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Yardangs in the semiarid central sector of the Ebro Depresion (NE Spain)
Abstract
This work reveals the existence of yardangs in the central sector of the semiarid Ebro Depression of Spain. Almost all the documented yardang fields are located in extreme arid environments. The yardangs are developed in horizontal tertiary gypsum and limestone. More than 100 closed depressions (solution dolines) have been formed in the soluble sediments of this area, some of them hosting saline lakes or playas. All the yardangs occur on the leeward margin of the larger playas and their mean orientation (N122E) coincides with the prevalent direction of the strong local wind called Cierzo. Two main types of yardang have been identified; 44 rock yardangs formed in the Miocene bedrock and six yardangs developed in unconsolidated lacustrine deposits. Nebkha dunes have been recognized and yardang-like morphologies in building rubble accumulations on the floor of a playa (<100 years old). The generation of yardangs in this semiarid area is related to the presence of playas, which constitute the source of abrading particles during dry periods. At the present time, the yardangs developed in the lacustrine terraces, nebkhas and rubble accumulations are active landforms, whereas the rock yardangs are considered to be inactive.
... Although the final sculpting of yardangs is aeolian, their initial development depends on fluvial and slope processes (Goudie, 2007;Gutiérrez-Elorza et al., 2002;Hu et al., 2017). The morphology and orientation of the structures is dependent on the prevailing wind directions and the mantling bedrock (e.g., Pelletier et al., 2018). ...
... Shallow fluvial incisions are believed to enhance the susceptibility of landscapes to aeolian abrasion and therefore mark an essential step in the process chain of yardang formation (Al-Dousari et al., 2009;Dong et al., 2012;Laity, 2009). This process chain is therefore determined by the preexisting relief, prevailing unidirectional wind directions, the erodibility of the exposed rock or sediment, as well as sufficient amount of erosive detrital material (Blackwelder, 1934;Goudie, 2007;Gutiérrez-Elorza et al., 2002;McCauley et al., 1977). The latter factor can also be limiting to yardang formation since a surplus of deflated sediment favours depositional aeolian landforms (Al-Dousari et al., 2009). ...
... Therefore, yardangs are mainly found in (peri-)desert regions (Al-Dousari et al., 2009;Dong et al., 2012;Ghodsi, 2017;Hu et al., 2017;Laity, 2009;Liang et al., 2019;McCauley et al., 1977;Ward and Greeley, 1984). Within the last two decades, however, yardang systems have also been identified in semi-arid (Gutiérrez-Elorza et al., 2002) and even in nowadays temperate-continental areas (Sebe et al., 2011). Nonetheless, aridity is required for yardang formation. ...
The foreland of the Russian Altai is dominated by the vast Ob loess plateau. The flat landscape exhibits striking linear features, partially more than 100 km in length and tens of km wide. The bottoms of these features are covered by forested dunes, whereas the loess ridges in between are intensively cultivated. To the north, the land cover changes due to gradual transition from the steppe towards the Siberian taiga. The genesis of these prominent features was debated within the last decades. Possible explanations cover tectonic lineaments, fluvial erosion, and landforms caused by outbursts of catastrophic floods from the Altai Mountains. Here, we present geomorphological evidence for the aeolian origin of these features based on field observations and geodata. These large lineaments do not show characteristic features of fluvial valleys, since the shape of the lineaments is too straight and does not show braided river characteristics as, e.g., the Ob or the Irtysh valley. The sheer size of these features also does not support the hypothesis of tectonic activity or a catastrophic flood since events like this would be imprinted in other environmental archives of the region. We show that these linear landforms show remarkable similarities with Pleistocene mega yardang systems throughout the world. These systems can usually be found in arid to hyper-arid environments, but were also described in, e.g., mid-latitude regions. We hypothesis that the Pleistocene glaciations of the Altai Mountains enhanced the strength and the influence of the westerlies in the Altai forelands. Therefore, we propose an erosive-aeolian origin of these remarkable landforms.
... Récemment, 175 dépressions endoréiques formées par un vent de nord-ouest localement appelé Tramontane ont été répertoriées et étudiées en détail dans le Languedoc-Roussillon (David & Carozza, 2013 ;Carozza et al., 2016). Des dépressions endoréiques ont également été identifiées au sud des Pyrénées dans les formations gypseuses de la vallée de l'Ebre où elles sont nommées localement « playas » (Gutiérrez-Elorza et al., 2002 ;Gutiérrez et al., 2013). ...
... In the lower Rhône Valley, their length exhibits strong variability and ranges between 61 to 1640 m (mean = 144 m), and their width ranges between 28 and 638 m (mean = 118 m). The average length to width (L:W) ratio is 4:1 (Fig. 4C) in agreement with the wind tunnel experiments of Ward and Greeley (1984) and the measured values for Pleistocene yardangs in Spain (Gutiérrez-Elorza et al., 2002). ...
... The spatial association between yardangs and pans indicates a genetic relationship between the landforms. Such an association is commonly observed in arid or semiarid environments (e.g., Gutiérrez-Elorza et al., 2002;Goudie, 2013). In the study area, wind erosion is assumed to be predominant in the formation of pans by most authors (Ambert, 1973(Ambert, , 2013Ambert and Clauzon, 1992;Carozza et al., 2016). ...
Depuis presque un siècle, d’importants dépôts éoliens pléistocènes ont été reconnus dans la branche méridionale du rift cénozoïque ouest-européen, i.e. la basse vallée du Rhône et celles de ses affluents alpins. Ces lœss, déconnectés des deux principaux systèmes éoliens européens (la ceinture lœssique nord-européenne et le bassin du Danube), constituent un enregistrement clé pour documenter l’évolution des paléoenvironnements glaciaires péri-méditerranéens. L’étude de leur répartition spatiale couplée à une analyse sédimentologique et géochimique a montré que leurs principales caractéristiques (mode grossier autour de 60 µm, distribution granulométrique polymodale, taux de carbonate élevé, épaisseur localement importante (> 5 m), distribution spatiale discontinue et bioturbation abondante) peuvent être expliquées par la persistance d’un couvert végétal arbustif dans un contexte climatique moins rigoureux que celui des régions nordiques, permettant la capture simultanée des particules transportées par saltation et par suspension depuis les sources alluviales. À l’échelle européenne, la composition géochimique des lœss varie d’une région à l’autre mais garde une certaine homogénéité au sein d’un même bassin versant. Nos résultats suggèrent que cette variabilité est principalement contrôlée par la lithologie des zones englacées. Une analyse multi-proxy et chronostratigraphique à haute résolution a été réalisée sur deux séquences de lœss-paléosols : la séquence de Collias (~ 8 m) qui couvre la quasi-totalité du dernier cycle glaciaire et celle de Lautagne (~ 4 m) qui fournit un enregistrement détaillé du Pléniglaciaire supérieur. À l’échelle régionale, la principale période de sédimentation éolienne a été datée entre 37,5 ka et 12 ka avec un maximum de sédimentation entre 26 ka et 25 ka, synchrone de l’avancée maximale de la calotte alpine. Ce résultat suggère que les fluctuations des glaciers ont été le principal moteur de l’accumulation lœssique en modulant la production de particules susceptibles d’être transportées par la déflation.
... Evolution of Earth yardangs plays a key role in understanding geomorphological processes on Earth (Goudie, 2008;Pullen et al., 2017) and Mars (de Silva et al., 2010;Xiao et al., 2017;Wang et al., 2018a), as well as global climatic changes (Kapp et al., 2011;Pullen et al., 2017), and dust transportation (Washington et al., 2006;Bristow et al., 2009;Pullen et al., 2017). Studies of yardang evolution have been conducted in Asia (Halimov and Fezer, 1989;Xia, 1987;Zheng et al., 2002;Al-Dousari et al., 2009;Dong et al., 2012;Wang et al., 2018a), Africa (Brookes, 2001), Europe (Gutiérrez-Elorza et al., 2002;Sebe et al., 2011), Northern America (Ward and Greeley, 1984;Clarke et al., 1996;Whitney et al., 2015;Pelletier et al., 2018), Southern America (Inbar and Risso, 2001), and even around the world (Goudie, 2007;Laity, 2011). These studies expounded yardang landform in terms of origins, morphologic changes and controlling factors, and obtained various achievements: (1) different influence factors have been examined (e.g., Ward and Greeley, 1984;Al-Dousari et al., 2009;Pelletier et al., 2018) and summarized (Laity, 2011); (2) several evolutionary patterns have been proposed based on local conditions (Halimov and Fezer, 1989;Brookes, 2001;Dong et al., 2012); ...
... Locations of 2, 11, 13 and 15 are related to the yardang records in Fig. 5A . Yardangs in Um Al-Rimam Depressions, Kuwait (square 'g' in Fig Fig. 2A, average annual rainfall of 400 mm/yr and average wind speed of 4.5 m/s derived from weather station 5 km to the north of the study area) are mainly developed in horizontal tertiary gypsum and limestone (Gutiérrez-Elorza et al., 2002). N100 closed depressions in Ebro area were caused by the combination of dissolution and subsidence by groundwater discharge and wind deflation during dry periods (Gutiérrez et al., 2013) and some of them host saline lakes or playas now (Gutiérrez-Elorza et al., 2002). ...
... Yardangs in Um Al-Rimam Depressions, Kuwait (square 'g' in Fig Fig. 2A, average annual rainfall of 400 mm/yr and average wind speed of 4.5 m/s derived from weather station 5 km to the north of the study area) are mainly developed in horizontal tertiary gypsum and limestone (Gutiérrez-Elorza et al., 2002). N100 closed depressions in Ebro area were caused by the combination of dissolution and subsidence by groundwater discharge and wind deflation during dry periods (Gutiérrez et al., 2013) and some of them host saline lakes or playas now (Gutiérrez-Elorza et al., 2002). The maximum speed measured in the weather station 50 km to the northwest of Ebro Depression reached 38 m/s in 1979 (Gutiérrez-Elorza et al., 2002). ...
Yardangs, as typical aeolian landforms, are extensively identified in arid/hyper-arid areas on Earth and other terrestrial bodies. In this review, based on analyses of morphology, distribution, age, climate, and geologic condition of yardangs worldwide, we (1) assess the impacts of various controlling factors on yardang development to generalize a model based on morphology; (2) illustrate the climate-driven mechanism for yardangs; (3) propose a geomorphological evolutionary model of yardang fields; and (4) discuss potential implications of the above two models of yardang evolution on Earth to Mars. It is demonstrated that both deflation and abrasion work on yardang development, and it is the lithology to determine which one will be more effective. In addition, the wind action dominates the general aerodynamic form of yardangs, while non-aeolian factors account for the diversity in yardang morphology. Thus, a model including four stages and initiating mechanisms is generalized for yardang processes, from initiation to demise. The synthetic analyses of meteorological data, chronology, and paleoclimatic proxies show that yardang development is mostly controlled by westerlies, and would be accelerated during glacial periods and interrupted by lake formation during interglacial periods due to dry-humid fluctuations and changes in atmospheric circulation, driven by variations in solar radiation at orbital scales. This leads to the zonal distributions of yardangs near 30° in both hemispheres. Depressions would be created or enlarged in yardang fields during glacial period, along with activities of dust release and transport, leading to aeolian deposition (e.g. loess) downwind. The depressions, then, could be turned into lakes during wet phase of interglacial period and evolved into new yardangs in the following glacial period due to wind erosion. This climate-controlled geomorphological process under glacial-interglacial cycles suggests that high accumulation rate in sediments from enclosed inland basins (e.g. the Qaidam Basin) doesn't mean high-resolution climatic record due to the hiatus by wind erosion, indicating the significance of robust chronology establishment before any climatic correlation. Under the rules of Earth-based yardang evolution, it is potential to identify the specific level of development and explore the geomorphological processes as well as the climatic background for yardangs on Mars.
... In the lower Rhône Valley, their length exhibits strong variability and ranges between 61 to 1640 m (mean = 144 m), and their width ranges between 28 and 638 m (mean = 118 m). The average length to width (L:W) ratio is 4:1 (Fig. 4C) in agreement with the wind tunnel experiments of Ward and Greeley (1984) and the measured values for Pleistocene yardangs in Spain (Gutiérrez-Elorza et al., 2002). ...
... The spatial association between yardangs and pans indicates a genetic relationship between the landforms. Such an association is commonly observed in arid or semiarid environments (e.g., Gutiérrez-Elorza et al., 2002;Goudie, 2013). In the study area, wind erosion is assumed to be predominant in the formation of pans by most authors (Ambert, 1973(Ambert, , 2013Ambert and Clauzon, 1992;Carozza et al., 2016). ...
... These systems share the following characteristics: (i) an upwind bypass area at the outlet of a corridor, dominated by deflation-related landforms (yardangs, pans, ventifacts); (ii) an accumulation area located downwind, characterized by coversands or dune fields and loess. Such aeolian systems have been described in the Ebro Valley in Spain (Gutiérrez-Elorza et al., 2002;Gutiérrez et al., 2013;Boixadera et al., 2015), the Orb Valley in southern France (Gottis, 1968;Ambert, 1974Ambert, , 1981Ambert, , 2013Ambert and Clauzon, 1992;Carozza et al., 2016), and the Carpathian Basin in Hungary Sebe et al., 2011Sebe et al., , 2015. The lower Rhône Valley belonged to this type of aeolian system. ...
In the Rhône Valley, a north-south oriented Cenozoic rift in southeast France, thick Pleistocene loess deposits have been recognized since the beginning of the last century. These loess records, which are disconnected from the North European Loess Belt (NELB), are of significant interest to document the evolution of perimediterranean landscapes and environments during the Last Glacial. To overcome the poor precision of available aeolian distribution maps, aeolian deposits were mapped using the topsoil textural database provided by the Land Use and Cover Area frame Statistical Survey project (LUCAS). The grain-size distribution of aeolian sand and loess was first determined using 116 samples taken from surveyed outcrops. Then, the areas showing a similar grain-size composition were extracted from the LUCAS rasters. The resulting map reproduces the conventional maps correctly but suggests a more significant extension of loess, in better agreement with the known distribution of outcrops. The map shows that the distinctive morphology of the valley dominantly controls the distribution of aeolian deposits. The deflation-related landforms, i.e., yardangs, closed depressions (pans), and desert pavements, are widespread south of narrowings of the Rhône Valley between latitudes 44°N and 45°N. They indicate palaeowinds blowing from the north/northwest. Aeolian sand, loessic sand, sandy loess, and loess deposits successively spread on both sides of the Rhône River. The loess is characterized by a coarse texture (main mode around 60 μm), strong local thickness (>5 m), limited extension, and abundant bioturbation. This preservation results from the persistence of a shrub vegetal cover during the coldest and driest phases of the Last Glacial that allowed for trapping the saltating and suspended particles close to the alluvial sources.
... The development of these basins results from subsurface dissolution of the gypsiferous bedrock and aeolian deflation, which operates when the floor of the playa-lakes is dry. The leeward side of the largest playas displays numerous yardangs oriented parallel to the dominant wind direction and carved in bedrock and lake terrace deposits (Gutiérrez-Elorza et al. 2002). To our knowledge, these are the only yardangs reported in Europe (Goudie 2007), together with some examples from Hungary (Sebe et al. 2011). ...
... A total of 50 yardangs with a dominant WNW-ESE (N122E) orientation were mapped in the leeward side of the largest playas, mainly La Playa (1.72 km 2 ), El Pueyo (0.14 km 2 ) and El Pito (0.35 km 2 ) lakes (Gutiérrez-Elorza et al. 2002). This spatial association indicates that the formation of these landforms is related to the increase in the concentration of wind-blown particles in the playas during dry periods, increasing significantly the abrasive capability of the air currents (Figs. ...
... Three lacustrine terraces have been identified in La Playa and El Pueyo by means of detailed geomorphological mapping; upper, intermediate and lower terraces situated at 9, 6 and 0.5 m above the lake bottom, respectively (Gutiérrez-Elorza et al. 2002;Gutiérrez et al. 2013) (Fig. 12.2). The intermediate terrace is the most extensive, and its upper surface merges with the top of the deposits The spatial distribution of this terrace indicates that during its accumulation, La Playa and El Pueyo used to form a single lake around 2.7 km 2 , suggestive of more humid conditions. ...
The Bujaraloz-Sástago endorheic area occurs on an exhumed structural platform in the central sector of the Ebro Cenozoic Basin, essentially underlain by subhorizontally lying gypsiferous and mudstone units with some limestones. The dominantly flat topography of this structural surface is interrupted by around 150 closed depressions, some of which host playa-lakes of outstanding ecological and geomorphological value. The origin of the depressions is related to subsurface dissolution of the gypsiferous bedrock and aeolian deflation caused by the strong local wind, called Cierzo. The leeward side of the largest playas displays yardangs carved on bedrock and unconsolidated Holocene lake terrace deposits. These are the only yardangs documented in Europe so far. Modern and relict lunette dunes also occur on the downwind margin of some playa-lakes. Lacustrine terraces preserved on the margins of the largest basins record alternating periods of aggradation and excavation, attributable to more humid and drier periods, respectively. The available radiocarbon dates from the most extensive terrace, allow us to infer deepening of the largest playa (La Playa) by wind erosion of 6 m over the last 2 ka, yielding an average lowering rate of ca. 3 mm/year. This figure compares well with those calculated in several arid regions of the world, mainly using yardangs carved in Holocene lake deposits.
... These yardangs are mainly distributed in hyper-arid regions, where annual precipitation is less than 50 mm and there is minimal vegetation cover. However, the yardangs in northeastern Spain (Gutiérrez-Elorza et al., 2002) and at Rogers Lake, California (Ward and Greeley, 1984) exist in a semiarid climate, and the yardangs in Hungary (Sebe et al., 2011) exist in a cold desert environment. The yardangs in these regions may also indicate the existence of an arid environment in the past, when they first evolved. ...
... Based on the results of wind tunnel experiments, Ward and Greeley (1984) proposed that the ideal (mature) form of a yardang would have an aspect ratio of 4:1, independent of scale. Gutiérrez-Elorza et al. (2002) found a mean aspect ratio of 4.1:1 for yardangs in the Ebro Depression of northeastern Spain, which corroborated the hypothesis of an ideal aspect ratio proposed by Ward and Greeley based on evidence from field observations. However, field observations by other investigators contradicted this hypothesis. ...
... The aspect ratio achieved by a mature yardang will be achieved only after long periods of time, and the actual aspect ratio may vary considerably within and between yardang fields. The aspect ratio of the whaleback yardangs agrees with previously published values of 3:1 in the Western Desert of Egypt (Grolier et al., 1980), 2.8:1 in the southern part of Yardang National Geological Park in China's Kumtagh Desert (Dong et al., 2012), 4.1:1 in the Ebro Depression (Gutiérrez-Elorza et al., 2002), and 4:1 at Rogers Lake in California (Ward and Greeley, 1984). Therefore, this value provides field evidence for the hypothesis that there is an ideal (mature) aspect ratio for a given yardang sub-type. ...
The northwestern Qaidam Basin exposes one of the largest and highest elevation yardang fields on Earth. The aim of the present study was to describe the distribution and morphology of these yardangs, and analyze the factors responsible for the distribution pattern of these aeolian landforms. The yardang fields are bounded by piedmont alluvial–diluvial fans from the mountain ranges surrounding the basin, except in the south, where they are bounded by dune fields, dry salt flats, lakes, and rivers. This distribution pattern can be attributed to regional tectogenesis and its corresponding environmental impacts. The morphology of the yardangs varies considerably in response to the diverse factors that control their formation and evolution. Long-ridge yardangs are mainly located in the northernmost part of the yardang field, and the long ridges are gradually dissected into smaller ridges in the downwind direction. Further downwind, the convergence of northerly and northwesterly winds and the effects of temporary runoff cause the ridges to gradually transition into mesa yardangs. Saw-toothed crests, and conical and pyramidal yardangs, occur in groups on folded brachyanticlinal structures. Typical whaleback yardangs are found in the southeast, at the northern margin of Dabuxun Lake. Morphological parameters vary among the yardang types. The orientation of the yardangs in the northernmost area is nearly N–S, with a transition towards NW–SE in the southernmost area in response to a change in the dominant wind direction that results from the orientations and positions of the mountain ranges that surround the basin.
... The formation of yardangs is affected by several factors, including wind and fluvial erosion, geological structure, lithology, gravity, humidity, temperature, salinity and time (Greeley and Iversen, 1985;Gutiérrez-Elorza, et al., 2002;Dong, et al., 2012). The relative importance of these factors is likely to vary between different regions due to differences in regional environmental factors (Dong et al., 2012). ...
... The relative importance of these factors is likely to vary between different regions due to differences in regional environmental factors (Dong et al., 2012). Wind erosion is one of the most significant factors shaping yardangs, and most researchers consider that strong unidirectional winds favor their development (Hobbs, 1917;McCauley et al., 1977;Inbar and Risso, 2001;Gutiérrez-Elorza et al., 2002). However, others observed that two sets of opposing wind directions could also form yardangs (Ritley, 2004). ...
Yardangs are a type of wind-sculpted landform which generally form in hyper-arid regions. Several factors affect the development of yardangs, and the relative importance of these factors likely varies with differences in regional environmental factors. In the Loulan region of Lop Nur, wind dynamics are the principal factor affecting the development of yardangs. However, layered yardangs, which have undergone a unique form of differential erosion, are common in the region. These erosional landforms differ from typical yardangs which are eroded solely by abrasion and deflation. We conducted field and laboratory investigations of layered yardangs to determine their origin. The results indicate that there are two types of strata comprising the yardangs: uncompacted sand-silt layers, with a lower carbonate content; and compacted clay-silt layers, with a higher carbonate content. Both types of strata are horizontal and occur in alternating layers. This type of structure enables the wind to more easily erode the less resistant sand-silt layers at different heights, leaving the more resistant compacted clay-silt layers relatively intact. Eventually the undercut remnant clay-silt layers collapse once the weight of the suspended strata exceeds their elastic resistance (more than 90% of the fallen blocks have length/thickness ratios between 1.2 and 2.5). Therefore, in addition to wind dynamics, the lithology and structure of the strata are important factors affecting the development of the layered yardangs. This type of differential erosion accelerates the development of the yardangs in the Loulan region.
... Yardangs are also present on Mars (Ward, 1979;Mandt et al., 2009;Bridges et al., 2010;Zimbelman and Griffin, 2010;Bishop, 2011;Day and Kocurek, 2015), Venus (Trego, 1990;Arvidson et al., 1991;Trego, 1992;Greeley et al., 1995), and Titan (Paillou and Radebaugh, 2013). Yardang fields are generated by the combined effects of wind erosion, gullying action, and slope movement (Greeley and Iversen, 1987;Gutiérrez-Elorza et al., 2002). Wind erosion, encompassing deflation and abrasion, likely plays the dominant role in sculpting yardangs (McCauley et al., 1977;Ward and Greeley, 1984). ...
... Scientists have studied yardangs through multiple perspectives, such as definition (McCauley et al., 1977;Ward, 1979;Goudie, 1989), morphology (Gutiérrez-Elorza et al., 2002;Ritley, 2004;Ehsani and Quiel, 2008;Al-Dousari et al., 2009;Al-Masrahy and Mountney, 2015;), and age and evolution (Brookes, 2001;Inbar and Risso, 2001;de Silva et al., 2010;Rohrmann et al., 2013;Barchyn and Hugenholtz, 2015). Geometrical characterization and analysis of yardangs such as orientation, length-to-width ratio (L/W), cross-cutting relationship, spacing, and spatial density, and their potential controlling factors, may shed light on interactions among geologic, surface, and climatic processes. ...
The hyperarid Qaidam Basin features extensive fields of yardangs (covering an area of ~ 40,000km²) sculpted in tectonically folded sedimentary rocks. We extracted the geometries of 16,749 yardangs, such as length-to-width ratio (L/W), spatial density, and spacing, from multi-source remote sensing data provided by Google Earth™. We classified the yardangs into four types based on their L/W: short-axis (1–2), whale-back (2–6), hogsback (6–10) and long-ridge (10 − 210). We interpreted the yardang geometries in the context of their geologic setting (bedding orientation, location along anticline crests or syncline troughs, and lithologic heterogeneity). Our results show that the yardang geometries in the Qaidam Basin are mainly controlled by the structural geology and rheology of the sedimentary rocks (e.g., strike and dip of bedding, the presence or absence of interbedded soft and hard beds, and structural position with folds), the angle between geomorphically-effective wind directions and the strike of bedding, and the relative cumulative wind shear force where two geomorphically-effective wind directions are present. Our analysis revealed the following: 1) nearly 69% of the yardangs with long-ridge and hogsback geometries are distributed in syncline areas whereas 73% of the yardangs with short-axis geometries are distributed in anticline areas; 2) the L/W ratio of yardangs exposed along the windward limbs of anticlines is lower than that of yardangs exposed along the leeward limbs; and 3) in the westernmost parts of the basin, yardangs are locally sculpted into mounds by two geomorphically-effective wind directions.
... Several of the geologic, hydrologic, and edaphic characteristics of those wetlands have been addressed during the last decades (e.g. Pueyo, 1978/79;Samper-Calvete and García-Vera, 1998;Sánchez et al., 1998;Schütt, 1998;Valero-Garcés et al., 2001;Gutiérrez-Elorza et al., 2002;Herrero, 2008;Mees et al., 2011;Domínguez et al., 2013a). Meanwhile, many of the Monegros saline wetlands have been degraded. ...
... In the map are drawn: i) the two red lutite layers from the lithological map of Salvany et al. (1996), ii) the flat-bottom valleys network from the geological maps (references in the text), and iii) the ghost drainage patterns (dashed lines). The left-upper box shows the limits between the ten wetlands sectors and the left-down box shows the wind rose (Gutiérrez et al., 2002) and lineaments direction (Arlegui and Soriano, 1998) in the area. (Table 4; Fig. 5). ...
Wetlands in semiarid regions have received less attention than wetlands in humid-temperate areas, and the limited amount of information has resulted in little regulatory recognition. A comprehensive map of the saline wetlands that occur in karstic depressions in the semiarid region of Monegros, NE Spain, was developed from historical data, topography, and surveys of vascular flora. Playa-lakes and other saline depressions are expressions of solution dolines largely founded on groundwater dynamics and favored by the limestone and gypsum-rich substrate. Substrate composition, groundwater dynamics, and the network of infilled valleys are key factors in the distribution of the wetlands. In spite of the anthropogenic imprint, wetlands morphometrics are the expression of geological processes. Significant correlations were found between basin area and depth, and between elongation and substrate composition. The predominantly subelongated shape of the Monegros saline wetlands reflects their origin and a geometry strongly influenced by fractures. Grouping these saline wetlands based on geological and vegetation features, provide a predictable relationship of surficial processes with the occurrence of otherwise complex and undetectable hydrological connectivity. Our ten geology-based Groups showed a high intra-group variation in depth, elongation, and vegetation cover. The eight vegetation-based categories mirror the gradation in flooding frequency and the soil salinity of the Monegros saline wetlands. The significant contrasts existing in-between the groups of wetlands and the disclosure of their causal factors provides a functional perspective at the landscape scale. This approach will help to monitor the ongoing environmental alterations associated with new on-farm irrigation developments.
... Dryland research has a long history of investigating the climatic and environmental processes that cause the development of natural landforms (Bull, 1977;Wasson and Hyde, 1983;Greeley and Iversen, 1985). The development of erosional landforms, such as yardangs, results from complex interactions between environmental conditions and the parent material (Greeley and Iversen, 1985;Gutiérrez-Elorza et al., 2002). For yardangs, erosion is predominately driven by aeolian processes (abrasion and deflation), with non-aeolian processes (gullying, fluvial processes) influencing the landform's profile (Laity, 2009;Dong et al., 2012;Barchyn and Hugenholtz, 2015). ...
The variable climatic and environmental conditions associated with dryland regions can cause rapid erosion to both natural and man‐made earthen structures. Whilst there is a long history of research into the evolution of erosional landforms such as yardangs, little research has investigated how dryland processes influence the erosion of built structures. Earthen heritage sites located in arid and semi‐arid environments experience rapid deterioration caused by exposure to environmental drivers such as wind and rain. Understanding how these environmental drivers interact with each other and cause deterioration to earthen material is vital for successful conservation strategies. To address this need, we present the Vegetation and Sediment TrAnsport model for Heritage Deterioration (ViSTA‐HD) that simulates the risk of polishing, pitting and slurry on earthen heritage in a spatially specific manner. A technical description of the model is provided and sensitivity and validation tests are reported. The model is then used to simulate the risk of deterioration occurring over centennial timescales at a Suoyang Ancient City, located in semi‐arid, northwest China. The modelled risk of deterioration is in good agreement with deterioration patterns found at Suoyang, with the risk of polishing predominantly occurring around the wall edges, areas at risk of pitting echoing the dune formation and the risk of slurry occurring in drape like patterns down the wall face. Consequently, ViSTA‐HD is a powerful and versatile model that can be used to help inform our understandings of long‐term interactions between dryland processes and deteriorative impact on earthen structures.
... Grain size is a basic, standard parameter used to determine the sedimentary environment, depositional mechanism, and development of aeolian landforms. It has been widely used in aeolian geomorphological research (He et al., 2009;Dong et al., 2011;Liu et al., 2014) but less so in yardang landform research (Gutiérrez-Elorza et al., 2002;J.Y. Li et al., 2012). ...
... The aspect ratio, which represents the ratio of the yardang length to its width, ranges between 1.0:1 and 26.3:1, with a mean of 8.6:1 . The aspect ratio of whaleback yardangs ranges between 1.2:1 and 5.4:1, with a mean of 3.2:1, agreeing with previously published values: 3:1 in the Western Desert of Egypt (Grolier et al., 1980), 2.8:1 in the southern part of Yardang National Geological Park in China's Kumtagh Desert (Dong, Lv, et al., 2012), 4.1:1 in the Ebro Depression in Spain (Gutiérrez-Elorza et al., 2002), and 4:1 at Rogers Lake in California (Ward & Greeley, 1984). The mesa yardangs have an aspect ratio between 1.0:1 and 3.2:1, with a mean of 1.7:1. ...
Aeolian processes and their role in desertification have been studied extensively at low elevations but have been rarely studied at high elevations in areas such as the Tibetan Plateau, where aeolian processes were active in the geologic past and remain active today. In this review, we summarize research that improves our understanding of aeolian processes on the Tibetan Plateau, including the distribution, characteristics, and provenance of aeolian sediments; the history of aeolian activity; aeolian geomorphology; and wind-driven land degradation. Contemporary aeolian processes primarily occur in dry basins, in wide river valleys, on lakeshores, on mountain slopes, and on gravel pavements. Sediment characteristics suggest a local origin and provide interesting contrasts with those of China's Loess Plateau. The history of aeolian activity and its paleoclimatic implications, reconstructed based on aeolian archives, is short (mostly since the Late Glacial) and shows wide regional differences. Aeolian geomorphology is simple and suggests short formation time. Wind-driven land degradation is less severe than previously thought, driven by different factors in different areas, and exhibited complex interactions with freeze-thaw processes. Aeolian research has been conducted within the general framework of aeolian science but addresses issues specific to the Tibetan Plateau that arise due to the low air temperature, low air density, and the presence of a cryosphere. We propose six priorities for future research: aeolian physics, the effect of freeze-thaw cycles, comparisons with other areas, regional differences, effects of wind-driven land degradation, and integrated observation and monitoring.
... This hypothesis is in stark contrast with the notable depth of the lacustrine depression and the steep slopes surrounding the lake. Moreover, aeolian sediments are very scarce and local in the area , indicating a limited deflational effect, the most important traces of which can be recognised in the form of some rocky yardangs (Gutiérrez et al., 2002a), ventifacts (Cuchí et al., 2012), and small local dunes . ...
The origin and nature of the numerous lakes in the central Ebro Basin have been interpreted according to the prevailing arid or semiarid conditions, the easily-eroded materials and the solubility of the gypsum- and/or carbonate-rich Tertiary/Cenozoic substratum, involving important dissolution (karstic) and/or Aeolian deflation. However, the origin of Sariñena Lake, the largest in the central Ebro Basin, remains unknown since the typical lake-generating processes in the region are not applicable. This work provides significant clues to the genesis and evolution of Sariñena Lake in a regional context. The combination of geomorphological mapping and high resolution LiDAR data together with sedimentological observations, the characterisation of soils and sediments around the lake, and the application of high-resolution geophysical techniques suggests that piping is the major genetic process driving the evolution of the Sariñena depression and lake. Field evidence demonstrates that piping is, at present, the most important erosive process in the region, generating significant collapse and surface lowering. Sariñena Lake is located within a deep endorheic depression excavated from Na-rich Tertiary materials. This work hypothesises that once an early, fluvially-originated palustrine area had developed, the progressive lowering of the regional water table linked to regional fluvial incision favoured the establishment of a hydrological gradient high enough to trigger piping processes within the claystones and siltstones underlying the original palustrine area. The Quaternary evolution of the Sariñena lacustrine basin was then controlled by successive water table fluctuations, linked to different phases of incision and alluvial deposition in the surrounding fluvial systems. All the evidence supporting a piping-related origin for this lake, together with examples of lakes generated by similar processes in different contexts, is used to propose a new genetic type of lacustrine depression, generated by piping processes under favourable conditions.
... Estas saladas se disponen sobre estratos miocenos de origen lacustre y palustre, ocupando pequeñas depresiones producidas por karstificación de los materiales calcáreos y yesosos infrayacentes. La similitud entre formas de erosión fósiles y actuales (Gutiérrez et al. 2002) indica la persistencia de las condiciones climáticas del pasado. Una descripción de las saladas y de las salmueras que contienen puede encontrarse en Pueyo (1978). ...
... Studies on yardangs under periglacial climates, the application of HiRISE cameras and the discovery of this kind of landform on Mars and Mercury have promoted further research on yardang landform greatly (Ehsani and Quiel 2008; Bridges et al. 2010; de Silva et al. 2010; Sebe et al. 2011). The evolution of yardangs in Dunhuang underwent prenatal stage, embryonic stage, adolescent stage, mature stage, recession stage and extinct stage, which was influenced by many factors including components , fluvial erosion, wind erosion and gravity (Gutiérrez-Elorza et al. 2002; Zheng et al. 2002; Xia 2007; Dong et al. 2011; Niu et al. 2011; Qu Keywords. Yardangs; Late Pleistocene; tectonic movement; river terrace; dating. ...
Developed in the Anxi-Dunhuang basin, the yardangs of Dunhuang (western China) are clearly affected by tectonic movement. Based on fieldwork, this study ascertained three levels of river terrace in the area for the first time. Through the analysis of river terraces formation and regional tectonic movement, the study ascertained that the river terraces were formed mainly by Late Pleistocene tectonic uplift, which had activated the evolution of yardangs in the study area. By electron spin resonance (ESR) dating and optically stimulated luminescence (OSL) dating, the starting time and periodicity of the evolution of the yardangs were determined. The river terraces designated T3, T2 and T1 began to evolve at 109.0 ∼98.5, 72.9 ∼66.84 and 53.2 ∼38.0 kaBP, respectively, which is the evidence of regional neotectonic movement. And, the formation of the yardangs was dominated by tectonic uplift during the prenatal stage and mainly by wind erosion in the following evolution, with relatively short stationary phases. This research focused on the determination of endogenic processes of yardangs formation, which would contribute to further understanding of yardangs formation from a geological perspective and promote further study of yardang landform.
... This allows high rates of horizontal aeolian transport in nebkhas that may be part of a positive feedback in which increased aeolian transport leads to increased sediment transport from interspaces to nebkhas ([19]). The nebkhas have been studied and described in Africa ([17] [20]), northern China ([21], Israel ([13] [22]), Spain ([23]), New Mexico [24]), and the arid zone of Argentina ([25][28] ), as common geomorphological-biological components of psammophilous environments. They have been indicated on crests of linear semi-fixed dunes [29] and in inter-dune valleys [30]. ...
Nebkhas, developed by the trapping of sand within the body of a plant, were
studied in the Medanos Grandes system, arid central Argentina, during the springs
of 2009-2010. The dynamics of nebkhas was studied in three draas (megadunes), and
considering both orientations: leeward and windward. The Drift Potential (DP) for
the study area was 42, evidencing the inactivity of the sand dunes or the scarce
activity only in crests. Dominant sand movement is in south-southeast direction,
with deflation processes at the southern side of the nebkhas. All nebkhas showed
uniformity in the morphometry. Tricomaria
usillo is the dominant plant species in the nebkha formation process; and results
showed a significant positive relationship between nebkha and canopy volumes.
... Here, there are extensive sand sheets and dune fields, as well as blowouts, deflation basins and lunette dunes (Bateman and Díez 1999;García-Hidalgo et al. 2007;Gutiérrez-Elorza et al. 2005b;Bernat-Rebollal and Pérez-González 2008). In the Ebro Basin, where there is very limited availability of sands, aeolian accumulations are very scarce, but the strong wind has carved yardangs in gypsiferous rocks and unconsolidated lake deposits in the leeward margin of playa lakes (Gutiérrez-Elorza et al. 2002;Chap. 12). ...
Spain has a remarkable geomorphological diversity largely due to its geological and climatic variety. From the geological perspective, the Iberian Peninsula may be divided in two broad geological domains; the Iberian Massif in the western sector, and the mountains belts and Cenozoic basins related to Alpine tectonics in the eastern sector. The Iberian Massif (Variscan Spain) mainly consists of Paleozoic metamorphosed sedimentary formations intruded by plutonic rocks. This region is characterised by extensive planation surfaces locally interrupted by inselbergs, and includes outstanding examples of granitic landscapes. The Alpine Mountain Belts, related to the convergence between Europe, the Iberian microplate, and Africa, contain excellent examples of landscapes controlled by active tectonics. In these Alpine orogens, extensive limestone outcrops have favoured the development of outstanding poljes, dolines and karren fields Glacial landscapes are best developed in the Pyrenees, which still contain a number of active cirque glaciers. The Cenozoic Basins include some of the finest areas to examine stunning conglomerate monoliths, dramatic badlands, dune fields, deflation basins associated with lunette dunes and yardangs, and a wide variety of features related to evaporite dissolution. The Canarian Archipelago is a late Cenozoic chain of hot-spot-related volcanic islands located in the Atlantic Ocean west of the Sahara coast. The evolution of the Canaries is characterised by the growth of large volcanic edifices, punctuated by the development of giant landslides. The Teide volcano (3,718 m a.s.l.) in Tenerife rises more than 7 km above the adjacent abyssal plain. A total of 18 eruptions have been documented over the last 500 years, some of them with great societal impact; the 1730-1736 Timanfaya eruption covered more than 20 % of Lanzarote island. The around 10,000 km-long coastline of the Spanish territory display a wide variety of coastal landscapes, including rías, estuaries sequences of raised beaches, deltas, lagoons and spit bars, and dune fields.
... Martian surface at several locations shows presence of cut and streamlined Yardang blocks. Yardangs are large, free-standing, streamlined blocks or wind eroded desert ridges commonly observed to form in terrestrial deserts with lengths ranging from micro to mega-scale aligned with the prevailing wind directions (Elorza et al. 2002;Carling 2013). Their presence reveals strong wind supply to be an effective agent in their formation and later on also in gradually modifying the surface morphology (Ward 1979). ...
Large scale landform modifications on varying temporal and spatial scales in response to past active processes are still evident on Mars providing important clues on the past climatic and environmental conditions. Nicholson crater on Mars is a ~100 km impact crater located near the dichotomy boundary. Based on a detailed morphological analysis using high resolution data, particularly from the Context Camera (CTX) onboard Mars Reconnaissance Orbiter, the present study attempts to bring out potential evidences of ancient fluvial processes and their subsequent modifications within this crater. This is manifested through the presence of inverted channel network, valley network, possible presence of a canyon/deep valley, presence of fluvially carved lobate features as well as fan/deltaic deposits observed at various locations throughout the crater. The yardang landform morphologies discussed here show prominent fluvial signatures on the intra-crater sedimentary mound, that are features shaped over millions of years. We recognise three distinct types of yardangs, for all of which, mostly wind direction and strength have played a major role in sculpting their distinct morphology. Although wind flow in different directions have played a role in sculpting varied yardang patterns, unidirectional wind strength seems to have played a common and predominant role in all the cases.
... Existe una nomenclatura muy numerosa sobre depresiones cerradas situadas en medios áridos que obedece en parte a la utilización de denominaciones vernáculas (Tricart, 1969;Cooke y Warren, 1973;Neal, 1975;Cooke et al., 1993;Currey, 1994;Rosen, 1994;Shaw y Thomas, 1997;Briere, 2000;Gutiérrez Elorza, 2001). El término playa es de origen español y fue introducido en el vocabulario anglosajón a raíz de la exploración española del suroeste de los Estados Unidos (Gutiérrez Elorza, 2001;Gutiérrez Elorza et al., 2002a). Recientemente, Briere (2000) ha llevado a cabo una minuciosa recopilación de los diferentes términos utilizados para designar a estas depresiones. ...
In the southern sector of the Duero Depression (Coca area) a significant number of playas has been developed. These playas show NNW and NW prevalent orientations subperpendicular to the main wind direction and are associated to the Quaternary aeolian deposits of Tierra de Pinares. The origin of the depressions is attributed to deflation processes acting on terrace surfaces and on the bottom of poorly drained creeks. The playas developed on the terraces seem to be controlled by the architecture (paleochannels) of the fluvial deposits. The generation of closed depressions in the creek bottoms is related with the abandonment of the main valley by a fluvial capture. In an initial stage the alluvial cover is differentially striped by aeolian erosion. Subsequently, the deepening of the playas progresses by deflaction of particles produced by weathering of the argillaceous bedrock. The weathering processes involved in the production of particles exportable by the wind include the aggregation of peloids by salt precipitation, wetting and desiccation cycles, haloclasty or biological activity. Aeolian erosion, especially effective during dry periods, leads to the lowering and enlargement of the playas and eventually to their coalescence.
... De hecho, en una reciente compilación cartográfica de loess a escala europea (Haase et al., 2007) no aparece señalado ningún depósito de este tipo en el territorio peninsular y a ello puede haber ayudado el que se haya relacionado tradicionalmente estos depósitos con la actividad glacial cuaternaria (Pye, 1995). A pesar de esta laguna, algunos autores (Torras y Riba, 1968;Mensúa y Ibáñez, 1975, Van Zuidam, 1976Gutiérrez et al., 2002) han mencionado formas y depósitos de limos en el NE español, en el marco de la cuenca del Ebro, a los que han atribuido un claro origen eólico. Modernamente, Iriondo y Kröhling (2004) describen diferentes perfiles de loess de naturaleza yesífera a lo largo del eje del Ebro, dos de ellos correspondientes al margen de la cuenca (Batea y Gandesa), que datan en la primera parte del Holoceno (entre 12.000 y 7.000 años A.P.) y los relacionan con los vientos dominantes procedentes del W. A pesar de lo anterior, no se conocen trabajos que describan estas acumulaciones, más al E, en la cubeta de Móra d'Ebre y áreas adyacentes, donde forman un tapiz muy extendido y su descripción constituye el objetivo de este trabajo. ...
... Lut, Saudi Arabia, northern Namibia) and may have been shaped over millions of years (Goudie, 2007). Gutierrez-Elorza et al (2002) studied the existence and generation of yardangs in the semiarid central sector of the Ebro Depression in Spain. They concluded that generation of yardangs in that area is related to the presence of playas, which constitute the source of abrading particles during dry periods. ...
This paper presents a robust approach using artificial neural networks in the form of a Self Organizing Map (SOM) as a semi-automatic method for analysis and identification of morphometric features in two completely different environments, the Man and Biosphere Reserve "Eastern Carpathians" (Central Europe) in a complex mountainous humid area and Yardangs in Lut Desert, Iran, a hyper arid region characterized by homogeneous repetition of wind-eroded landforms. The NASA Shuttle Radar Topography Mission (SRTM) has provided Digital Elevation Models (DEM) for over 80% of the land surface. Version 3.0 SRTM data provided by the CGIAR-CSI GeoPortal are the result of substantial editing effort on the SRTM DEM produced by NASA. Easy availability of SRTM 3 arc second data promoted great advances in morphometric studies and numerical description of terrain surface features as shown by many literature references. The goal of this study was to develop a new semi-automatic DEM-based method for geo-morphometric feature recognition and to explore the potential and limitation of SRTM 90 meter data in such studies. The 3 arc seconds data were re-projected to a 90 m UTM grid. Bivariate quadratic surfaces with moving window size of 5×5 were fitted to this DEM. The first derivative, slope steepness and the second derivatives minimum curvature, maximum curvature and cross-sectional curvature were calculated as geo-morphometric parameters and were used as input to the SOMs. Different learning parameter setting, e.g. initial radius, final radius, number of iterations, and the effect of the random initial weights on average quantization error were investigated. A SOM with a low average quantization error was used for further analysis. Feature space analysis, morphometric signatures, three-dimensional inspection and auxiliary data facilitated the assignment of semantic meaning to the output classes in terms of geo-morphometric features. Results are provided in a geographic information system as thematic maps of landform entities based on form and slope. Geo-morphometric features are scale-dependent and the resolution of the DEM limits the information, which can be derived. The results demonstrate that a SOM is an efficient scalable tool for analyzing geo-morphometric features as meaningful landforms under diverse environmental conditions. This method provides additional information for geomorphologic and landscape analysis even in inaccessible regions and uses the full potential of morphometric characteristics.
The Pannonian mega-yardang system is the northernmost mega-yardang field in Europe, exerting a profound influence on the relief and surface hydrographic structure of Western Hungary. Despite its significance, the precise timing and climatic conditions under which this mega-yardang system formed remain elusive. The geological setting was studied, the former humidity and biome patterns were modelled, the climatically analogous regions of the past climates of the region were identified, and the Köppen Aridity Indices of the region were compared to the aridity values of other yardang fields from five continents in the last 3.3 mys. The geological structure of the yardangs suggests an origin post-dating the Tortonian period but preceding the mid-Pleistocene era. In the Zanclean and in the warmer periods of the Piacenzian, the studied region had humid subtropical climatic conditions that were not suitable for the formation of large-scale aeolian landforms. Similarly, in the Quaternary period, the climate varied between boreal and humid continental, which was not conducive to the formation of mega-yardang fields. Cold semi-arid climatic conditions only existed in the mid-Pliocene cold period and perhaps at the transition of the Pliocene to the Pleistocene. This paleoclimatic hypothesis is in accordance with the Pliocene fossil record of the region, which contains the remains of such arid habitat-dwelling fauna elements as camelids and struthionids. The comparison of the development of Köppen Aridity Indices among yardangs in other territories also suggests that the Pannonian mega-yardang system may have formed around the transition of the Neogene to the Quaternary epochs.
Graphical abstract
This chapter focuses on the Neogene evolution of the Pyrenean mountain range. Based on methods of denudation chronology and landform analysis, the transition from the ancestral (Paleogene) to the modern Pyrenees is analysed. During the ~25 Ma following the definitive relaxation of tectonic convergence, the sediment record reveals a state of transience in which rates of crustal deformation and crustal denudation were neither steady through time at any given location, nor spatially uniform. Transience is also encoded in landform assemblages within the mountain range, whether in the Axial Zone or the outer fold belts. Slope systems, interfluve summits and valleys responded in characteristic ways to relative lulls in the history of crustal deformation, and/or were at certain places buffered from base-level changes. The mountain landscape, accordingly, consists of (i) a mosaic of range-top, low-gradient erosion surfaces of varying sizes and states of conservation, often in abrupt juxtaposition with (ii) a population of younger incisional landforms, themselves displaying evidence of successive stages of downcutting and drainage reorganisation such as dry valleys, wind gaps, mountain-flank pediments and rock benches. All are situated above (iii) staircases of Quaternary alluvial terraces, fans and debris cones. The entire landscape evolution sequence was driven by post-orogenic regional uplift and was enhanced more locally by active fault tectonics.
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.
European aeolian deposits have been mapped using the LUCAS topsoil texture database and the location, thickness and grain size mode of loess sections described in the literature have been compiled. The map, in fairly good agreement with those based on a classical geological approach, has the advantage of being of equal precision for the whole study area and makes it possible to highlight grain size gradients. It shows three main types of aeolian systems, i.e. systems associated with ice sheets (FIS, BIIS, AIS) and the rivers they feed, continental systems related to the erosion of sedimentary rocks, and coastal systems. Each aeolian system is comprised of a band of coversands of varying extent close to the sources and bands of sandy loess and loess away from the sources. The thickness of loess is highly variable throughout Europe. The areas of greatest accumulation are associated with the rivers draining the AIS (Rhine, Danube, Rhône). The Middle Danube, which combines sources of glacial origin (AIS) and local sources, constitutes the main accumulation area in Europe. Conversely, continental and coastal systems generated only limited loess cover. The latitudinal gradient of vegetation probably played an important role in loess sedimentation during the LGM. Weak interference between particle transport and vegetation (periglacial desert in the sand belt, cryptogamic crusts and steppe in the north European loess belt) allowed for particle sorting through aeolian transport to develop. The loess band is extensive in low-relief areas and is typified by a low sand content. In southern Europe, on the other hand, the capture of saltating and suspended particles by the shrubby steppe vegetation led to the dominant accumulation of sandy loess near sources.
Parameters of geomorphological characteristics are critical for research on yardangs. However, which are low-cost, accurate, and automatic or semi-automatic methods for extracting these parameters are limited. We present here semi-automatic techniques for this purpose. They are object-based image analysis (OBIA) and Canny edge detection (CED), using free, very high spatial resolution images from Google Earth. We chose yardang fields in Dunhuang of west China to test the methods. Our results showed that the extractions registered an overall accuracy of 92.26% with a Kappa coefficient of agreement of 0.82 at a segmentation scale of 52 using the OBIA method, and the exaction of yardangs had the highest accuracy at medium segmentation scales (138, 145). Using CED, we resampled the experimental image subset to a series of lower spatial resolutions for eliminating noise. The total length of yardang boundaries showed a logarithmically decreasing (R2 = 0.904) trend with decreasing spatial resolution, and there was also a linear relationship between yardang median widths and spatial resolutions (R2 = 0.95). Despite the difficulty of identifying shadows, the CED method achieved an overall accuracy of 89.23%with a kappa coefficient of agreement of 0.72, similar to that of the OBIA method at medium segmentation scale (138).
The identification of karst sinkholes in Mars may provide evidence of dissolution processes caused by liquid water and information on paleoclimatic and paleohydrological conditions. This work presents a comprehensive cartographic inventory of 513 closed depressions developed on evaporite-bearing Equatorial Layered Deposits (ELDs) within Kotido crater, Arabia Terra. Detailed mapping, morphometric analyses and spatial distribution relationships reveal a number of features supporting that the depressions correspond to collapse sinkholes related to evaporite dissolution: (1) suitable topographic and litho-structural conditions for the development of a fracture-controlled epigene evaporite karst; (2) presence of open fissures at the foot of the scarped margins; (3) dimensions and frequency-size distributions comparable with those reported on Earth; (4) spatial association with high-permeability zones (i.e., fractures). Some characteristics of the depressions indicate that they have been re-shaped and enlarged by wind erosion: (1) dominant orientation consistent with the prevalent one-directional winds; (2) differing morphological characteristics on the downwind- and upwind-sides; and (3) nested depressions associated with the upwind sector. The relatively fresh appearance of the depressions and the lack of impact craters suggest a poorly constrained Amazonian karstification phase in the region.
Yardangs are typical aeolian erosion landforms, which are attracting more and more attention of geomorphologists and geologists for their various morphology and enigmatic formation mechanisms. In order to clarify the aeolian environments that influence the development of long-ridge yardangs in the northwestern Qaidam Basin of China, the present research investigated the winds by installing wind observation tower in the field. We found that the sand-driving winds mainly blow from the north-northwest, northwest and north, and occur the most frequent in summer, because the high temperature increases atmospheric instability and leads to downward momentum transfer and active local convection during these months. The annual drift potential and the ratio of resultant drift potential indicate that the study area pertains to a high-energy wind environment and a narrow unimodal wind regime. The wind energy decreases from northwest to southeast in the Qaidam Basin, with the northerly winds in the northwestern basin changing to more westerly in the southeastern basin. The strong and unidirectional wind regime for the long-ridge yardangs in the northwestern Qaidam Basin results from the combined effects of topographic obstacles such as the Altun Mountains and of the interaction between the air stream and the yardang bodies. Present study suggests that yardang evolution needs such strong and unidirectional winds in high- or intermediate-energy wind environments. This differs from sandy deserts or sandy lands, which usually develop at low- or intermediate-energy wind environments. Present study clarifies the wind regime corresponding to the long-ridge yardangs’ development, and lays firm foundation to put forward the formation mechanisms for yardangs in the Qaidam Basin.
'Wind erosion' is something of a misnomer, because the formation of wind‐eroded landforms is not the work of the wind itself, but of the mineral grains that the wind carries. This chapter uses the term 'wind erosion' to describe how landforms are shaped by wind‐blown particles. It describes the major landforms developed as a consequence of wind erosion. These landforms are best manifested on bedrock or boulder‐covered land surfaces, which retain more evidence for wind erosion. Ventifacts are loose surface clasts or upstanding bedrock protrusions that have been shaped by the bombardment by wind‐blown particles. The term yardang is a transliteration of a Turkic (central Asian) word to describe streamlined ridges, and was originally used to refer to landforms identified in the cold‐desert Taklamakan region of north‐west China. Deflation basins are shallow and sometimes very large land surface depressions that are likely to have experienced periods of wind erosion.
Yardangs are ridge features of various scales indicating the direction of prevailing wind when they evolved. In this paper, we apply an integrated approach for the automatic extraction of yardangs in typical areas of the Qaidam Basin. Canny’s edge detection algorithm is used for yardangs in 15-m Landsat 8 Operational Land Imager (OLI) data to acquire the spatial distribution patterns of yardangs, and ellipse-fitting algorithm is used to get the morphological parameters of yardangs in 1.7-m Digital Elevation Model (DEM) generated by unmanned aerial vehicle (UAV) images. Both the spatial distribution patterns and morphological parameters can improve knowledge of yardangs. The experimental results illustrate the accuracy and feasibility of the proposed extraction approach for the spatial distribution patterns of yardangs with an average relative error no more than 10%. The statistical analysis shows that most of yardangs developed near anticlines and few in synclines; most yardangs are in Pliocene strata, and a few in lower and middle Pleistocene strata. For the automatically extracted morphological parameters, the relative error of each parameter is less than 6%, which is meaningful for the type identification of yardangs.
As a member of UNESCO Global Geoparks, Dunhuang Global Geopark has developed a great quantity of landforms formed under special geological background and extremely droughty climate, which integrate together with specific geographic location and cultural relics on the "Silk Road Economic Belt". The main geoheritage in Dunhuang Global Geopark is Yardang landform, which is formed by loose Quaternary sediments. According to different shapes, the Yardang landform were divided into five types, namely, ridge-shaped Yardang, wall-shaped Yardang, tower-shape Yardang, column Yardang and Yardang monadnock. In order to monitor and protect the unique morphological features of Yardang landforms, a spatial information management platform is established, using SPOT 6 remote sensing image, with object oriented approach and manual interactive interpretation. Study shows that the maximum area, perimeter, length and width of Yardang were 324843.1 m2, 3447.52 m, 1508.41m, and 285.81 m, respectively. Additionally, the aspect ratio of Yardang has a certain positive correlation, with the coefficient of correlation being 0.675. Furthermore, the relationship between length and width of Yardang is calculated using formula Y=2.546X, where Y = length, X = width.
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.
the volume presents all the data collected during the cycle of research conducted by the italian Archaeological Mission in the Farafra oasis between 1990 and 2005. the 29 multidisciplinary essays contained in this book provide a detailed picture of the population of the Farafra oasis, hitherto one of the least well known within the Western Desert. Farafra became particularly important during the middle Holocene, the period when climate conditions were most favourable, with later brief humid episodes even in the historic periods. the results of the long-term research cycle presented here, combined with data from the survey of the whole Wadi el obeiyid still in progress, allow the authors to identify changes in the peopling of the oasis and to define various occupation phases. the new chronology for the Wadi el obeiyid is one of the main achievements of the book and, as demonstrated in the final chapter, is in complete agreement with the main cultural units of other territories in the Western Desert. on this chronological basis, the contacts between the latter and the populations established on the nile are brought into sharper focus. the importance of the archaeological documents discovered at Farafra and, at the same time their fragility due to the deterioration of the physical environment and the uncontrolled human activities, make us fear for their conservation. We hope that this book, with its complete documentation of the precious nature of the Farafra oasis landscape and its archaeological heritage, may help to promote more effective policies for its safeguard.
the volume presents all the data collected during the cycle of research conducted by the italian Archaeological Mission in the Farafra oasis between 1990 and 2005. the 29 multidisciplinary essays contained in this book provide a detailed picture of the population of the Farafra oasis, hitherto one of the least well known within the Western Desert. Farafra became particularly important during the middle Holocene, the period when climate conditions were most favourable, with later brief humid episodes even in the historic periods. the results of the long-term research cycle presented here, combined with data from the survey of the whole Wadi el obeiyid still in progress, allow the authors to identify changes in the peopling of the oasis and to define various occupation phases. the new chronology for the Wadi el obeiyid is one of the main achievements of the book and, as demonstrated in the final chapter, is in complete agreement with the main cultural units of other territories in the Western Desert. on this chronological basis, the contacts between the latter and the populations established on the nile are brought into sharper focus. the importance of the archaeological documents discovered at Farafra and, at the same time their fragility due to the deterioration of the physical environment and the uncontrolled human activities, make us fear for their conservation. We hope that this book, with its complete documentation of the precious nature of the Farafra oasis landscape and its archaeological heritage, may help to promote more effective policies for its safeguard.
IntroductionThe physical setting: conditions for wind erosionConclusions
References
http://www.insegnadelgiglio.it/prodotto/from-lake-to-sand/
The volume presents all the data collected during the cycle of research conducted by the Italian Archaeological Mission in the Farafra Oasis between 1990 and 2005. The 29 multidisciplinary essays contained in this book provide a detailed picture of the population of the Farafra Oasis,
hitherto one of the least well known within the Western Desert. Farafra became particularly important during the middle Holocene, the period when climate conditions were most favourable, with later brief humid episodes even in the historic periods. The results of the long-term research cycle presented here, combined with data from the survey of the whole Wadi el Obeiyid still in progress, allow the authors to identify changes in the peopling of the oasis and to define various occupation phases. The new chronology for the Wadi el Obeiyid is one of the main achievements of the book and, as demonstrated in the final chapter, is in complete agreement with the main cultural units of other territories in the Western Desert. On this chronological basis, the contacts between the latter and the populations established on the Nile are brought into sharper focus. The importance of the archaeological documents discovered at Farafra and, at the same time their fragility due to the deterioration of the physical environment and the uncontrolled human activities, make us fear for their conservation. We hope that this book, with its complete documentation of the precious nature of the Farafra Oasis landscape and its archaeological heritage, may help to promote more effective policies for its safeguard.
Endorheic basins are typical landscape elements of various parts of the Ebro basin/Spain. Their origin is in some cases explained by karstification, in other cases by wash processes occurring on slightly inclined surfaces in weakly consolidated claystones. Cores were taken from these endorheic basins which are ideal sediment traps. Based on geochronological and mineralogical analysis of the cores palaeoenvironmental conditions have been reconstructed. Differences in morphology and lithology of the various study areas allow to calibrate these informations. A first analysis of sediments from two endorheic depressions of the central Ebro basin shows that climatic conditions during the Holocenc changed from rather humid conditions in the past to more arid conditions today, subrecently interrupted by a rather humid phase.
Several evaporitic units, found among distal alluvial-fan deposits, are made up of bioturbated microlenticular primary gypsum, secondary nodular and meganodular gypsum after anhydrite, chert nodules, and charophyte limestones. These evaporites formed in shallow saline lakes along the basin margin. Concentration of these waters led to deposition of microlenticular gypsum and charophytic carbonates, which were then intensely bioturbated. Early-diagenetic chert nodules and nodular anhydrite also formed. With more advanced burial the primary gypsum was profoundly anhydritized in some areas . -from Authors
In the central part of the Ebro basin, filled with Neogene beds, more than 6000 lineaments have been mapped at a 1 : 100000 scale, using a combination of bands 2, 4 and 7 of the Landsat Thematic Mapper (TM). With an automatic exploration program, geometric features of the lineament population were analysed, identifying four sets of lineaments trending NW-SE, ESE-WNW, NNE-SSW and NE-SW. Field studies reveal that most lineaments are normal faults. The highest density lies over a lax NW-trending syncline, where the Tertiary thickness is lower and the basement shows NW-trending faulting. It is thus concluded that the lineaments developed as a result of cover deformation above large basement faults.
Yardangs are streamlined, wind-eroded hills common to most deserts. Yardangs at Rogers Lake, Mojave Desert, California, have streamlined forms characteristic of objects eroded by moving fluids, a teardrop shape that approaches an ideal 1:4 width-to-length ratio. In wind-tunnel simulations, miniature forms of various shapes changed sequentially by erosion of the windward corners, erosion of the windward slope, erosion of the leeward corners and flanks, and erosion of the leeward slope. Prominent mechanisms in yardang evolution apparently are abrasion at the windward end and deflation and reverse air flow near the middle and at the downstream end. Width-to-length ratios of yardangs are grossly similar to those of some fluvial and glacial streamlined landforms. Refs.
RESUMEN La zona endorreica de Bujaraloz-Sástago constituye un relieve ta-bular, compuesto por depósitos detnticos, yesíferos y carbonatados, correspondientes al Oligoceno superior y Mioceno inferior. Se han distinguido seis unidades litológicas superpuestas. De base a techo son: 1) Unidad Detrítica Basal, 2) Unidad Carbonatadada Inferior, 3) Unidad Detrítica Intermedia, 4) Unidad Yesífera Intermedia, 5) Uni-dad Detrítica Superior, y 6) Unidad Yesífera Superior. Todos estos materiales presentan muy baja permeabilidad y con-tienen aguas subterráneas con elevada salinidad. En el área estudiada se pueden distinguir dos diferentes acuíferos superficiales: 1) Acuífe-ro inferior, constituido por la Unidad Yesífera intermedia; 2) Acuífero Superior, constituido por la Unidad Yesífera Supenor. Ambos acuífe-ros quedan separados por la Unidad Detntica Superior que actúa co-mo nivel semiconfinante. El Acuífero Inferior es un acuífero libre en la mitad meridional de la zona endorreica. El flujo se dirige principal-mente hacia las depresiones y lagunas que caracterizan la zona endo-rreica, donde tiene lugar su descarga. En la mitad septentrional cons-tituye un acuífero confinado bajo las unidades Detrítica Superior y Yesífera Supenor. E1 Acuífero Superior constituye un acuífero libre en toda su extensión. Su componente principal del flujo se dirige desde el SO hacia el NE, con descarga en las lagunas y barrancos próximos a Bujaraloz.
Physico-chemical characteristics of water from a group of pools and their geological environment were studied. Results are considered in relation to groundwater flow in the area. A hydrogeological functioning scheme has been established, assuming a low permeability geological medium, and with consideration of the phreatic surface morphology. This scheme explains the origin of closed depressions, origin and dynamics of pool water and the high variability of the physico-chemical parameters. Groundwater flow digital models have reaffirmed the functioning scheme. -after English summary
Wind eroded ridges, called yardangs, occur by the millions arranged in parallel ranks and oriented to the unidirectional wind in the Western Desert of Egypt. They stand in the interzones between corridors that have been consistently windraked and lowered for many millenia while aerodynamic forces, and to lesser degree, sandblasting have segmented the intervening ridges, shaped the remnants to streamlined forms and sculptured their surfaces with beautiful burnished fluting. In size, yardangs range from a few meters in length to several kilometers long. In Egypt, yardangs occur on soft lake-bed materials to hard granite. The greatest number occur in the dense siliceous limestone on the top of the Limestone Plateau. -from Author
Lineaments belong to two main sets trending NW-SE and WNW-ESE, and two secondary sets trending NNW-SSE and NE-SW. Most of them are normal faults. Lineaments determine some of the Quaternary landscapes in the area, mainly by its influence in karst, drainage network and slope processes.
Most lakes are well flushed and the chemical constituents of their waters do not accumulate beyond the potable range. In unusual circumstances, however, the solute load may be increased and the lake then becomes saline. This is caused either by evaporation exceeding inflow or by the inflow being saline or both. Saline lakes are quite common in certain parts of the earth, but they have neither the size nor abundance of normal lakes. For this reason, few extensive studies have been made of such lakes and consequently their hydrologic, geochemical, sedimentological, and biological environments remain little explored. This is a pity, for saline lakes have much to teach us about processes under extreme conditions and, once we understand them, we will have greatly improved our understanding also of normal lakes. This paucity of studies is also surprising because saline lakes not only are of economic significance but are important in the geologic record as sensitive indicators of past tectonic and climatic events.
Deflation, mass movement and abrasion of hydro-aeolian deposits within former lake basins of the North-eastern Sahara have produced a series of spectacular yardang landforms which attest to high rates of aeolian abrasion and erosion in response to strong prevailing northerly trade winds. The deposits were previously interpreted in a descriptive fashion via both field and remote sensing survey. We analysed the yardang features of the Kharga Oasis by undertaking a morphometric survey using high precision Global Positioning System Kinematic Surveys. Yardang morphology is more complex than has previously been interpreted. Alignment of clusters of yardangs as en echelon suites of N-S swarms is commonplace. To fully understand the morphology and evolution of these features a more detailed assessment of yardang sets rather than individual forms is required. Erosion of the yardangs is observed to be occurring via aeolian abrasion at the windward tip, but more significantly via mass failures which exploit cracking within the fines-rich sediment matrix.
In this extremely arid basin of Central Asia, wind transport and corrasion are the strongest geomorphological forces. In the eastern part, wind sedimentation prevails, in the western part is the largest yardang field on earth. Weathered material is removed and the bedrock sandblasted. The bizarre hills are totally destroyed, others remain for a while as low, streamlined whalebacks. The ratio of their length, width and height is 10:2:1,; this is the same as that found in wind tunnel experiments after 200 hours. -from Authors
An aerodynamic system is far more complex than many investigators have realized and varies with every alteration in shape and detail. Therefore, in a multidirectional wind, a different aerodynamic system results on any one feature with every wind shift, because the relationship of wind direction to feature orientation is altered and so alters the division of the streamlines and the subsidiary flow patterns. This amounts to a shape change which has concomitant effects on patterns of vorticity.-after Author
Yardangs are perhaps, the most neglected and misunderstood of the Earth's landforms. The existence of large fields of yardangs ranging from meters to kilometers in length is not well known to American geologists. In the United States many early workers overstated the role of the wind in landscape development, leaving a legacy of uncertainty about the effectiveness of the wind as an agent of surface sculpture. New field and experimental modeling data suggests that deflation is far more important in yardang development than previously realized. -from Authors
Builds on the classic work by Bagnold and applies his principles in a planetary context with reference not only to Earth, but also Mars, Venus and Titan, the largest of the satellites of Saturn. The chapters are: wind as a geologic process; the aeolian environment; physics of particle motion; aeolion abrasion and erosion; aeolian sand deposits and bed-forms; interaction of wind and topography; windblown dust. In each chapter the bulk of the material is devoted to Earth but there are sections on the other planets. Appendix B describes the small scale modelling of aeolian phenomena in a wind tunnel.-K.Clayton
LE BORKOU, ASPECTS D'UN MODELE EOLIEN. - Resume. - Bordure meridionale d'un bassin sedimentaire paleozoique et mesozoique, le Borkou presente une morphologie en marches d'escalier dont les degres s'elevent du SW vers le NE tandis que les paliers sont inclines dans la meme direction. Les principaux traits du paysage de cette region desertique sud-saharienne sont donnes par l'alternance: escarpement-plateau-plaine trilogie de formes qui se reproduit plusieurs fois et qui resulte d'une action fluviatile paleoclimatique a laquelle se superpose un curieux faconnement eolien en barres, couloirs, yardangs et cuvettes hydroeoliennes, dont evolution se poursuit activement de nos jours pour donner un des plus etonnants modeles eoliens du Sahara.
Experiments and field observations indicate that both the gross shaping and the sculpturing of lineation and pit details on wind-eroded surfaces may be accomplished by the impact of particles which have been impelled aerodynamically by interfacial flow or by vorticity. The vorticity operates along lines of positive, negative, and secondary flow over all surfaces of an object. The axis of vorticity is typically normal to or at some high angle to the surface undergoing erosion. Therefore, it is not the so-called roller vortex type which has been postulated by other investigators, for one typical erosion pattern left abundantly by the vortex configuration is a round pit either helically or radically scored. Often such pits occur in chains along the beds of the lineations. A second type of erosion pattern common in channels is parallel transverse lineation, which this writer has seen in development in snow flutes under influence of normal-axis vortices. Such vortices travel singly along lines of flow, pulling particles centripetally into the vortex configurations. In snow flutes, for example, vortices become visible due to suspended snow, and the secondary flow can be delineated. Where general windflow is essentially unidirectional, the erosional result of such vorticity is the creation of cross-lineated, essentially symmetrical grooves.
An 11-year study provides data on saltating sand driven across a barren, bouldery alluvial plain by strong unidirectional wind. By weight, 50 per cent of the grains travel within 5 inches, and 90 per cent, within 25 inches of the ground. Maximum height attained exceeds 10 feet, even for grains of 3–4 mm.
Weight of each grain-size fraction decreases with height, except for particles smaller than 0.062 mm which are presumably in suspension. The weight percentage of grains smaller than 0.125 mm increases with height, that of grains between 0.25 and 0.125 mm decreases with height, and that of still larger grains peaks out at heights increasing with size. This suggests that grains smaller than 0.125 mm are affected by turbulence, and that the rebound height of particles larger than 0.25 mm varies directly with grain size. Grains between 0.125 and 0.25 mm are too large to be significantly affected by turbulence and too small for high rebound. Arithmetic mean grain size within the saltating curtain attains its maximum at heights between 10 and 40 inches, the higher values occurring with stronger winds.
Maximum wear of 0.038 inch in 10 years on a vertical lucite rod occurred 9 inches above the ground. This represents the height at which grain size, number, and velocity combine to give maximum impact energy. Common red bricks were worn 1.6 inches in 6 years and 2.25 inches in 11 years. Cubes of commercial gypsum cement (hydrocal) showed maximum wear of 1.9 inches in 10 years. Sound gneissic boulders displayed no measurable wear in 10.5 years.
Cubic hydrocal blocks became tilted upwind by ground scour, and one 6-inch cube turned over in 6 years. Smaller cubes similarly turned over and in addition fell sidewise several times in 11 years. Rotation up to 90° on a near-vertical axis accompanied some overturns. Movement of blocks was principally southward, indicating that overturns were due to unusually strong winds from the north. Wind scour is a major mechanism for the shifting of ventifacts in this environment.
Yardangs are streamlined erosional wind forms, similar in form to inverted boat hulls, that in terrestrial deserts range from meters to kilometers in length. On Mars the best examples are seen in the equatorial region. In the Amazonis region, hundreds of ridges and sawtooth-edged mesas have been wind sculptured in layered rocks. Individual ridges are tens of kilometers long with intervening valleys nearly 1 km wide. The wind-stripped surface seems to be relatively young and therefore must be easily erodible. Possible lithologies include ignimbrites, mudflows, or lithified regolith. Other wind-sculpted features occur in the Aeolis region, in Ares Valles, and in the Iapygia region. White Rock, a light-colored plateau inside a crater, is interpreted to be a yardang cluster eroded in a deposit inside the host crater. White Rock may be a jointed, wind-eroded pyroclastic deposit. Yardangs on Mars, especially when they are sculpted in young geologic units, demonstrate that much of the observed eolian erosion is recent. Yardang azimuths often are not parallel with wind streak directions, indicating that the yardangs were formed by different (older or weaker) winds from those that formed the streaks.
Aeolian erosion develops through two principal processes: deflation (removal of loosened material and its transport as fine grains in atmospheric suspension) and abrasion (mechanical wear of coherent material). The relative significance of each of these processes appears to be a function of the properties of surface materials and the availability of abrasive particles. The landforms that result from aeolian erosion include ventifacts, ridge and swale systems, yardangs, desert depressions, and inverted relief.
Three regional joint sets striking N–S, E–W and WNW–ESE affect the Tertiary rocks of the central Ebro basin. From analysis of their chronological relationships and spatial distribution, it is concluded that they correspond to two different tectonic events. The N–S set (oldest) and the E–W set (younger) are present in the southern and central sectors, while the WNW–ESE joint set predominates in the northern one. The N–S joints propagated in response to joint-normal and fluid loads under an intraplate stress field with SHmax oriented near N–S (related to forces caused by the convergence of Africa, Iberia and Europe and rifting at the Valencia trough) during the sedimentary infilling of the basin. These joints are only present in the southern part of the area. The E–W joint set in the southern-central sector records the same fracturing event as the WNW–ESE set does in the northern one. Its orientation was modified by the presence of the older N–S set in the south, which perturbed the regional stress field. The younger WNW–ESE and E–W joint sets are interpreted as unloading joints. These propagated as a consequence of flexural uplift and exhumation related to isostatic rebound at the Pyrenees and the Ebro foreland basin. A numerical approach is used to explain the inhomogeneous distribution of the N–S joint set in terms of their absence being controlled by the depth of the water table at the time of their formation.
Potassium-rich feldspars separated from aeolian sands from the East and West Cronese Basin have been dated by infra-red stimulated luminescence. Sands from shoreline-reworked aeolian dune ridges in the West Cronese Basin suggest that Late Holocene playa lakes have existed in the basin within the last 2000 years. Four dates from these ridges suggest that a phase of aeolian deposition and stabilisation occurred 150–250 years ago. A date of 250 ± 75 years on a yardang formed from horizontally bedded sand, however, implies that at least a metre of erosion in the West Cronese Basin has occurred since that time, in response to a dominant southwesterly wind. Late Pleistocene dates ranging from 18,850 to 23,350 years were obtained from an aeolian unit in the East Cronese Basin giving limiting ages on wavecut shorelines formed in the deposit. This aeolian sand appears to have been deposited and stabilised at a similar time to part of the nearby Cat Dune sand ramp.
Ventifacts are found in several physical settings in California: in formerly glaciated areas, in periglacial areas above or beyond glacier limits, in presently semiarid areas, along the coast, and in true deserts. In several localities, both active and fossil forms are found. Ventifacts and abraded surfaces develop wherever strong winds, laden with abundant sediment, erode resistant boulders or bedrock. In California, as elsewhere, the abrasive agent is most commonly a fine- to medium-grained aeolian sand. In arid areas, ventifacts occur near to Pleistocene lake shorelines, downwind of alluvial rivers, near dune fields, or in corridors of regional sand transit. They formed principally during a drier middle Holocene period from 8 to 5 ka. Staining and discoloration of the abraded face, patchy granular disintegration and spalling of abraded surfaces, and stabilized aeolian sand provide evidence for current inactivity of wind erosion. Ventifacts found on moraines beyond receding ice fronts are much older, dating from Pleistocene cold stages.
A genetic and evolutionary model is established for saline depressions in continental areas. These depressions are located in arid or subarid areas, and are developed on low permeability geological mediums (K<10 mm/day) with a lack of streams reaching the small lakes. The phenomenon of evaporation is fundamental, since it is the basic requirement for the presence or absence of a free water surface in the lake, and also for depression of the phreatic surface, which causes inflow of groundwater towards the lake. With these conditions, the proposed model includes the following stages: (i) initiation of the close depression; (ii) deepening of the depression; (iii) formation of the lake basin and the end of the deepening; and (iv) levelling and lateral extension of the lake basin. The combined effects of groundwater flows and aeolian action offer a coherent explanation for the origin and evolution both of the closed depressions found in the Ebro Valley, and of the salt lakes that subsequently form. The processes described form morphologies of oval shape with the main axis parallel to the direction of the wind, flat floors and evaporitic sedimentation, although they act on geological materials with different lithologies. © 1998 John Wiley & Sons, Ltd.
The early evidence of aridity in Australia is provided by the contraction of large lakes and extensive faunal changes towards the end of Tertiary time. Although not well dated some indication of probable age is derived from the trends of sea-surface temperature in the Southern Ocean which played an important part in the evolution of the continental climatic pattern. Early changes probably date from the growth of the west Antarctic ice cap about 10–17 m.y. ago. By 2.5 m.y. a substantial cooling had taken place and Australia's climatic pattern probably had begun to resemble that of today.
The Western Desert of Egypt is one of the most arid regions on earth and is probably the closest terrestrial analog to the surface of Mars. An expedition to the area in 1978 revealed an abundance of quartzite and basalt rocks that have been pitted and fluted by wind erosion and deflation of the desert surface. These pitted rocks are internally homogeneous, show no internal holes or vesicles, and are considered an important but neglected type of ventifact. They bear a striking resemblance to the pitted and fluted rocks seen by the Viking Landers, rocks that have generally been interpreted as vesicular basalts only slightly modified by wind erosion. Wind tunnel studies of the air flow over and around nonstreamlined hand specimens from the Western Desert show that windward abrasion coupled with negative flow, secondary flow, and vorticity in a unidirectional wind can explain the complex arrays of pits and flutes. These field and laboratory observations suggest that the pitted rocks at the Viking Lander sites are also ventifacts, and thus the Martian surface may be far more wind eroded than previously thought.
A review of the Martian eolian regime suggests that it is a more powerful agent of surface sculpture than its terrestrial counterpart. At least seven different types of probable erosional windforms are now recognized on Mars from Mariner 9 television data. Saltation is the principal mechanism by which these erosional features are produced. Suspension transport also occurs and removes fine detritus from most of the equatorial and mid-latitude regions. These fines become trapped at least temporarily in the polar regions and in local large basins and crater floors, and thus deflation of many plains surfaces is allowed to proceed. Detailed comparisons between many of the inferred erosional windforms on Mars and those of the rainless coastal desert of central Peru from the Paracas peninsula to the Rio Ica show remarkable similarities in form and scale. These similarities strengthen the conclusion that the surface of Mars has been extensively modified by the wind and that eolian erosion and sedimentation are important geologic processes. Mariner revealed a wide variety of features that can be attributed to wind activity. Sagan et al. [1972] deal with the problem of the light and dark markings that are interpreted as surficial wind streaks consisting of mobile sand. The role of the wind in producing linearions and other landforms in the polar regions is described by Cutts [1973] and by Sharp [1973a]. Depositional features produced by the wind such as dunes are the subject of a companion paper by Cutts and Smith [1973]. Mariner 4 revealed that Martian craters are generally softer and shallower appearing than
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