model, continental margin, Antarctic Peninsula, swath bathymetry.

Hemipelagic, sediment drift deposits have been discovered and mapped on the Antarctic Peninsula shelf in 300-500 m water depth. The drift located adjacent to Andvord Bay covers 44.5 [km.sup.2] and exhibits continuous and discontinuous parallel reflections that conform to peaks and valleys in the acoustic basement as observed in deep-tow boomer and sparker seismic records. This style of drift deposit is a common feature of deep oceanic sediments, but is not normally found in continental shelf environments. Measured sedimentation rates of 1-3 mm/yr on the Andvord drift indicate that the total 40 m drift thickness observed in the seismic records is probably postglacial. The drift contrasts with the basin-fill style of sedimentation that is normally associated with the Antarctic continental shelf and may play an important role in the carbon cycle. On the basis of an isopach map of drift sediments and previously published core information, the rate of carbon accumulation in the Andvord drift is estimated to be about 1.7 g/[cm.sup.2]/k.y., which is comparable to the highest rates reported for the southwestern Ross Sea.

Cosmogenic 36Cl/Cl ratios measured from glacially eroded bedrock provide the first quantitative constraints on the magnitude, rate, and spatial distribution of glacial erosion over the last glacial cycle. Of 23 36Cl/Cl ratios, 8 yield exposure ages that predate the well-constrained deglaciation of the Puget Lowland, Washington, and are inferred to result from 36Cl inherited from prior exposure during the last interglaciation where ice did not erode enough rock ([approximately]1.80-2.95 m) to reset 36Cl/Cl ratios to background levels. Surfaces possessing inherited 36Cl evidently were abraded only 0.25-1.06 m, corresponding to abrasion rates of 0.09-0.35 mm [multiplied by] [yr.sup.-1]. These results indicate that in the absence of glacial quarrying, the Cordilleran ice sheet may have abraded as little as 1-2 m of bedrock near its equilibrium-line altitude over the last glacial cycle, equating to only tens of meters over the entire Quaternary.

The Sirius Group comprises a suite of consolidated glacial deposits that are widespread in the Transantarctic Mountains; some of the deposits contain Pliocene marine diatoms. Because of the interpretation that the Sirius Group tills and the marine diatoms were deposited by the East Antarctic Ice Sheet, it has been inferred that the East Antarctic Ice Sheet was much reduced in area and volume during the Pliocene. These interpretations were evaluated by studying Sirius Group lodgment tills on Mount Fleming in the Dry Valleys sector of the Transantarctic Mountains. We infer that the Sirius Group lodgment tills at Mount Fleming do not support the hypothesis that the East Antarctic Ice Sheet was much reduced during the Pliocene. We demonstrate that Sirius Group lodgment tills on Mount Fleming were deposited by alpine ice. This conclusion is founded on ice-flow directions inferred from rattails, gravel fabric, and the orientation of bullet boulders, as well as on sediment and gravel characteristics of the tills. Only a very few unidentifiable diatom fragments were recovered. Better preserved, identifiable, Pliocene marine diatoms occur in moderate abundances in a 5-10-cm-thick unconsolidated and discontinuous drift that caps the lodgment tills. On the basis of the decline in diatom preservation with depth into the till, we dismiss transport of the better preserved diatoms by the ice that deposited the underlying Sirius Group lodgment till. Instead, we attribute diatom presence to eolian deposition and recycling into lodgment tills at a later stage.

Erosional and depositional evidence on Mount Olympus, Greece, and across the adjacent piedmont provides clear indication that the mountain was more extensively glaciated over a longer period of time than has been previously reported. The stratigraphic record of Pleistocene-Holocene events on Mount Olympus is most clearly documented on the eastern piedmont, where three discrete sedimentary packages (units 1-3), each capped by a distinctive soil, reflect glacial and nonglacial activity in the Mount Olympus region. A working stratigraphic framework for sediments and softs is proposed and is tentatively correlated with a dated alluvial succession south of Mount Olympus. We suggest that the oldest sedimentary package (unit 1) predates 200 ka (isotope stage 8?). Lithologic and pedologic equivalents of the piedmont stratigraphy are found within major valleys draining Mount Olympus, as well as within cirque basins and on the summit plateau surface. These deposits can be clearly tied to three stages of cirque development on the upland and at valley heads. Taken together, upland and piedmont glacial features and deposits indicate the following general scenario: (1) earliest glaciation (isotope stage 8?) produced upland ice and valley glaciers that extended as piedmont lobes east, north, and west of Mount Olympus; (2) nonglacial (interglacial) conditions (isotope stage 7?) were accompanied by extensive erosion and subsequent pedogenesis; (3) a second glaciation (isotope stage 6?) involved production of upland ice and valley glaciers that did not reach the piedmont; (4) interglacial (interstadial) conditions (isotope stage 5?) provided time for stream erosion and substantial pedogenesis; (5) final(?) glaciation (isotope stages 4-2?) was restricted to valley heads (no upland ice) and glaciers that extended to mid-valley positions; (6) nonglacial conditions (isotope stage 17) were associated with additional pedogenesis and stream incision. The largest cirque on the mountain (Megali Kazania) may contain depositional evidence for neoglaciation. Study of the neotectonic history of the Mount Olympus region indicates that uplift has persisted throughout the mid-Pleistocene and Holocene at a rate of about 1.6 m/k.y.; the total uplift since deposition of unit 2 is approximately 200 m.

The Shed Brook Discontinuity in the western Mohawk Valley region of central New York is an unconformity separating lacustrine beds of the Valley Heads drift (Port Bruce stade) from pre-Valley Heads lacustrine deposits (Nissouri stade). The Little Falls Gravel is a fluvial unit that can be found draped by Valley Heads lacustrine deposits in the Mohawk Valley. The discontinuity and gravel represent subaerial erosion and braided river deposition in response to initial late Wisconsinan ice recession and lake drainage in the Mohawk Valley. These events were followed by Valley Heads lake impoundment and glacial read-vances. The Shed Brook Discontinuity and Little Falls Gravel have a 14C age constrained to 17-14.1 ka on the basis of stratigraphic relationships in central New York and the correlation of paleomagnetic declination records from central New York and the 14C-calibrated New England varve chronology. The Shed Brook Discontinuity and Little Falls Gravel appear to be features equivalent in age to the Erie interstade (about 14.5-16 ka), an interval during which eastward drainage to the Mohawk Valley has been inferred for lakes in the Erie basin. Evidence for a river in the Mohawk Valley (Little Falls Gravel) indicates that it may have served as an eastern outlet for the Erie and Ontario basins, and thus allowed the first eastward diversion of meltwater away from the Mississippi Valley during the last deglaciation. Drainage down the Mohawk Valley entered Lake Albany in the Hudson Valley, which drained into Long Island Sound and not along the present Hudson River course to the New Jersey continental shelf. Meltwater drainage across New York during the latest stages of the Erie interstade may have introduced large volumes of meltwater directly to the western Atlantic, creating a situation that needs to be more fully explored in terms of its possible influence on thermohaline circulation in the North Atlantic and climatic change marked by the Port Bruce stade.

Do glaciers enhance or inhibit chemical weathering rates relative to other environments? The importance of glaciers in the global carbon cycle and climate change hinges on the answer. We show that catchments occupied by active alpine glaciers yield cation denudation rates greater than the global mean rate but do not exceed rates in nonglacial catchments with similar water discharge. Silica denudation rates are distinctly lower in glacier-covered catchments than in their nonglacial counterparts. Because sediment yields are high from glaciers, this suggests that water flux, rather than physical erosion, exerts the primary control on chemical erosion by glaciers. Potassium and calcium concentrations are high relative to other cations in glacial water, probably due to dissolution of soluble trace phases, such as carbonates, exposed by comminution, and cation leaching from biotite. Preferential weathering of biotite may result in higher 87Sr/86Sr in glacial runoff than expected from whole-rock compositions. Thus, although glaciers do not influence total chemical denudation rates at a given runoff, they may yield compositionally distinctive chemical fluxes to the oceans. Disruption of mineral lattices by grinding increases dissolution rates; this and high surface area should make glacial sediments exceptionally weatherable. Weathering of glacial erosion products in environments beyond the glacier margin deserves attention because it may figure prominently in global chemical cycles.

10Be and 26Al data from sandstone boulders in three Neogene glacial deposits in the McMurdo Sound-Dry Valleys region of southern Victoria Land, Antarctica, indicate minimum exposure ages of [approximately]3 Ma and maximum long-term erosion rates of [approximately]5-12 cm/m.y., supporting the suggestion that polar desert conditions have persisted in the Dry Valleys since at least late Pliocene time. Variation of cosmogenic nuclide production rate with altitude also allows constraints on past uplift rates. Model calculations employing 10 Be data indicate little or no uplift in the Dry Valleys region in the past 3 m.y., precluding rapid ([approximately]1 km/m.y.) late Pliocene uplift previously suggested for some parts of the Transantarctic Mountains.

Marine geologic evidence from the western Barents Sea shelf leaves little doubt that the area was covered by grounded ice during the last glaciation, but the pattern and timing of the subsequent deglaciation were not well determined. Here we reconstruct the timing and mechanism of ice-sheet retreat as constrained by seismic stratigraphy and lithostratigraphy and accelerator-mass-spectrometer 14C dating of foraminifera and mollusc shells in sediment borings from the southeastern Barents Sea, [approximately]1000 km from the western shelf edge and former ice-sheet margin. The deposition of ice-proximal glaciomarine sediments upon till began at or shortly before 12.7 ka, indicating that the ice-sheet retreat which most likely commenced along the western margin of the Barents Sea shelf at 15-14.5 ka - reached the southeastern part of the shelf in [less than or equal to]2 ka. The subsequent accumulation of glaciomarine sediments took place in two distinct pulses ([approximately]12.7-12.1 ka and [approximately]10.5-9.4 ka) separated by an interval of nondeposition lasting [greater than or equal to] 1.5 ka. The two pulses of sedimentation were coeval with periods of increasing surface-water and air temperatures in the northern Atlantic region and accelerated eustatic sea-level rise, suggesting that the remaining retreat of the Barents Shelf ice sheet was paced by these factors.

Radiocarbon-dated glacial-geologic evidence documents an abrupt advance of the northern margin of the Labrador sector of the Laurentide Ice Sheet during the last deglaciation. Ice-flow directional indicators, together with ice-marginal features found onshore and offshore, delimit an ice stream that advanced north-northeast >300 km, crossed the mouth of Hudson Strait and outer Frobisher Bay, and overran summits approximately 400 m above sea level on outer Hall Peninsula, southeast Baffin Island. Fifty-eight 14C ages, including 27 accelerator mass spectrometry ages on single shells, indicate that the advance was extremely rapid. The entire advance-retreat cycle took place in an approximately 300 yr (14C) interval, 9.9-9.6 ka. At its maximum extent, the ice stream supported a calving margin >200 km long terminating in open water approximately 500 m deep, implying a massive iceberg release. Marine evidence for the outflow is preserved along the Labrador Sea shelf as thick carbonate-rich glacial-marine drift but has not been recognized farther east in the North Atlantic. Either the discharge of icebergs was insufficient to produce a trans-North Atlantic, carbonate-rich (Heinrich) layer, or the icebergs tracked southward where they encountered warming sea-surface temperatures. Although the advance seems to have been coincident with a brief interval of minor regional cooling, it failed to trigger a significant climate perturbation, suggesting that the ocean-atmosphere system had attained its stable interglacial condition by that time.

The steep-sided valleys and overdeepened basins of alpine landscapes are well-known products of glaciation, yet relatively little is known about how the dynamics of ice flow and glacial erosion interact to give rise to such landforms. By linking a finite-element model for ice flow through a glacier cross section with an erosion model, it is possible to investigate the development of one of the most striking glacial landforms, the U-shaped valley. In addition to providing a detailed understanding of landform development, such modeling provides a way to test current understanding of the controls on glacial sliding and erosion. To simulate valley development, I first model flow through an initial glacier cross section and calculate the glaciological parameters that govern erosion. I then numerically simulate erosion to produce a modified transverse profile, for which a new flow field and erosion pattern are computed. A number of iterations permits examination of the progressive transformation of cross-section form, which can be compared with field data. Model predictions of the cross-section flow field are in close accord with data from the Athabasca Glacier and include marked lateral variations in sliding velocity. With an erosion law dependent on basal velocity, the model predicts the rapid transformation of a V-shaped cross section into a recognizably glacial form over a time period on the order of 10 to the fourth power yr and the eventual development of a steady-state, quasi-parabolic glacier cross section. Better agreement with empirical data from glaciated valleys is obtained by including temporal variations in ice discharge, in order to mimic the characteristics of 100,000-yr glacial cycles. The high-discharge phase dominates form development, and, at low discharges, cross-section form is essentially inherited from the central part of the form that developed during the preceding high-discharge phase.

The principal points of water input to a glacier are the bergschrund in cirques, and crevasse fields lower on the glacier. Crevasse fields commonly occur over convexities at the heads of overdeepening in glacier beds. The amplitude of subglacial water-pressure fluctuations is large just down-glacier from these points of water input. Erosion by quarrying is likely in such areas. Erosion is thus inferred to be localized on the headwalls of cirques and overdeepenings. In the case of overdeepenings, this leads to a positive feedback process in which a perturbation in the bed causes crevassing at the surface, resulting in erosional forces that accentuate the perturbation. When subglacial water flows up an adverse bed slope leading out of a cirque or overdeepening, much of the viscous energy dissipated is used to warm the water to keep it at the pressure melting temperature as the ice thins and pressure decreases. In such situations, subglacial conduits are maintained by high water pressures rather than by melting of conduit walls. In the limit, water pressures apparently become so high that water is forced out along the ice-bed interface and the conduits collapse. The products of erosion are then no longer flushed out, and a protective till layer accumulates. By limiting erosion on such adverse bed slopes, this till layer controls the geometry of these over-deepened basins.

A class-supported, massive diamicton to poorly sorted, massive gravel (Gm/Dcm) bed in the area around Dinosaur Provincial Park, southern Alberta, Canada, provides a means of identifying early deglacial flood courses. This is especially useful where flood and spillway courses have very subtle geomorphic definition and are difficult to map from aerial photographs. The Gm/Dcm bed conformably overlies a brown, late Wisconsinan till and rarely overlies bedrock and lacustrine sediments. The geographic distribution of the bed is restricted to flat to gently undulating prairie on either side of the Red Deer River where shallow floods and spillway melt waters were concentrated upon deglaciation. The bed has also been observed along the banks of the South Saskatchewan River at Medicine Hat, 90 km to the south, and is therefore regionally extensive. Because the Gm/Dcm bed has none of the attributes of the supraglacial sediment and possesses a clast fabric and sphericity very similar to those of underlying till, it is interpreted as a winnowed gravel/diamicton lag, which was produced by low-competence melt-water floods along shallow depressions in the prairie surface after the drainage of local proglacial lakes. After enough of the fine-grained matrix of the late Wisconsinan till was removed to create a lag

ABSTRACT Studies of modern glaciers have recently drawn attention to the importance of subglacial sediment deformation to glacier dynamics and processes. Inferences regarding the probable shear strength of this sediment imply that large clasts may in some cases sink to underlying rigid sediment, where abrasion by overlying deforming sediment could occur. This scenario provides a formative mechanism for striated clast pavements commonly described from the base of fine-grained massive diamictons associated with the late Pleistocene Laurentide ice sheet. Such a mechanism indicates that, at the time of formation of clast pavements, overlying diamictons associated with pavements had a low yield strength ([is less tha or equal to]0.5 kPa) and were deforming mechanically like a debris flow. Clast pavements may therefore be an important criterion for recognition of sediments deposited by subglacial deformation transport.

In 1899, T. C. Chamberlin proposed that the [CO.sub.2] content of the atmosphere decreased during times of enhanced continental erosion, ultimately resulting in glacial epochs. He ascribed the increase in the rate of chemical weathering (relative to the rate of supply of [CO.sub.2] from Earth's interior) to increased orogenic activity and globally higher average elevations, which promoted rapid chemical erosion of silicates. The oceanic record of strontium isotopes, preserved in marine sediment, supports his suggestion that glacial climates during the Phanerozoic are in part linked to increases in the rate of global chemical erosion relative to outgassing from Earth's interior. Further, the close correspondence of the major tectonic episodes of the Late Ordovician and Early Silurian, the Devonian, the Carboniferous and Permian, and the late Cenozoic to times of increased continental erosion and glaciation suggests that Chamberlin's hypothesis of the cause of glacial periods should be revived.