Geomorphology. Raked linear dunes in the Kumtagh Desert, China

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1 Geomorphology 123 (2010) Contents lists available at ScienceDirect Geomorphology journal homepage: Raked linear dunes in the Kumtagh Desert, China Zhibao Dong a,b,, Zhenhai Wei a, Guangqiang Qian a, Zhengcai Zhang a, Wanyin Luo a, Guangyin Hu a a Key Laboratory of Desert and Desertification, Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, China b Department of Geography, Shaanxi Normal University, China article info abstract Article history: Received 13 January 2010 Received in revised form 30 June 2010 Accepted 2 July 2010 Available online 8 July 2010 Keywords: Dune geomorphology Linear dunes Raked linear dunes Linear dunes are extensive in sand seas and dune fields around the world, but they take a range of forms due to the complex factors that control their development. Raked linear dunes, composed of primary ridges and subsidiary ridges that lie almost perpendicular to the primary ridges, were recently identified in the northern part of China's Kumtagh Desert. The primary ridges are typical linear dunes, but the subsidiary ridges display vestiges of barchan dunes. The subsidiary ridges are sufficiently short that they do not greatly affect the general appearance of the linear dunes. However, the raked linear dunes in the Kumtagh Desert have several unique characteristics that distinguish them from typical linear dunes. These dunes develop in an environment that is deficient in available sediment, and under a wind regime typical of linear dunes: an environment with a high wind energy and a directional variability index (RDP/DP) around 0.5. The raked linear dunes appear to have evolved from barchans following a modified form of Tsoar's (1984) model. Barchans formed under a northern wind regime were modified by an eastern wind regime oriented at an oblique angle to the barchans. The strengths of the two wind regimes are similar. Under these conditions, the barchans became reoriented, with the limbs farthest from the eastern winds extending to form subsidiary ridges and the limbs closest to the eastern winds forming the primary ridges, which appear to form mainly from dune collisions Elsevier B.V. All rights reserved. 1. Introduction Aeolian sand dunes result from complex interactions between a number of factors, including the wind regime, sediment availability, and presence of surface obstacles such as vegetation (Mabbut, 1977; Livingstone and Warren, 1996). A range of dune types develop as a result of differences in these control factors. In extremely arid sand seas and dune fields, where vegetation is scarce, the shapes of free dunes can usually be explained primarily in terms of the wind regime, which includes the directional variability, strength, and duration of the wind, as well as by sediment availability (Fryberger, 1979; Wasson and Hyde, 1983; Lancaster, 1994; Livingstone and Warren, 1996). Three fundamental dune categories are usually recognized: transverse (barchanoid), linear (longitudinal) and star dunes. Fryberger (1979) showed that transverse dunes were associated with unimodal wind regimes, linear dunes with wide unimodal or bimodal regimes, and star dunes with obtuse bimodal regimes or complex regimes. Wasson and Hyde (1983) showed that dune types occur in areas uniquely defined by both the equivalent sand thickness of the sediments held in the dunes (sediment availability) and a measure of Corresponding author. Cold and Arid Regions Environmental and Engineering Research Institute, Chinese Academy of Sciences, No. 322, West Donggang Road, Lanzhou, Gansu , China. Tel.: ; fax: address: zbdong@lzb.ac.cn (Z. Dong). directional variability of the main sand-moving winds. They stated that barchans occurred where there were little sand and almost unidirectional winds; transverse dunes occurred where sand was more abundant and the wind was only moderately variable; longitudinal dunes occurred where the winds were more variable but there was little sand; and star dunes occurred where sand was abundant and wind variability was at a maximum. However, these descriptions only provide simplified general views of a highly complex process. The complex dune morphologies that appear in the field often make it difficult to explain their formation. For example, transverse dunes can be subdivided into transverse, barchan, dome, and reversing types, and linear dunes be subdivided into sand-ridge, seif, and other types (Livingstone and Warren, 1996). As a result of the complex interactions between the factors that control dune formation, convincing explanations of dune formation are only available for a few simple dune types, such as typical barchan dunes and barchanoid ridges, despite more than a century of unremitting research (Pye and Tsoar, 1990; Lancaster, 1995; Livingstone and Warren, 1996). The formation of most dune types has only been hypothesized, usually based on overly simplistic assumptions. To some extent, this situation can be attributed to insufficient meteorological data, because most sand seas and dune fields are too remote to permit detailed, long-term collection of scientific data. Linear dunes that are generally characterized by their considerable length, relative straightness, parallelism, regular spacing, and low ratio of dune to inter-dune areas are more disputed than other dune types (Tsoar, X/$ see front matter 2010 Elsevier B.V. All rights reserved. doi: /j.geomorph

2 Z. Dong et al. / Geomorphology 123 (2010) ; Lancaster, 1995) because they have several varieties such as lee dunes, seifs and vegetated linear dunes, each of which has a different shape and mechanism of formation and elongation (Tsoar, 1989; Livingstone, 1996). Moreover, there are other varieties to be found. China's Kumtagh Desert remained largely unexplored until recently. As a result, mysteries about the unique dune geomorphology in this desert are being uncovered. The unique linear dunes in the north of the Kumtagh Desert led researchers to use other terms such as feathery dunes, pseudo-feathery dunes to characterize them (Qu et al., 2007; Dong et al., 2008). Dong et al. (2008) analyzed morphology of the seif dunes in the north of the Kumtagh Desert and proposed a tentative interpretation about their formation in accordance with Bagnold's (1941) evolution model. However, further investigation revealed that even the seif dunes of the Kumtagh Desert exhibited several varieties. A unique raked form of linear dunes (Fig. 1) is identified and its formation cannot be accounted for by Dong et al.'s (2008) simplified explanation. In the present paper, we attempt to provide a preliminary analysis and discussion of the morphology and development of these raked dunes based on field investigations, remote-sensing images and detailed wind information. 2. Physiographic settings The Kumtagh Desert is located in the hyper-arid area of Northwest China, between E and E longitude, and between N and N latitude, covering an area of 22,800 km 2.Itis bounded by the Lop Nur Depression to the northwest, by an eastern branch of the Tianshan Mountains to the north, and by the Altyn Tagh mountains, which have been experiencing rapid uplift since the late Pliocene, to the south (Li et al., 1979) (Fig. 2). Sand dunes have developed on sediments that mainly originated from the Altyn Tagh mountains and have been subsequently reworked by winds that primarily flow from the north and east. Kumtagh means sand mountains in the Uighur language; the name derives from the general appearance of the dune fields that cover the alluvial diluvial plain north of the Altyn Tagh Mountains, which slopes downward from southwest to northeast. There exist pronounced spatial variations in dune geomorphology. Dune types become more complex and dune height increases moving from north to south. Linear dunes have developed in a continuous area of about 3400 km 2 in the northern part of the desert, whereas star dunes, compound dunes, and complex mega-dunes have developed in the south (Dong et al., 2009). The morphology of the linear dunes also exhibits spatial variation, with the raked linear dunes having developed in an area of about 2400 km 2. The region's annual precipitation averages less than 30 mm. Six wind towers were established in 2007 to provide data on the groundlevel wind-flow patterns that would explain the formation of sand dunes and their spatial variation. Two wind towers (W1 and W2 in Fig. 2) were located about 9 km apart in the area of linear dunes, and W2 lies at the eastern edge of the raked linear dune area. We estimated drift potential using the method proposed by Fryberger (1979). Fig. 3 shows that the linear dunes and raked linear dunes in the study area developed under an environment with high wind energy, and that the drift potential (DP) ranged from 448 to 528 VU. The resultant drift direction (RDD) varied between 33 and 44, and the directional variability index (RDP/DP) is around 0.5. This is a typical wind environment in which linear dunes evolve, which agrees with Fryberger's (1979) conclusions. 3. Morphology and morphometry of the raked linear dunes Raked linear dunes are mostly distributed north of N. Typical raked linear dunes evolve into other dune types with increasing distance south from this area. We analyzed their morphology and morphometry based on field investigations, aerial photographs, and high-resolution images provided by Google Earth. Fig. 1 shows that a raked linear dune is composed of a primary ridge and subsidiary ridges that are oriented almost perpendicular to the primary ridge. It is worth noting that all the subsidiary ridges in the study area developed on the northwestern flank of the primary ridge Primary ridges Fig. 1. Typical landscape of raked linear dunes in the northern Kumtagh Desert. (A) An overview of the geomorphology. (B) A close-up of a single dune. (Location of the center of the photograph: N, E.) The primary ridges are linear dunes that extend several hundred meters to several kilometers, with the longest reaching several tens of kilometers in length. The dunes are generally aligned from NE55 to SW235 north of N, and from NE40 to SW220 south of N, with the orientation rotating counterclockwise moving from north to south in response to changes in the local wind regime. The dunes are usually sharp-ridged, and their ridges undulate (rise and fall vertically) so that the raked linear dunes take on a moniliform shape, retaining the vestiges of barchans and suggesting a genetic relationship with barchans. In most cases, the raked linear dunes clearly show (Fig. 1B) that the primary ridges form as a result of linking of the southeastern limbs of barchans with the stoss slopes of barchans located farther downwind. The crests of the barchans form the rises of the raked linear dunes and the lower stoss slopes of the barchans constitute the falls. The barchans that combine to form the raked linear dunes show a long-axis oriented from NE80 to SW260. A small number of raked linear dunes show connections between the extending southeastern limb of an upwind barchan with the northwestern limb of a downwind barchan (Fig. 4). A few raked linear dunes show coalescence of the extending southeastern limbs of the barchans. Although raked linear dunes in the Kumtagh Desert developed under a typical wind regime that produces linear dunes, they have some unique characteristics compared with other linear dunes. First, they lack the typical spatial patterns of such dunes, which are characterized by the strong correlation between height and spacing that has been found

3 124 Z. Dong et al. / Geomorphology 123 (2010) Fig. 2. The location of the raked dune fields and wind towers (W1 and W2) in the present study. for typical linear dunes (Lancaster, 2006). Fig. 5 shows a lack of correlation between height and spacing for the primary linear ridges of the raked linear dunes based on measurements from aerial photographs. Similarly poor correlations between height and spacing were also found for other linear dunes in this desert (Dong et al., 2008). Ewing et al. (2006) introduced cumulative frequency plots of dune spacing and crest length to create statistical populations, and used this approach in pattern analysis of dune-field parameters. They recognized discrete dune populations as line segments separated by inflection points in the cumulative frequency plots for the dune spacing and crest length measurements. They found that the presence of single statistical populations characterized simple dune fields, whereas multiple populations characterized compound and complex dunes or dune fields. We adopted their pattern analysis method to analyze the height and spacing of the primary ridges of raked linear dunes in our study area. Fig. 6 shows that the cumulative probability plots of both primary ridge height and spacing have two segments separated by an inflection point. A transitional segment occurs between the two segments in both graphs, but is narrow. The inflection points occur about at a height of 6 m and a spacing of 300 m. Hence, two populations can be identified for both height and spacing. The inflection points have morphometric significance, but do not correspond to each other. The raked linear dunes occur in bunches due to the truncation of some primary ridges, so that their spacing can be divided into a within-bunch spacing and an inter-bunch spacing. The inter-bunch spacing is usually greater than 300 m, and the withinbunch spacing is usually less than 300 m, though some overlap occurs. The overlap is partly due to the approximate definition of the two spacing populations. The primary ridges taller than 6 m usually have well-developed subsidiary ridges, whereas those shorter less than 6 m- high usually lack well-developed subsidiary ridges, extend no more than 1 km, and are truncated at both ends. Although these results are not precise, the pattern analysis method of Ewing et al. (2006) clearly provides some significant information on dune morphometry. We also found that the orientations of the raked linear dunes deviated from the predicted resultant drift direction (RDD). The orientations of the dunes are rotated clockwise compared with the resultant drift direction, showing that the eastern group of winds is Fig. 3. Drift potential roses calculated at two sites in the area of raked dunes. Site W1: wind tower W1. Site W2: wind tower W2.

4 Z. Dong et al. / Geomorphology 123 (2010) Fig. 4. An example of the connection of the extending southeastern limb (A) of an upwind barchan with the northwestern limb (B) of a downwind barchan, producing raked linear dunes that show the connection between these limbs. (Location of the center of the photograph: N, E.) more significant than the northern group of winds in shaping the raked dunes. This is confirmed by the sand-driving-wind rose calculated using data from a wind tower established in 2004 in the northern part of the Kumtagh Desert (Qu et al., 2007) Subsidiary ridges Subsidiary ridges are modified limbs of barchans (Fig. 1) that have clear slipfaces, and are arranged in a ladder pattern. They are almost perpendicular to the primary ridges, and extend 20 to 80 m from the ridges. In most cases, the subsidiary ridges are the elongated northwestern limbs of the asymmetrical barchans that constitute the primary ridges. A small number of subsidiary ridges are newly developed barchan and transverse dunes on the northwestern flanks of primary ridges. Some subsidiary ridges are straight, but most are curved. Some are rounded, but others have a turning point that results from the response of slipfaces to winds from several directions. Subsidiary ridges are spaced at 30 to 140 m. Some are regularly spaced, but others appear to be randomly spaced. Their height and spacing are poorly correlated (Fig. 7), which differs from the strong correlation that characterizes typical barchan and transverse dune fields. We used Ewing et al.'s (2006) pattern analysis method to obtain more geomorphological information about these ridges. Fig. 8A shows that the cumulative probability plots for subsidiary ridge height have three segments, separated by two inflection points. Hence, the height of the subsidiary ridges can be roughly divided into three statistical Fig. 6. Cumulative probability distribution plots for (A) height and (B) spacing of the primary ridges of the raked dunes. populations. The first height population has heights of less than 3 m, representing newly developed barchans on the northwestern flanks of the primary ridges. The second height population ranges from 3 to 5 or 6 m in height, and represents the majority of the subsidiary ridges that formed from modified barchans. The third height population has heights greater than 5 or 6 m, and represents several subsidiary ridges at the end of large raked linear dunes and those that formed by the convergence of barchans in two directions. Fig. 5. The relationship between the height and spacing of the primary ridges. Fig. 7. The relationship between height and spacing for the subsidiary ridges.

5 126 Z. Dong et al. / Geomorphology 123 (2010) Tsoar, 1990), but there is rich evidence that barchans may evolve into linear dunes both on the Earth and on other planets such as Mars and planetoids such as Titan, and the barchan asymmetry that results from extension of one limb downwind supports this hypothesis (Bourke, 2010). Four causes of barchan asymmetry have been identified (Bourke, 2010): responses to bidirectional winds, dune collisions, asymmetry in sediment availability, and variations in topography, such as local breaks in the slope. The possibilities of asymmetry in sediment availability and of variations in topography can be excluded from the causes of raked linear dunes in the Kumtagh Desert. Asymmetrical sediment availability has been proposed mainly based on Rim's (1958) laboratory experiments. Only Lancaster (1982) has provided potential field evidence to suggest that the increased availability of sediment near a dune field in the Namib Desert caused asymmetry in the barchan limbs closest to the margin of the sand sea. In accordance with Lancaster's results, the southeastern limbs of the barchans that constitute the primary ridges of the raked dunes should be extended because sediment is more available in the south, as is suggested by the fact that the dunes increase in size and the dune fields become more continuous in the southern part of our study area. However, in most cases, the northwestern limbs have extended to form subsidiary ridges, though some extended southeastern limbs were also found. The interaction between sediments and local topography could distort dune shape and lead to asymmetry (Long and Sharp, 1964). However, it seems unlikely that variations in topography cause the asymmetry of barchans to evolve into the raked linear dune field in the Kumtagh Desert, because few topographic variations can be found in the study area. There are several old stream channels that have been exposed, and that are evident as gravel bodies (Dong et al., 2010), but they show little influence on the overall development of the raked linear dunes due to their limited distribution. This suggests that the asymmetry of the barchans and their evolution into raked linear dunes are caused primarily by the wind regime and by dune collisions. Fig. 8. Cumulative probability distribution plots for (A) height and (B) spacing of the subsidiary ridges. Fig. 8B shows that the spacing of the subsidiary ridges is divided into two segments, with a narrow transitional segment. The first population is spaced at less than 80 or 90 m, and accounts for more than 80% of the subsidiary ridges. The second population is spaced at more than 90 m, and represents subsidiary ridges that developed near the ends of primary ridges and the northwestern flanks of relatively widely spaced raked linear dunes where sediment availability is relatively low. 4. Formation of raked linear dunes The above description of the morphology and morphometry of raked linear dunes indicates that they are a unique variety of linear dunes which are composed of primary ridges and subsidiary ridges. What processes are responsible for the development of such a unique dune type? Four questions concerning the morphology and morphometry must be answered to clarify the formation of raked linear dunes: (1) Why do the raked linear dunes retain clear vestiges of the barchans that form the primary ridges? (2) Why have the original barchans evolved into raked linear dunes arranged in a ladder-like pattern? (3) Why have all the subsidiary ridges developed on the northwestern flanks of the primary ridges? (4) Why do the primary ridges lack the strong correlation between height and spacing that characterizes typical linear dunes? The remains of the barchans on the primary ridges suggest that the raked linear dunes have evolved from barchans. Several hypotheses have been proposed to explain the formation of linear dunes (Pye and 4.1. Dune collision Dune collisions include both the effect of the proximity of an upwind dune to a downwind dune and the merging, absorption, and lateral linking of adjacent dunes (Bourke, 2010). Previous researchers have noted that dune collisions had the potential to distort dune morphology (Close-Arceduc, 1969; Grolier et al., 1974). Bourke et al. (2009) suggested that collisions between small barchans and larger downwind barchan limbs contributed to the elongation of the downwind barchan limbs. This kind of dune collision does not appear in the raked linear dune field in the Kumtagh Desert. However, there are extensive examples of barchan collisions of similar size. Fig. 9 Fig. 9. Collision and merging between an upwind southeastern limb and the windward slope of the downwind dune to form primary ridges. (Location of the center of the photograph: N, E.)

6 Z. Dong et al. / Geomorphology 123 (2010) Fig. 10. Models to explain the evolution of barchan dunes into linear dunes (thickness of the arrows indicates the relative strength of wind). (A) Bagnold's (1941) model (after Bourke et al., 2009). (B) Tsoar's (1984) model. provides an example of how the collision and merger between an upwind southeastern limb and the windward slope of the downwind dune has formed the primary ridges. Similar dune collisions have been reported for asymmetrical dunes in Antarctica and the Sudan (Bourke et al., 2009; Bourke, 2010). There is also dune collision in which the southeastern limb of a larger upwind barchan merges with the southeastern limbs of smaller downwind barchans to form primary ridges and with the northwestern limbs to form subsidiary ridges. At present, except for a few specific examples, little is known about the role of such collisions in turning barchans into linear dunes (Bourke et al., 2009) Wind regime We have attributed the formation of raked linear dunes in the Kumtagh Desert to dune collision and wind regimes, but the latter is more important because dune collisions can only occur under specific wind regimes, and their role in shaping dunes changes when the wind regime changes. Fig. 3 shows that the annual drift potential (DP) and wind direction variation index (RDP/DP) represent typical bidirectional wind regime of linear dunes. It is hard to explain the uniqueness of raked linear dunes solely according to the average annual wind regime. Further analysis reveals that significant differences in wind groups may be responsible for the raked linear dunes. Fig. 3 shows that there are three main groups of sand-driving winds in the raked linear dune field of our study area: the first group (which we have called the northern group) includes the NNE and NE winds, the second (eastern) group includes the ENE and E winds, and the third (western) group includes the W and WSW winds. The northern and eastern groups are dominant. Wind regime varies with season. Spring and summer are dominated by the northern group wind, autumn is dominated by the eastern group wind, and winter is dominated by the western group wind (Fig. 3). Predominantly bidirectional wind regimes are considered to cause the preferential extension of a barchan's downwind limb to form linear dunes. However, the effect of variable wind directions and strengths has been discussed by researchers in terms of two models (Bagnold, 1941; Tsoar, 1984). In Bagnold's model, the windward barchan limb is extended by the dominant wind, and is sustained and enhanced by gentler winds that blow parallel to the barchan form (Fig. 10A). Dong et al. (2008) proposed tentatively that the formation of seif dunes in the northern Kumtagh Desert could be explained by the evolution model proposed by Bagnold (1941) according to limited wind data. But detailed wind data obtained in the last two years reveals the formation of raked linear dunes better follows Tsoar's (1984) model. Tsoar (1984) proposed that the long axes of the barchans are oriented parallel to the strongest wind regime and are modified by the gentler winds that blow at an oblique angle to the barchan. The limb farthest from an approaching gentle wind extends, whereas the limb closest to the wind becomes eroded (Fig. 10B). The longer limb is extended by the oblique winds in a manner similar to the development of seif dunes (Tsoar, 1978, 1982, 1983). The effect of wind regime on the formation of raked linear dunes can be explained by a modified version of Tsoar's (1984) model (Fig. 11). In this model, barchans that formed under the influence of the northern group of winds are modified by the eastern group of winds, which blow at an oblique angle to the barchans. As a result, the barchans are reoriented, with the limbs farthest from the approaching eastern group of winds extending to form the subsidiary ridges and the limbs closest to the eastern group of winds being eroded. This explains why the subsidiary ridges develop on the northwestern flanks of the primary ridges. The present model differs from Tsoar's model in three aspects. First, the difference in the strength of the two wind groups in our model is not as large as in Tsoar's model. This at least can explain why the alignment of the raked linear dunes is rotated clockwise compared with the resultant drift direction. Second, the eastern group of winds not only extends the limbs farthest from the wind and erodes the limbs closest to the wind, but also reorients the barchans. Consequently, the modified barchans are oriented neither parallel to the northern group of winds nor parallel to the eastern group of winds. Instead, they are oriented from roughly SW255 to SW260 towards roughly 75 NE to 80 NE. Third, the extension of the barchan limbs is very limited, leading to the formation of short subsidiary ridges with bright tailing dune-like drifts due to a deficiency in sediment availability. Dong et al. (2008) suggested that the lag sediments in the corridors between linear dunes in the northern Kumtagh Desert were difficult to erode, that few dunes developed under these conditions, and that only bright dune-like drifts with indistinct height differences from their surroundings formed. The western group of winds also plays a role in shaping the raked linear dunes in that they help to accumulate sediments on the primary ridges. Although the raked dunes developed in an area deficient in sediment availability, they appear to be in dynamic equilibrium with the wind regime. Erodible sediments move between the dunes, as indicated by the bright dune-like drifts in the inter-dune corridors. Both erosion and deposition occur on the dune ridges. Because the western groups of winds are usually not strong enough to cause significant erosion on the ridges, but can to some extent counter Fig. 11. A modified version of Tsoar's model that can explain the formation of the raked dunes in the Kumtagh Desert. The barchans form under the dominant influence of the northern group of winds (N), then the eastern group of winds (E) causes erosion of the limbs closest to the wind and elongation of the limbs farthest from these winds.

7 128 Z. Dong et al. / Geomorphology 123 (2010) forming the primary ridges as a result of dune collision. The proposed role of collision in turning barchans into linear dunes will require further study, since there is currently little understanding of this process. Dune types in the Kumtagh Desert exhibit spatial variation that depends on the regional variation in the factors that control dune formation. A comprehensive investigation of the spatial variation in dune types and in the corresponding control factors will provide a deeper understanding of how the raked linear dunes developed in part of the study area. Acknowledgments Fig. 12. An example of the convergence of bright dune-like drifts (arrows) and their relationship to linear dune formation. (Location of the center of the photograph: N, E). sediment transport from the primary dune ridges by the eastern group of winds, they help sediment to accumulate on the primary ridges. Based on these observations, we can propose an explanation for why the original barchans are arranged in a ladder-like pattern along strips. Fig. 12, which shows the raked linear dunes and bright dunelike drifts at the northern edge of the raked linear dune field, provides some clues to the relationship between bright dune-like drift and raked linear dunes. The dune-like drifts are convex roughly towards the resultant drift direction, and each can be divided into two segments. The southern segment represents the action of the eastern group of winds, and the northern segment represents the action of the northern group of winds. A convergence zone parallel to the primary ridges of the linear dunes is formed by these two contiguous segments. Dunes develop when there is sufficient accumulation of sediments in the convergence zone. 5. Conclusions Recent field investigations in China's last explored desert, the Kumtagh Desert, in combination with high-resolution satellite images provided by Google Earth and aerial photographs, enable us to identify a new variety of linear dunes, which we have called raked linear dunes. The geographic location of the Kumtagh Desert has produced complex spatial variations in the factors that control dune geomorphology. The raked linear dunes have developed in the northern part of the desert, where the wind regime is a relatively simple bidirectional pattern (with a minor third component), where sediment availability is relatively low, and where topography has had little influence on dune development. The raked linear dunes in the Kumtagh Desert have several unique characteristics compared with other linear dunes, even though they developed under a wind regime typical of other linear dunes. The relative strength of the wind from different directions appears to have played a significant role in shaping these dunes. The effect of the region's wind regime on the formation of these dunes can be explained by a modified version of Tsoar's (1984) model. Barchans that formed under the influence of the northern group of winds are modified by the eastern group of winds, which blow at an oblique angle to the barchans. As a result, the barchans are reoriented, with the limbs farthest from the approaching eastern group extended to form subsidiary ridges and the limbs closest to these winds eroded to We gratefully acknowledge the funding received from the National Science Foundation of China ( ). We thank the staff of Google Earth for the use of high-resolution satellite images. References Bagnold, R.A., The Physics of Blown Sand and Desert Dunes. William Morrow & Company, New York. Bourke, M.C., Barchan dune asymmetry: observations from Mars and Earth. Icarus 205, Bourke, M.C., Ewing, R.C., Finnegan, D., McGowan, H.A., Sand dune movement in Victoria Valley, Antarctica. Geomorphology 109, /j.geomorph Close-Arceduc, A., Essai d'explication des formes dunaires sahariennes. Etudes de Photo-Interpretation. Institut Geographique National, Dong, Z., Qian, G., Yan, P., Su, Z., Gravel bodies in the Kumtagh Desert and their geomorphological implications. Environmental Earth Science 59, Dong, Z., Qu, J., Lu, J., Qain, G., Luo, W., Wang, X., Zhou, Q., Geomorphic Map of the Kumtagh Desert. Science Press, Beijing. Dong, Z., Qu, J., Wang, X., Qian, G., Luo, W., Wei, Z., Pseudo-feathery dunes in the Kumtagh Desert. Geomorphology 100, Ewing, R.C., Kocurek, G., Lake, L.W., Pattern analysis of dune-field parameters. Earth Surface Processes and Landforms 31, Fryberger, S.G., Dune forms and wind regime. In: McKee, E.D. (Ed.), A Study of Global Sand Seas. U.S. Government Printing Office, Washington, pp Grolier, M.J., Ericksen, G.E., McCauley, J.F., Morris, E.C., The Desert Landforms of Peru: A Preliminary Photographic Atlas. USGS, Flagstaff, AZ, p Lancaster, N., Dunes on the Skeleton Coast, Namibia (South West Africa): geomorphology and grain size relationships. Earth Surface Processes and Landforms 7, Lancaster, N., Dune morphology and dynamics. In: Abrahams, A.D., Parsons, A. (Eds.), Geomorphology of Desert Environments, pp Lancaster, N., Geomorphology of Desert Dunes. Routledge, London. 290 pp. Lancaster, N., Linear dunes on Titan. Science 312, Li, J., Wen, S., Zhang, Q., Wang, F., Zheng, B., Li, B., The time, magnitude and modes of uplift of the Qinghai Tibetan Plateau. Science in China (Series A) 6, (in Chinese). Livingstone, I., Warren, A., Aeolian Geomorphology: An Introduction. Addison Wesley Longman Limited, England. Long, J.T., Sharp, R.P., Barchan-dune movement in Imperial Valley. California. Geological Society of America Bulletin 75, Mabbut, J.A., Desert Landforms. ANU Press. Pye, K., Tsoar, H., Aeolian Sand and Sand Dunes. Unwin Hyman Ltd. 396pp. Qu, J., Liao, K., Zu, R., Xia, X., Jing, Z., Dong, Z., Zhang, K., Yang, G., Wang, X., Dong, G., Study on the formation of feather-shaped sand ridge in Kumtagh Desert. Journal of Desert Research 27, (in Chinese with English abstract). Rim, M., Simulations by dynamical model, of sand tract morphology occurring in Israel. Bulletin of Research Council of Israel 7-G, Tsoar, H., The Dynamics of Longitudinal Dunes. Final Technical Report, US Army. European Research Office, London. Tsoar, H., Internal structure and surface geometry of longitudinal (seif) dunes. Journal of Sedimentary Petrology 52, Tsoar, H., Dynamic processes acting on a longitudinal (seif) sand dune. Sedimentology 30, Tsoar, H., The formation of seif dunes from barchans a discussion. Zeitschrift fur Germanistik 28, Tsoar, H., Linear dunes forms and formation. Progress in Physical Geography 13, Wasson, R.J., Hyde, R., Factors determining desert dune type. Nature 304,