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On the west side of Lee Bay on the northeast coast of Stewart Island, ventifact cobbles of pyrite-coated granite occur on the beach near the high tide mark and appear to be derived from a sand-cemented gravel deposit that forms a low bank at the back of the beach. The pyrite coat (up to 1 mm thick) completely covers the granitic cobbles and is zoned, with an inner zone of fine-grained colloform pyrite and an outer framboidal zone. Framboidal pyrite is typically formed in anoxic sedimentary environments. Subrounded grains of hematite, ilmenite with hematite blebs, magnetite, feldspar, biotite, quartz and zircon are present in the outer framboidal zone, with some ilmenite and hematite grains being partially replaced by pyrite. The assemblage of ilmenite-hematite-magnetite-biotite-zircon is similar both in mineralogy and size range to that found in heavy mineral beach sands. Sulphur isotope values of the pyrite coat are consistent with formation of the pyrite by microbial sulphate reduction of seawater sulph...

Meltwater flows emanating from the Pyrenees during the Pleistocene constructed a braided outwash plain in the Ebro Basin and led to the karstification of the Neogene gypsum bedrock. Synsedimentary evaporite dissolution locally increased subsidence rates and generated dolines and collapses that enabled the accumulation and preservation of outwash gravels and associated windblown deposits that were protected from erosion by later meltwater flows. In these localized depocentres, maximum rates of wind deceleration resulted from airflow expansion, enabling the accumulation of cross-stratified sets of aeolian strata climbing at steep angles and thereby preserving up to 5 m thick sets. The outwash plain was characterized by longitudinal and transverse fluvial gravel bars, channels and windblown facies organized into aeolian sand sheets, transverse and complex aeolian dunes, and loess accumulations. Flat-lying aeolian deposits merge laterally to partly deformed aeolian deposits encased in dolines and collapses. Synsedimentary evaporite dissolution caused gravels and aeolian sand deposits to subside, such that formerly near-horizontal strata became inclined and generated multiple internal angular unconformities. During episodes when the wind was undersaturated with respect to its potential sand transporting capacity, deflation occurred over the outwash plain and coarse-grained lags with ventifacts developed. Subsequent high-energy flows episodically reached the aeolian dune field, leading to dune destruction and the generation of hyperconcentrated flow deposits composed in part of reworked aeolian sands. Lacustrine deposits in the distal part of the outwash plain preserve rhythmically laminated lutites and associated Gilbert-type gravel deltas, which developed when fluvial streams reached proglacial lakes. This study documents the first evidence of an extensive Pleistocene proglacial aeolian dune field located in the Ebro Basin (41˙50° N), south of what has hitherto been considered to be the southern boundary of Pleistocene aeolian deposits in Europe. A non-conventional mechanism (evaporite karst-related subsidence) for the preservation of aeolian sands in the stratigraphic record is proposed.

Surface textural features – pits, grooves, flutes, and scallops – of sand-blasted ventifacts have been attributed to various erosional mechanisms and to variations in material hardness. This study develops a numerical model of abrasion to examine feature formation by direct grain impacts. We demonstrate that abrasion by a continuous supply of impactors arriving from a single direction, each hitting the surface once, transforms existing cavities into transient features (similar to scallops, flutes, and pits), which shrink and eventually vanish. However, by incorporating additional impacts within the features from rebounding particles, persistent, continuously growing features emerge. We conclude that: (1) rebounding particles concentrate their effect within features, and therefore increase their size; (2) the development of features is sensitive to material type; and (3) the ultimate maximum feature scale is related to the size of the rock, and the size and velocity of impactors. Our model helps organize diverse ventifact field observations into an initial framework of pattern formation and confirms field and laboratory observations that the most common features are linear in wind-parallel directions.

The origin, formation and evolution of volcanic sands are less well known than the formation of the much more common quartz-rich sand sheets. Combining active volcanism and a cold climate, Iceland is covered for about 21% of its surface by sandy areas. The sands were analyzed in detail at two sites and results reveal their diverse origins. The first site is Dyngjusandur, located north of Vatnajokull, and the second site is the Lambahraun area, located south of Langjokull. At both sites, the sand origin is determined from field observations (wind directions from ventifacts), chemical and mineralogical analyses of rocks and sands. At Dyngjusandur, the sand is dominated by glass grains, a situation typical of sand plains in Iceland. Hyaloclastite ridges presently buried beneath Vatnajokull are the dominant source of the sand, and only large size plagioclase crystals (0.5 cm) in sands seem to be derived from the lava flows. Hyaloclastite ridges were crushed by glaciers and mechanically eroded sediments were washed out by melt-water onto flood plains. The sand chemical composition is spatially homogeneous and similar to the average composition of neighboring sub-aerial lava flows, reflecting efficient mixing of distinct sources below the glacier. The presence of sand north of Dyngjujokull can be taken as a way to explore the average chemical composition of non-exposed volcanic material beneath the glacier. In the case of Lambahraun, prevailing winds indicate several potential sources of sand at the north of the sand sheet. Comparison of chemical and mineralogical analyses of sands and rock samples helped to refine the exact origin. In contrast with the first site, the sand is dominated by crystals and is chemically consistent with a mixture of material derived from the lava flows of Eldborgir and Skersli shield volcanoes. Analysis of the contact between the lava flows and the glacier reveals that basaltic sand grains formed as the result of recent advances of the glacier abrading the rocks. The direct interaction of glacial and fluvio-glacial activity with basaltic plains appears to be necessary to produce a large amount of sands in a relatively short period of time (

Soil preservation from three glacial thermal regimes was examined in the Transantarctic Mountains (TAM) using the University of Wisconsin Antarctic Soils Database (http://nsidc.org/data/ggd221). Glacial thermal regimes included temperate (wet-based) glaciers from overriding of the TAM prior to c. 15Ma bp and subsequent polar (dry-based) glaciers. The glacial thermal regimes were distinguished from landform, sediment and erosional features. Buried soils were most common from deposition by dry-based glaciers (44 of 51 pedons). Several of these buried soils had a desert pavement intact with in situ ventifacts. Fifteen percent of the pedons contained recycled ventifacts in relict and buried soils that ranged from late Quaternary to Miocene in age, particularly in drift from dry-based glaciers (56 of 77 pedons). Overall 84% of the buried soils and 78% of the pedons with recycled ventifacts originated from dry-based glaciers. The proportion of soils with recycled clasts on a particular drift was greatest where the ratio of drift thickness to soil thickness (“recycling ratio”) was the least. These data illustrate the effectiveness of Antarctic dry-based glaciers in preserving underlying landforms and deposits, including soils. Moreover, the data imply that Antarctic glaciers have been recycling clasts for the past c. 15Ma. These findings have important implications in selecting surface boulders for cosmogenic dating.

The surface topology, surface area, and volume of rock simulant targets abraded in a boundary layer wind tunnel are quantified using results from a laser scanner with a spatial resolution of ~300 µm, an order of magnitude improvement over previous measurement techniques. Laplacian smoothing enables delineation of topography at even finer scale. The surface area to volume ratio generally increases at a rate greater than log (2/3), indicating roughening with time. Sub-millimeter- to millimeter-scale changes provide information on the relationships between natural abrasion and the resulting textures. Pit size frequency statistics reflect a balance between pit production from fresh impacts and net loss when two or more pits merge into a single pit. Most targets show a rollover in curves, whereby the number of pits smaller than a few hundred µm in size decrease with continuing abrasion whereas the abundance of larger pits increases. This is evidence that smaller pits merge into larger ones over time. There is a weak correspondence between elongation and diameter, with larger pits tending to have a more elliptical shape, especially among more abraded samples. For angled front faces and with increasing abrasion, pits tend to dig into the targets at an angle greater than that of the facet. Most facets have a greater fraction of elongated pits aligned downwind at the conclusion of abrasion compared to the start, with variable trends at intermediate stages. Profiles of layered targets show a progressive retreat of the front face, with angled targets developing stair-stepped topography, with 60° angles showing the greatest development and 30° the least. Natural abraded rocks in terrestrial desert environments and probably on Mars show many of the textural characteristics documented in the wind tunnel rock simulants. Correlation of these parameters promises to provide information on integrated abrasion duration, offering a new tool for constraining weathering history on Earth and Mars.

Strong westerly winds that emanate from San Gorgonio Pass, the lowest point between Palm Springs and Los Angeles, California, dominate aeolian transport in the Coachella Valley of the western Sonoran Desert. These winds deposit sand in coppice dunes that are critical habitat for several species, including the state and federally listed threatened species Uma inornata , a lizard. Although wind directions are generally defined in this valley, the wind field has complex interactions with local topography and becomes more variable with distance from the pass. Local, dominant wind directions are preserved by growth patterns of Larrea tridentata (creosote bush), a shrub characteristic of the hot North American deserts, and ventifacts. Exceptionally long-lived, Larrea has the potential to preserve wind direction over centuries to millennia, shaped by the abrasive pruning of windward branches and the persistent training of leeward branches. Wind direction preserved in Larrea individuals and clones was mapped at 192 locations. Compared with wind data from three weather stations, Larrea vectors effectively reflect annual prevailing winds. Ventifacts measured at 24 locations record winds 10° more westerly than Larrea and appear to reflect the direction of the most erosive winds. Based on detailed mapping of local wind directions as preserved in Larrea , only the northern half of the Mission-Morongo Creek floodplain is likely to supply sand to protected U. inornata habitat in the Willow Hole ecological reserve.

An area north of Lake Fryxell in the Taylor Valley, Antarctica, was surveyed to determine the frequency of occurrence of rocks that show strong evidence of wind abrasion and ventifaction, which is defined here as rocks having well-developed faceting, to define their relative abundance in this area of the ice-free McMurdo Dry Valley system and to provide an indication of the role of wind as a geomorphic agent in this area. The orientation of abrasion-caused features (facets, keels, and grooves) with respect to the present day wind regime is also described. Rocks were examined on five linear transects ranging from 300 to 510 m in length. A total of 1324 rocks were examined. On average, 60% of all rocks exhibited distinct wind abrasion features with polish being the most common feature and polished rocks were distributed equally between survey lines, suggesting abrasion was ubiquitous in the study area. Approximately 4% of the rocks had distinct facets and/or keels, and fine-grained ultramafic peridotite-type rocks produced the most finely-featured forms (i.e., sharp facet edges and keels). A larger percentage, ≈ 12.5%, had grooves. Grooves were typically associated with a tabular form of mafic diabase-type igneous rock. The distribution of faceted ventifacts and grooved rocks was not uniform for the five transects, suggesting that the distribution mechanism for the surface rocks and the source areas determined, to a large extent, what form of ventifact could be produced at a location. The orientation of the grooves and dip directions of the facets indicates the direction of the abrasive winds had a strong westerly component, which coincides with the modern wind regime of winter katabatic flows that move down valley toward the Ross Sea. The orientation of the facets and grooves suggests that the rate of formation of the ventifacts proceeds at a pace greater than other surficial processes (e.g., down-slope soil movement, cryoturbation), which should tend to remove trends in the facet and groove orientations, or that the down-slope movement of the surface is approximately perpendicular to the wind allowing preservation of the alignment.

Terrestrial ventifacts – rocks that have been abraded by windblown particles – are found in desert, periglacial, and coastal environments. On Mars, their abundance suggests that aeolian abrasion is one of the most significant erosional processes on the planet. There are several conflicting viewpoints concerning the efficacy of potential abrasive agents, principally sand and dust, and the relationships between wind direction and ventifact form. Our research, supported by a review of the literature, shows that sand, rather than dust or other materials, is the principle abrasive agent on Earth and Mars. Relative to dust, sand delivers about 1000× the energy onto rock surfaces on a per particle basis. Even multiple dust collisions will do little or no damage because the stress field from the impact is much smaller than the spacing of microflaws in the rock. The abrasion profiles of terrestrial ventifacts are consistent with a kinetic energy flux due to saltating sand, not airborne dust. Furthermore, Scanning Electron Microscope images reveal surfaces that are fractured and cleaved by sand grain impact. With respect to their distribution, ventifacts are found in regions that contain sand or did so in the past, but are not found where only dust activity occurs. Contrary to some published reports, our evidence from field studies, analytical models, and wind tunnel and other experiments indicates that windward, not leeward, abrasion is responsible for facet development and feature formation (pits, flutes, and grooves). Leeward abrasion is confined to fluvial conditions, in which the high viscosity and density of water are able to entrain sand-size material in vortices. Therefore, ventifacts and abraded terrain provide an unambiguous proxy for the direction of the highest velocity winds, and can be used to reconstruct palaeowind flow.

Frost-shattered bedrock and ventifacts interpreted to be abraded by drifting snow or ice particles occur frequently in the wooded areas of northernmost Sweden. Ice-wedge casts and periglacial involutions are encountered more sporadically. The phenomena cannot be explained by the present or the Holocene climate and demonstrably pre-date the last deglaciation. The relation to glacial and non-glacial stratigraphy, and to ice flow during the latest glacial stade, strongly suggests that the periglacial activity dates from the local Tarendo Interstadial, tentatively correlated with Odderade. The presence of the periglacial features implies not only that different morphological formations survived the erosive impact of the last continental ice sheet but also that in wide areas the very ground surface, including ventifacts in primary position, is the same today as during the interstadial. A similar situation appears to prevail in northernmost Dalecarlia and parts of Harjedalen and Jamtland in central Sweden.

Ventifacts (wind-abraded clasts or rock surfaces) are common features of many terrestrial hot and cold deserts, coastal, periglacial and mountain environments, as well as on Mars, and have a long history of investigation. This review paper discusses some of the main themes in terrestrial ventifact research including their formation and geomorphic controls. The varied morphological forms of ventifacts in terrestrial environments, and their environmental significance as indicators of past and present wind direction and sediment mobility, are critically evaluated. Future research directions are identified.

Ventifacts occur frequently in extensive areas in northern and central Sweden. They arose during severe periglacial conditions in the Weichselian Tarendo interstadial and their presence indicates that the latest ice sheet was incapable of reshaping earlier deposits or even significantly after the interstadial ground surface in these areas. Accordingly, the ventifacts not only witness to harsh climatic conditions during part of the Tarendo interstadial but also reflect the limited erosional capacity of the latest ice sheet. Their significance as stratigraphic markers and means in the search for pre-Late Weichselian organic sediments, serving as archives for past climate and environment, is emphasized.

A thin sequence of in situ fossiliferous gravels and silts overlain by a glacigenic bed is described from a temporary exposure in a working silica sand quarry. The sequence directly overlies unweathered Mercia Mudstone. Locally the bedrock surface forms a shallow palaeovalley some 15 m deep and almost 1 km wide. The sequence in question lies close to the valley axis. The silts, pollen, plant macrofossils, Mollusca, Coleoptera, and Ostracoda assemblages each suggest a similar environment of sedimentation: a shallow pool within a treeless open landscape. Much of the bedrock within the working quarry appears to be draped by the Oakwood till and this unit is co-extensive with the glacigenic sediments over the silts and gravels. In parts at least, the till has been subject to redeposition and its upper surface has scattered ventifacts. Above this wind-deflation horizon lies the Chelford Sands Formation, in the middle of which is the Chelford Interstadial stratotype, the Farm Wood member. The biota preserved in the gravels and silts suggest a stadial prior to the Early Devensian Chelford Interstadial. This is the first unambiguous stratigraphic evidence from Cheshire of a glacial event antedating the Chelford Interstadial.

Context. We present an investigation of the surface properties of areas on the nucleus of comet 67P/Churyumov-Gerasimenko. Aims: We aim to show that transport of material from one part of the cometary nucleus to another is a significant mechanism that influences the appearance of the nucleus and the surface thermal properties. Methods: We used data from the OSIRIS imaging system onboard the Rosetta spacecraft to identify surface features on the nucleus that can be produced by various transport mechanisms. We used simple calculations based on previous works to establish the plausibility of dust transport from one part of the nucleus to another. Results: We show by observation and modeling that "airfall" as a consequence of non-escaping large particles emitted from the neck region of the nucleus is a plausible explanation for the smooth thin deposits in the northern hemisphere of the nucleus. The consequences are also discussed. We also present observations of aeolian ripples and ventifacts. We show by numerical modeling that a type of saltation is plausible even under the rarified gas densities seen at the surface of the nucleus. However, interparticle cohesive forces present difficulties for this model, and an alternative mechanism for the initiation of reptation and creep may result from the airfall mechanism. The requirements on gas density and other parameters of this alternative make it a more attractive explanation for the observations. The uncertainties and implications are discussed.

There are sand dunes and stony desert surfaces (gobi) on the piedmont along the western side of the Puruogangri Icefield in central Qinghai–Tibet Plateau. The dunes formed on the gobi, probably after 16 ka. Their sediment derived from the underling till deflated by mid-latitude westerly winds and the Qiangtang high-pressure system. The gobi contains granitic moraine boulders, ventifacts of limestone and secondary CaCO3 precipitates at subsurface. Occasional humus layers are present within the sand dunes. A total of 13 humus layers were collected for radiocarbon dating and 20 samples for particle size analysis. The dune particle size distribution is generally negatively skewed. The radiocarbon ages range from 12,709 to 922 calendar years BP. It is likely that the humus layers correspond to warm times and can be correlated to the results from oxygen isotopic data in the ice cores from Guliya in the western Kunlun Mountains and Dunde in the Qilian Mountains. These results initially provide an outline of Lateglacial climate history for the ice field area.

A pavement of faceted pebbles of haematite ore has been discovered in Amazonia. It occurs on the top of a lateritic plateau in Carajas, beside lakes and marshes where the available palynological data indicate dry periods during the Pleniglacial. The pebbles have pyramidal tops and characteristic polished facets. They are interpreted as ventifacts additionally worn by fluvial action. The possible occurrence of ventifacts may represent the erosional manifestation, hitherto unrecognised, of the Ice Age aridity recorded in these Amazonian lacustrine sediments.

Field measurements have been made since 1951 on hundreds of ventifacts abraded by strong, southerly, katabatic winds that blow in winter and summer past Paulatuk, a small western Arctic coastal settlement. Sand is commonly entrained by the strongest winds in winter. The ventifacts, all glacial erratics deposited prior to 12 ka BP, have been gradually rotated by the southerly winds until the long axes of most ventifacts now trend approximately east–west, normal to the katabatic winds. In contrast, pebbles have a slightly preferred north–south orientation, parallel to the katabatic winds. The facets on sandstone and diabase ventifacts are almost planar, but are rounded on granites and hackled on limestones, reflecting the influence of both solution and abrasion. Abrasion is evident on the built structures in Paulatuk, but despite the over 50 years of observation, abrasion of the ventifacts has been virtually undetectable. The extremely slow abrasion rate has been estimated from: observations on two ventifacts from 1951 to 2003; photographic comparisons and observations of 60 ventifacts from 1968 to 2003; optical examination of 14 granite slabs, polished and unpolished, exposed to abrasion from 1967 to 1976; and comparisons of the windward and leeward sides of six large rock caches built with ventifacts probably long before 1900. If the present rates of abrasion are representative of Holocene conditions, ventifact formation has probably taken much of postglacial time. The increase in vegetation cover around many rocks between 1968 and 2003 suggests that the climate has changed in the last three decades.

A diagenetic silcrete horizon from c. 1 to >6 m thick is commonly present within fluvial quartz sandstones and pebble conglomerates of Eocene Hogburn Formation and early‐middle Miocene Dunstan Formation in Central Otago. The silcrete was formed by precipitation of crystalline quartz‐overgrowth cement from groundwater focused within permeable, moderately well sorted or well‐sorted fluvial sands and silts that were deposited on floodplains that broadened ahead of marine and lacustrine transgressions. The silcrete is hosted near the base of these sandstone and siltstone beds in both formations, and extends downward into the conglomeratic, incised‐channel strata that locally underlie them. Downward advance of the cementation front was inhibited by reduced porosity arising from the higher clay content of the basal sandy or conglomeratic beds. Late Miocene‐Recent uplift has exposed the highly resistant silcrete horizons at the range margins. In situ silcrete undergoes cantilevering and tensile failure ...

Ventifacted boulders are present within the intertidal zone of a mixed sand and boulder beach in Gweebarra Bay, northwest Ireland. The boulders show features typical of wind abrasion by sand including polished surfaces, pits and grooves. Orientation of ventifact keels was measured and direction of prevailing winds responsible for ventifaction was inferred from a sample of 50 boulders in each of two adjoining locations on the beach (30 m apart). The keel orientations and inferred wind direction are both strongly clustered but results from each location differ by 90° from one another, and neither corresponds closely to the present-day regional wind regime. Since wind flow patterns were not significantly different during the Little Ice Age, when the ventifacts were likely formed, the orientation of ventifact keels cannot be used uncritically, as in many studies, as a proxy record of prevailing wind direction. It is likely that ventifact development in Gweebarra Bay was controlled by sediment availability rather than by wind direction.

Examples of ventifacts and forms of unusual patterned ground that differ in situation, characteristics and/or origin to types reported previously are described from the Falkland Islands, South Atlantic. The ventifacts, represented by clasts projecting from an eroding tillite shore platform, have been grooved by the passage of windblown sand. The patterned ground forms comprise stone polygons on clay loam regolith, stone polygons on peat, peat-filled desiccation cracks on peat, and clusters of on-edge clasts (stone packings) on a gravel beach. Aeolian transport of particles is considered responsible for the infill of the polygonal cracks. The clusters of on-edge clasts are thought to have developed as a result of wave action, in particular backwash and/or undertow. Although these features occupy small surface areas they provide useful information about local geomorphic processes and testify to the marked diversity of such features in nature.

Wind tunnel studies are integrated with field observations to better understand the processes and rates of rock abrasion on Earth and Mars and how these factors affect ventifact morphology. The wind tunnel work consists of controlled experiments at terrestrial and Martian pressures in which known fluxes of sand are blown onto abradable targets of various geometric shapes. Mass loss and dimensional changes are measured and shape evolution observed as a function of total sand flux, wind speed, target shape, and target composition. To provide ground truth to these experiments, the same types of targets were placed in a field plot at a Mojave Desert ventifact locality for 6 months and measurements and observations like those in the wind tunnel were made. Weather data recorded by a co-located station provided wind speed and direction during this time. These data and results from the abraded field targets were compared to flute directions of local ventifacts. Initial results from this work are: (1) initial rock shape controls the rate of abrasion, with steeper faces abrading faster than shallower ones, (2) targets also abrade via slope retreat, with intermediate angled faces becoming shallower (flatter) at a greater rate than initially flat or steep faces, (3) the direction of maximum velocity winds exerts a greater control on ventifact flute orientations than the direction of average velocity winds, (4) irregular targets with pits or grooves abrade at greater rates than targets with smooth surfaces, with indentations generally enlarging and faces becoming rougher with time, and (5) there are many similarities between the experimental and terrestrial ventifacts, as well as rocks interpreted as ventifacts on Mars. The pitted and faceted appearance of many Martian rocks is easily attributable to aeolian abrasion. Many Martian rocks appear pitted or vesicular, characteristics which our laboratory experiments show enhance abrasion. Although measured Martian wind speeds are generally below those necessary to induce saltation, occasional gusts above threshold may be sufficient for some rock abrasion. Ventifact formation is potentially a common geomorphic process on Mars provided there are sufficient supplies of sand and high velocity winds needed for saltation.

The unusual location of ventifacts, on a boulder-built jetty at the mouth of the Siuslaw River, Oregon coast, western USA, allows ventifact age and wind abrasion rates to be estimated with some precision. The jetty was built mainly between 1892-1901 and extended throughout the twentieth century. Consideration of historical shoreline position and the history of jetty construction and repair suggests the ventifacts have formed since about 1930.Morphologically the ventifacts are aligned south-to-north reflecting winter winds and sediment transport from the adjacent beach. Wind-parallel grooves and ridges with sharp, sinuous crests are developed on inclined boulder surfaces on top of the jetty and reflect suspended sand transport in wind vortices: Deeply pitted surfaces on steep boulder surfaces nearest the beach reflect impact by saltating sand grains.Based on present wind regimes (1992-2000) from three regional weather stations, southerly winds above the sand transport threshold occur for 21.9-29.6 per cent of the time. Based on estimated depth of loss from boulder surfaces, wind abrasion rates are calculated to be on the order of 0.24-1.63 mm a(-1). This is the first well-constrained field estimate of ventifact age and ventifaction rate from a modern coastal environment. Copyright (C) 2003 John Wiley Sons, Ltd.

Late Pleistocene and Holocene landscape evolution around Lake Sulivan, Falkland Islands, South Atlantic, is assessed using morphological, stratigraphical, palaeoecological and 14C-dating evidence. Data indicate a complex landscape history involving frost-related activity, ventifact development, growth and degradation of peat terrain, expansion and contraction of lakes, and the development, stabilisation and subsequent reworking of alluvial and aeolian sand bodies. Late Pleistocene cold conditions favoured production of sandy diamicts on hillslopes, and ventifacts on outcrops and blockfield clasts. Lateglacial climatic amelioration was marked by early growth of peat (ca 16,500 cal. yr BP). Since then, peat accumulation, changes in lake areas and depths, and alluvial and/or aeolian sand deposition have been recurring and dominant features of landscape change which throughout the Late Pleistocene and early Holocene may have been a response to near-synchronous climatic fluctuations along the South America–Antarctica axis. Marked landscape changes around Lake Sulivan coincided with intervals of glacier readvance in South America and South Georgia at ca 3200–1900 and 700–100 cal. yr BP: with lower lake levels and aeolian sand transport from exposed lakebeds and shorelines implying greater aridity in the Falklands than at present.

The Quaternary succession in west Norfolk contains two previously unrecognizedaeolian sand units whose ages have been determined by the infra-red stimulated luminescence procedure. The ‘Leziate coversand’ dates from the Loch Lomond Stadial (Late Devensian); large numbers of well-formed, lithologically varied, gravel ventifacts are found within this material. The pre-Holocene age of a ventifact-rich, windborne sand at Blackborough End is evident (in spite of an unsuccessful attempt to obtain a luminescence date) from its association with a former ice wedge and with involutions; and an aeolian sand with ventifacts accumulated against the flank of a cryogenic mound which developed at Walton Common during the Loch Lomond Stadial. Renewed aeolian activity in the Holocene led to the deposition of the IRSL-dated ‘Ling Common drift sand’.

International audience; Ventifacts, rocks abraded by wind-borne particles, are found in Gale Crater, Mars. In the eastwarddrive from “Bradbury Landing” to “Rocknest,” they account for about half of the float and outcrop seen byCuriosity’s cameras. Many are faceted and exhibit abrasion textures found at a range of scales, fromsubmillimeter lineations to centimeter-scale facets, scallops, flutes, and grooves. The drive path geometryin the first 100 sols of the mission emphasized the identification of abrasion facets and textures formedby westerly flow. This upwind direction is inconsistent with predictions based on models and the orientationof regional dunes, suggesting that these ventifact features formed from very rare high-speed winds. Theabsence of active sand and evidence for deflation in the area indicates that most of the ventifacts are fossilfeatures experiencing little abrasion today.

Aeolian features observed on the surface of Mars provide insight into current, and potentially past, surface wind systems. In some cases the features are clearly transient and related to the lifting and settling of atmospheric dust. Other features, like dunefields, yardangs, and ventifacts, are more persistent and likely require significant time to form. In this study we analyze the observed directions of selected aeolian features with the aid of the Geophysical Fluid Dynamics Laboratory Mars general circulation model (GCM). Initially, we examine bright and dark streaks which have been observed to form in association with global dust storms. The ability to match these features with Mars GCM wind directions provides an important validation of the model. More important, we are able to define best fit seasons and local times for both types of features which provide the basis for extension and modification of the Veverka et al. [1981] model of bright and dark dust streak formation. In addition these best fit times correspond well with the dark streak “wind storm” model of Magalhaes and Young [1995]. The primary focus of this paper is to provide constraints on the range of mechanisms proposed to explain inconsistencies between current wind direction patterns and long-term wind indicators (for example, the misalignment of rock tail and ventifact orientations at the Mars Pathfinder landing site). Specifically, we assess whether changes in planetary obliquity, precession, or global dust opacity could significantly alter patterns of surface wind directions. In all cases we find the seasonal and annual average wind direction patterns to be highly invariable. While changes in the dust loading (hence the partitioning of solar absorption between the surface and atmosphere) and in the surface latitude of maximum solstitial insolation cause the vigor of the large-scale circulation to increase (especially the Hadley cell), the spatial patterns of the surface wind orientations remain essentially unchanged. In the case of perihelion during northern summer (opposite of the current perihelion position), the northern summer Hadley cell remains weaker than the southern summer cell despite the strengthened heating in the northern hemisphere. Taken together, these results cast significant doubt on orbital explanations for surface wind changes. It is thus suggested that significant changes in topography (e.g., Tharsis uplift, true polar wander) or climate (e.g., the existence of a significantly thicker atmosphere or an ocean at some point in the past) are more likely explanations for long-term wind indicators such as the ventifact orientations at the Mars Pathfinder landing site.

Wind‐etched rocks (ventifacts) at Gweebarra Bay, County Donegal, northwestern Ireland, are located

Surface features related to the wind are observed in the vicinity of the Mars Pathfinder (MPR landing site data from the lander and in data from orbit by the Viking Orbiter and Mars Global Surveyor missions. Features seen from the surface include wind tails associated with small rocks, barchanoid duneforms, ripplelike patterns, and ventifact flutes cut into some rocks. Features seen from orbit include wind tails associated with impact craters, ridges inferred to be duneforms, and modified crater rims interpreted to have been eroded and mantled by windblown material. The orientations of these features show two prevailing directions. One is inferred to represent winds from the northeast, which is consistent with strongest winds predicted by a general circulation model to occur during the Martian northern winter under current conditions. A second wind blowing from the ESE was responsible for modifying the crater rims and cutting some of the ventifacts. The two wind regimes could reflect a change in climate related to Mars' obliquity or some other, unknown factor. Regardless of the cause, the MPF area has been subjected to a complex pattern of winds and supply of small particles, and the original surface formed by sedimentary processes from Tiu and Ares Vallis flooding events has been modified by repeated burial and exhumation.

The observation that the Mars Pathfinder landing site looks very similar to its appearance after it was deposited by catastrophic floods around 1.8-3.5 Ga allows quantitative constraints to be placed on the rate of change of the site since that time. The abundance of erosional features such as an exhumed former soil horizon, sculpted wind tails, ripplelike and other tag deposits, and ventifacts (fluted and grooved rocks) all suggest the site has undergone net deflation or loss of 3-7 cm of material. The presence of barchan dunes and ventifacts argues for erosion by saltating crystalline sand-size particles entrained in the wind. Most ventifacts probably formed soon after the catastrophic flood, which likely introduced a large, fresh supply of sand-size particles distributed across the rocky plain. The strongest winds blew toward the northwest during this time, resulting in the sculpting of ventifacts, deflation of the surface, collections of dunes within Big Crater and other lows, and possibly preferentially eroding small crater rims. The predominant wind direction changed to blow toward the southwest, similar to today. These winds further deflated the surface, completed the deposition of sand-size material in dunes and ultimately trapped these dunes in lows. The erosional features observed by Pathfinder indicate extremely low long-term deflation rates of 0.01-0.04 nm/yr since the end of the Hesperian (1.8-3.5 Ga) similar to less precise rates of

The ice-free areas of the Northern Foothills, Antarctica, represent an ice-marginal, high-latitude periglacial environment. In addition to extreme cold and aridity, they are characterized by exceptionally strong winds. The effectiveness of traditional freeze–thaw and mass-wasting (solifluction) processes are limited because of the lack of moisture and the shallow active layer. Mass wasting mainly occurs through in situ rock disintegration and associated gravity processes. Bare rectilinear bedrock slopes are the result and reflect a balance between debris production and debris removal. The most active landscape-modifying processes appear to be (1) wind erosion and (2) taffoni and honeycomb weathering. Ventifacts are used to suggest a very tentative rate of wind abrasion of approximately 0.10–0.30 cm per 1000 years. Rates of taffoni and honeycomb weathering appear to be an order of magnitude greater, estimated to be between 2–3 and 8–10 cm per 1000 years. Copyright © 1999 John Wiley & Sons, Ltd. Les regions libres de glace des Northern Foothills, dans l'Antarctique, representent un environment periglaciaire de haute latitude et de marge glaciaire. En plus d'un froid et d'une aridite extremes, cette region est caracterisee par des vents exceptionnellement violents. Les processus traditionnels de gel-degel et de mouvements de masse (solifluxion) sont limites a cause de l'absence d'humidite et du peu d'epaisseur de la couche active. Des mouvements de masse se produisent principalement a l'occasion de la desagregation de la roche en place et des processus de gravite associes. Des pentes rectilineaires de roche en place refletent un equilibre entre la production de debris et leur transport. Les processus les plus actifs modifiant le paysage semblent etre (1) l'erosion eolienne et (2) l'alteration creant des taffonis et des nids d'abeilles. Les cailloux eolises suggerent une vitesse d'abrasion eolienne d'approximativement 0.10–0.30 cm par 1000 ans. Les vitesses de formation des taffonis et des nids d'abeille semblent etre un ordre de grandeur plus grand, et sont estimees entre 2–3 et 8–10 cm par 1000 ans. Copyright © 1999 John Wiley & Sons, Ltd.

Toroidal-shaped gold particles, extracted from a sample of the Basal reef orebody in the Welkom gold field that contained crossbedded gold, have been recognized in thin section, demonstrating that the form in question is not an artifact of extraction but is a genuine detrital form. Similar forms have been recovered in great abundance from an eolian paleosurface in Yakutia and from a recent storm-beach placer in New Zealand, indicating the likelihood that eolian processes produced these forms. This has been confirmed by means of wind-tunnel experiments. Toroids in various states of preservation have been extracted from a number of other Witwatersrand paleoplacers where there is an association with ventifacts, indicating that wind deflation might have played a role in concentrating gold from placer-sand deposits.

About half of the rocks at the Mars Pathfinder Ares Vallis landing site appear to be ventifacts, rocks abraded by windborne particles. Comparable resolution images taken by the Imager for Mars Pathfinder (IMP) camera and the Viking landers show that ventifacts are more abundant at the Pathfinder site. The ventifacts occur in several forms, including rocks with faceted edges, finger-like projections, elongated pits, flutes, grooves, and possible rills. The trends of elongated pits, flutes, grooves, and rills cluster at ∼280–330° clockwise from north and generally dip 10–30° away from their trend direction. These orientations are indicative of southeast to northwest winds and differ from the trend of wind tails at the landing site, the direction of local wind streaks, and predictions of the Global Circulation Model, all of which indicate northeast to southwest winds. The disparity between these data sets strongly suggests that local circulation patterns have changed since the abrasion of the ventifacted rocks. The greater number of ventifacts at the Pathfinder site compared to either of the Viking sites is most easily explained as being due to a larger supply of abrading particles, composed of either sand-sized grains or indurated dust aggregates, and higher surface roughness, which should increase the momentum of saltating grains. The Pathfinder ventifacts may have formed shortly after the deposition of outflow channel sediments nearly 2 Gry ago, when a large local supply of abrading particles should have been abundant and atmospheric conditions may have been more conducive to rock abrasion from saltating grains. Based on how ventifacts form on Earth, the several ventifact forms seen at the Pathfinder site and their presence on some rocks but not on others are probably due to local airflow conditions, original rock shape, exposure duration, rock movement, and to a lesser extent, rock lithology. The abundance of ventifacts at the Pathfinder site, together with other evidence of weathering, indicates that unaltered rock surfaces are rare on Mars.

The Mars Pathfinder (MPF) spacecraft landed on relatively young (late Hesperian-early Amazonian; 3.1–0.7 Ga) plains in Chryse Planitia near the mouth of Ares Vallis. Images returned from the spacecraft reveal a complex landscape of ridges and troughs, large hills and crater rims, rocks and boulders of various sizes and shapes, and surficial deposits, indicating a complex, multistage geologic history of the landing site. After the deposition of one or more bedrock units, depositional and erosional fluvial processes shaped much of the present landscape. Multiple erosional events are inferred on the basis of observations of numerous channels, different orientations of many streamlined tails from their associated knobs and hills, and superposition of lineations and streamlines. Medium- and small-scale features, interpreted to be related to late-stage drainage of floodwaters, are recognized in several areas at the landing site. Streamlined knobs and hills seen in Viking orbiter images support this inference, as they seem to be complex forms, partly erosional and partly depositional, and may also indicate a series of scouring and depositional events that, in some cases, further eroded or partially buried these landforms. Although features such as these are cited as evidence for catastrophic flooding at Ares Vallis, some of these features may also be ascribed to alternative primary or secondary depositional processes, such as glacial or mass-wasting processes. Close inspection of the landing site reveals rocks that are interpreted to be volcanic in origin and others that may be conglomeratic. If such sedimentary rocks are confirmed, fluvial processes have had a greater significance on Mars than previously thought. For the last several hundred million to few billion years, eolian processes have been dominant. Dunes and dune-like features, ventifacts, and deflation and exhumation features around several rocks probably are the most recent landforms. The relatively pristine nature of the overall landscape at the MPF site suggests weathering and erosion processes on Mars are exceptionally slow.

The Mars Pathfinder landing site contains abundant features attributed to aeolian, or wind, processes. These include wind tails, drift deposits, duneforms of various types, ripplelike features, and ventifacts (the first clearly seen on Mars). Many of these features are consistant with formation involving sand-size particles. Although some features, such as dunes, could develop from saltating sand-size aggregates of finer grains, the discovery of ventifact flutes cut in rocks strongly suggests that at least some of the grains are crystalline, rather than aggregates. Excluding the ventifacts, the orientations of the wind-related features correlate well with the orientations of bright wind steaks seen on Viking Orbiter images in the general area. They also correlate with wind direction predictions from the NASA-Ames General Circulation Model (GCM) which show that the strongest winds in the area occur in the northern hemisphere winter and are directed toward 209°.

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Studies of the auriferous Witwatersrand placers and associated rocks have revealed that certain palaeo-environments, especially braided fluvial, are particularly well represented in this part of the rock record. However, there is a paucity of lithofacies indicating certain other palaeo-environments. Possible reasons for their scarceness or absence are suggested in this paper. It is generally assumed that pre-vegetational fluvial systems would have been characteristically braided, because of the absence of land plants necessary to stabilise river banks. A question arising is what kind of downstream changes such pre-vegetational braided streams underwent. A recent study of a braided stream placer revealed that the depositing system retained its braided character right down to the palaeo-shoreline. However, gravel did not reach the palaeo-beach. As with many modern examples, beach conglomerates are rare in the Witwatersrand rocks. The paucity of conglomeratic beach placers is ascribed to the low probability of gravel being transported across coastal plains, because of the relatively low slopes or depths of rivers here. The Witwatersrand fluvial channels are generally considered to have had high width-to-depth ratios, because of the absence of land vegetation to stabilise channel banks. However, two examples of deep, relatively narrow scour features, with predominantly fine-grained fills, occur near the base of the Central Rand Group. The low width-to-depth ratios of these scour features, which probably represent palaeo-valleys, are ascribed to severe incision during a rapid sea-level fall. The auriferous fluvial systems of the Witwatersrand are generally considered to have been entirely braided, due to the lack of bank stability. However, the fluvial B placer of the Welkom goldfield is confined to discrete channels. Their banks are thought to have been stable, due to the cohesive nature of the lutite into which the gravelly streams incised. In addition, braiding sensu stricto was probably inhibited by initial incision and a low sediment supply. No deposits of specifically deltaic sub-environments have been found in the Central Rand Group. Their absence is attributed to the following factors: (a) the paucity (absence?) of well-defined palaeo-river mouths; (b) low concentrations of suspended sediment; (c) intermittent sediment supply to the palaeo-coastline; and (d) reworking by tidal and longshore currents. Alluvial-fan deposits are also apparently absent in the Witwatersrand rocks. The absence of fan deposits is attributed to the poor potential for development and preservation of fan deposits in the compressive tectonic setting proposed for the Witwatersrand Basin. Although ventifacts have been found in the Witwatersrand rocks, no aeolianites have been reported. Their apparent absence is probably due to (a) reworking in a predominantly humid climate, and (b) transport of sand by dominant winds to areas unfavourable for the preservation of aeolianites.

Fossil windpolished stones and boulders in in situ position have been found in various parts of Denmark. Typical wind abrasion microforms and mesoforms, such as facets and flutes, and the overall distribution of windpolished surfaces, have been studied on 31 in situ boulders at 20 locations in Denmark. Based on these investigations it is concluded that the direction of the former abrading winds was mainly from the ESE, from the sector 105°±20°. The ESE winds dominated during the deglaciation period, most probably in the period from 22 to 17 ka BP, when first katabatic and later zonal winds from the shrinking Weichselian Ice Sheet abraded surfaces of boulders in the periglacial environment outside the ice margin. During the subsequent Holocene period westerly winds dominated in a temperate climate, as is the case today. Best preserved windpolish is primarily found on boulders in boulder concentrations, whereas on the large erratic boulders the windpolish is often almost totally removed, owing to the many visitors climbing the boulders as well as weathering. Even on boulders transgressed by the sea for some time, windpolish has proved able to survive in low-energy littoral environments. This yields opportunities for reconstructing palaeo-wind directions in areas that are or have earlier been transgressed by the sea. Comparing the presented Danish palaeo-wind evidence with neighbouring parts of north-west Europe shows a regional variation in the wind regime, controlled by the extension of the Weichselian Ice Sheet. Dominating easterly palaeo-winds prevailed as katabatic and/or zonal winds in a narrow zone in front of the wind-controlling ice sheet during its maximum extension, and in some areas further to the east during the subsequent deglaciation. © 1998 John Wiley & Sons, Ltd. Des pierres et des blocs qui ont ete polis par le vent et qui sont restes dans la meme position depuis leur faconnement, ont ete trouves en differents endroits du Danemark. Des formes d'abrasion eolienne typiques de petite et moyenne dimension (comme des facettes et des rainures), ainsi que la distribution d'ensemble des surfaces polies ont ete etudiees sur 31 gros blocs de 20 sites du Danemark. Sur la base de ces recherches, il est demontre que les vents qui ont faconne les blocs sont venus principalement de l'ESE, depuis le secteur 150°, ±20°. Les vents venus de cette direction ont ete dominants pendant la periode de deglaciation, tres probablement entre 22 et 17 ka BP. A ce moment, les vents, d'abord catabatiques et ensuite zonaux, venus de la calotte glaciaire en retrait, ont faconne les surfaces des blocs dans un environnement periglaciaire typique. Pendant la periode holocene ulterieure, des vents d'ouest ont domine dans un climat tempere comme cela se passe aujourd'hui. Les blocs polis les mieux conserves ont ete avant tout trouves dans des accumulations de blocs, tandis que, sur les plus grands erratiques, le poli eolien est presque totalement disparu par alteration et par l'action des nombreux visiteurs qui escaladent les blocs. Meme sur des blocs qui ont ete sous le niveau de la mer pendant quelques temps, le poli eolien a resiste pour autant qu'ils aient ete dans un environnement littoral de faible energie. Ceci donne la possibilite de retrouver des directions anciennes de vents dans des regions qui sont, ou qui ont ete recouvertes par la mer. En comparant les directions des vents fossiles reconnus au Danemark avec celles retrouvees dans les parties voisines du nord-ouest de l'Europe, il apparait une variation regionale dans le regime des vents, en relation avec l'extension de la calotte glaciaire vistulienne. Des vents d'est ont ete dominants comme vents catabatiques ou zonaux dans une zone etroite pendant l'extension maximale de la calotte glaciaire; ils se sont exerces plus a l'est pendant la deglaciation qui a suivi. © 1998 John Wiley & Sons, Ltd.

Ventifacted limestone clasts and ventifacted outcrops of both limestone and quartz-dolerite are present on the coast of Northumberland at Holy Island and Budle Point respectively. Clasts display well-developed facets while outcrops are grooved, fluted, facetted and polished. Facet and groove orientations suggest development was in association with westerly and northwesterly winds. The abrasive agent is most likely to have been sand. Age and length of time over which ventifact formation occurred is not known with certainty. Outcrops may have gained their features over the last 15–13 ka, following removal of glacier ice, under both periglacial and temperate climatic conditions; the clasts are probably Holocene storm beach deposits and facet development may relate to sand movements linked to aeolian dune construction. At both locations, present-day sand movements may be sufficient to maintain ventifact sculpture.

Cold-climate environments include periglacial environments where the mean annual air temperature is less than +3 °C and seasonally cold environments where the coldest mean monthly temperatures are below −3 °C. In these environments, temperature and moisture conditions significantly affect the efficacy of sediment transport by wind. Specific wind conditions operating in cold-climate environments that promote aeolian processes include frontal winds, pressure-gradient winds, katabatic winds, and foehn winds, as well as large-scale pressure gradient winds associated with persistent stationary high-pressure cells centered over ice sheets. Sediment source supply is a limiting factor in present-day cold-climate aeolian environments, particularly within formerly glaciated regions of North America and Europe. In addition, in cold-climate environments, even though the wind speed may be high, entrainment and transport of sediment may not occur because surface soil moisture and phase-state conditions, vegetation cover, or other factors including organic matter content or soluble salts limit the availability of sediment. Aeolian processes and features within cold-climate environments include deflation, loess and dust, sand dunes and sand sheets, as well as niveo-aeolian processes, sand wedges, ventifacts and oriented lakes. Four major settings where cold-climate aeolian processes occur include cold-polar environments, glacially proximal environments, cold high-altitude environments, and seasonally-cold environments. In addition, although contemporary cold-climate aeolian environments are largely restricted in extent, even at high latitudes, relict cold-climate aeolian environments were considerably more extensive during full glacial and early Holocene times.

An abraded landscape forms predominantly by the mechanical wear of coherent material by saltating sand grains, with deflation removing particles and soils by wind shear. Abraded systems range from small wind-scoured areas to the vast landscape assemblages of the Sahara Desert and the planet Mars. Landforms include ventifacts, yardangs, deflation basins, and inverted terrain. Wind-abraded landscapes require strong and generally sustained winds, abrasive sediment, and sparse vegetation. These conditions occur in desert, cold (periglacial and paraglacial), and coastal environments, with the most extensive features on Earth formed in the hyperarid environments of Asia and Africa. Many of the same landforms occur on Mars.

International audience; Ventifacts (wind-worn stones) are typical of terrestrial environments remained very long without any vegetation, under hot or cold climates. Therefore, within the sedimentary record, they can allow recognizing desert conditions, even where no aeolian dune deposits are preserved. There seems that, in the recent literature, pebbles and cobbles from various palaeoenvironments were mistaken for ventifacts. This may partly be explained by the scatter and relative scarcity of illustrations to which refer. The aim of this paper is to help recognizing ventifacts in the sedimentary record, based on a critical review of the diagnostic properties generally used, and on new studies permitting to suggest additional criteria. After an evaluation of the sedimentary contexts favourable to preserving ventifacts, the distinctive characters that could be seen on each one are treated in order of increasing alteration of the original appearance: surface features, medium-scale features (new types of pit especially), and general shape. Finally, the problem of distinguishing between ventifacts and aquafacts is approached.

Over the last half century, spacecraft visits to the many worlds in our Solar System have revealed that the surfaces of no fewer than four planetary bodies are subject to aeolian processes. These worlds beyond Earth include the planets Venus and Mars, as well as a moon of Saturn, Titan. Each body shows the influence of the wind in a unique way, with our understanding strongly controlled by the quantity and type of data returned from spacecraft investigations. Of these worlds, the best studied is Mars, which is prone to dust storms and is freckled with bedforms, yardangs, and wind streaks. Venus is spanned by thousands of wind streaks, but bedforms and yardangs are rare (or unresolved by available data). Now known to be the sandiest world in the Solar System, Titan's equator is belted by vast sand seas made of hydrocarbon grains. Although conditions vary widely across the Solar System, the depositional and erosional processes acting on these worlds are in many ways similar to those observed on Earth. As a result, concepts and methods developed for studying terrestrial aeolian landforms can be applied to their planetary counterparts. The factors controlling dune field sediment state are the same (sediment supply, availability, and wind transport capacity), although they are commonly controlled by very different processes on other worlds. Emergent structures (i.e., bedforms) are self-organized; thus, they are controlled by dune-scale processes regardless of where they form, so that extraterrestrial dune field patterns may be analyzed with the same techniques used on Earth (e.g., pattern analysis, gross bedform-normal transport). Scaling relationships for elementary bedforms have been developed, which correlate well with bedforms formed under varying conditions. However, these relations do not appear to hold everywhere (particularly on Venus and Titan); this may be a result of low resolution data that cannot resolve elementary features of the predicted size. Although not well observed on Venus or Titan, deflation and abrasion are active processes on Mars, scouring dusty surfaces and eroding materials into yardangs and ventifacts. However, it is likely that the timescales of erosion are much longer on Mars than on Earth. Continuing studies show that aeolian landscapes beyond Earth can appear at once hauntingly familiar and exotically alien. Although there are many fundamental unanswered questions about aeolian processes on Mars, Venus, and Titan, it is clear that this juxtaposition occurs because the processes that produce aeolian landscapes on Earth also operate elsewhere, but they do so under vastly different conditions and over very different time- and length-scales.

Periglacial environments were dominated by aeolian processes periodically in the past but under present-day periglacial conditions, although contributing to local landscape development, are not as widespread. Wind speed, duration, efficiency in sediment transport, and sublimation mean that aeolian activity can take place even where substrates have high moisture contents or are frozen. Active layer processes and the hydrological regimes of rivers can act to replenish sediment availability. As a result, aeolian processes can lead, in addition to landscape elements common to other aeolian environments, to some more distinct to periglacial environments (e.g., elongated thaw lakes, glacier nourishment, and niveo-aeolian deposits).

Aeolian abrasion is the process whereby rocks and landforms are eroded by saltating particles, contributing to the formation of ventifacts, yardangs, deflation basins, and inverted relief. Sand is the predominant or sole abrasive agent. Abrasion is sensitive to the highest velocity winds in a region, which controls the directionality of ventifact features. As high-velocity winds are infrequent, many decades, centuries, or even millennia may be required for ventifact formation. Ventifacts, yardangs, and inverted relief are also common on Mars, and although abrasion is infrequent, the process does not require winds or atmospheric densities different than those found today.

Exhumed sand wedges and ventifacts in the upper Churchill watershed of north-western Saskatchewan indicate that aeolian and periglacial processes were dominant from c. 11 to c 10.5 ka BP. Nineteen sand wedges were identified that display vertical foliation, form polygonal nets, and penetrate either till, glaciolacustrine sediment or glaciofluvial diapiric sand. The sand wedges, and sand lenses within the till and glaciolacustrine sediments, indicate permafrost aggradation after glaciation and lake drainage. Ventifacts that overlie the sand wedges have preferred keel orientations that correlate with palaeo-katabatic wind directions interpreted from stabilized parabolic dune orientations. Rat-tail lineaments on faceted surfaces of coarse-grained, granitic ventifacts also indicate palaeo-wind flow direction. Strong and weakly developed lineaments on the same einkanter are explained by opposing flow from prevailing winds, katabatic winds and anticyclonic air flow from the adjacent Laurentide Ice Sheet, or by flow separation at the ventifact keel under unidirectional air flow. Two geomorphic surfaces are identified: (1) an older surface with sand wedges and weakly developed ventifacts indicative of permafrost and brief aeolian activity; and (2) a lower and younger segment of the former Lake Agassiz basin, without permafrost indicators but with well-developed ventifacts. The upper surface, developed in association with the Beaver River Moraine and glacial Lake Meadow-McMurray, is 11–10.5 ka BP in age, synchronous with the Younger Dryas cold period. The younger, lower surface was last modified beginning at 9.9 ka BP, when the north-west outlet of Lake Agassiz opened, was susceptible to greater aeolian reworking than the upper surface, and postdates the Younger Dryas. © 1996 John Wiley & Sons, Ltd.

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.