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Quaternary Geomorphic Processes and Landform Development in the Thar

Desert of Rajasthan

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Quaternary Geomorphic Processes and LandformDevelopment in the Thar Desert of Rajasthan

Amal Kar1

Abstract: Evolution of landforms in the Thar Desert of Rajasthan is very much influencedby the exogenic and endogenic processes operating in the region during the Quaternaryperiod. Studies have revealed that several fluctuations in climate between drier and wetterphases and periodic earth movements decided the type and intensity of geomorphicprocesses. The paper describes the broad sedimentation pattern in the desert, knownfacets of Quaternary climate and landform characteristics. It also discusses the influenceof Quaternary climate change, neotectonism and human activities on landform evolution.

IntroductionThe Thar, or the Great Indian Sand Desert, is situated in the arid western part ofRajasthan state in India and the adjoining sandy terrain of Pakistan. It forms adistinctive, but integral part of the arid lands of western India that runs through thestates of Punjab, Haryana, Rajasthan and Gujarat. The eastern limit of the desert canbe marked along the calculated moisture availability index (also called the aridityindex) of –66.6, which roughly passes through the foothill zone of the degraded,NNE to SSW-trending Aravalli mountain ranges. In the west, the desert extends upto the fertile alluvial plains of the Indus in Pakistan. The Aravalli hill ranges, whichpartially control the spatial pattern of present-day rainfall in the region, and throughit the efficiency of different geomorphic processes, were formed more than 2500million years ago. It underwent at least three cycles of orogenesis and planationsince the Proterozoic, and is now one of the oldest hill ranges in the world. TheAravalli orogenic cycles and the attendant widespread igneous activities wereresponsible for the construction of much of the basement for subsequent sedimentaccumulation. The basement in much of the desert area today is made up of graniteand rhyolite and the gneissic complex. From the upper Proterozoic period onwardsedimentation here took place in several basins, under continental and marineconditions. The identified basins are: (i) Marwar basin, (ii) Lathi basin, (iii) Jaisalmerbasin, (iv) Barmer basin, (v) Palana-Ganganagar Shelf, and (vi) Sanchor basin. These

1Central Arid Zone Research Institute, Jodhpur 342 003

Landforms Processes & Environment ManagementEditor: S. Bandyopadhyay et al.

ISBN 81-87500-58-1 2011 (223-254)

acb publicationsKolkata, [email protected]

224 Landforms Processes and Environment Management

basins, separated from each other by major tectonic features, including faults, formedtogether a shelf region that merged with the Indus geosyncline further west. Excellentreviews on the geological and structural frameworks of the Aravalli and the desertarea to the west of it are available in Heron (1953), Narayanan (1964), Sen (1970),Chatterji (1977), Dasgupta and Chandra (1978), Pareek (1984), Roy (1988) and SinhaRoy et al. (1998).

Broadly, the landforms in the Thar are fluvial, aeolian and lacustrine in nature(Fig. 1; Table 1). Fluctuating climate since the beginning of the Quaternary periodabout 2 million years ago played a major role in their evolution. There is also growingevidence of periodic tectonic activities during the Quaternary, which accelerated thesubaerial processes and left their imprints on the landforms. The basement for

Quaternary sedimentation in Rajasthan part of the Thar was essentially a vastpediplaned surface which was composed largely of the Pre-Cambrian metasedimentsand igneous in the east and a gradually thickening sedimentary deposits of Mesozoicand Tertiary periods in the west. We propose to discuss here the present understandingof landform development in the region, the related geomorphic processes, as well asthe possible forcing mechanisms. Earlier reviews on the geomorphology of the desert

Figure 1. Landforms of Thar desert, Rajasthan


are available in Ghose et al. (1977a), Allchin et al. (1978), Kar (1992, 1995), Singhviand Kar (1992), and Singh et al. (1997).

Climate change during the Quaternary and sedimentation patternThe discovery of the wide, dry bed of the Ghaggar in the northern fringe of the Thar,its suggested link with the legendary Saraswati river from the Himalayas andestablishment of the contribution of the Sutlej in its survival (Oldham, 1893), provokedthe earth scientists to search for clues on climate change and tectonism. Ghose (1964)reconstructed from aerial photographs the early integrated drainage systems in thesouth-central part of the desert between Pali and Jalor, and concluded that some timein the past the climate in the desert was wetter. Several multi-disciplinary studiessince then have established that the Quaternary climate fluctuated many time betweenwetter and drier phases (Bryson and Baerries, 1967; Singh et al. 1974; Ghose et al.,1977b; Allchin et al., 1978; Wasson et al., 1983; Singhvi and Kar, 1992, 2004; Kar etal., 2001, 2004). Observation of several shallow and deep Quaternary sequencesconfirms that the Quaternary lithofacies in the desert comprise essentially of thealternate fluvial and aeolian sedimentary deposits. Lacustrine and fluvio-lacustrinedeposits are noticed in some favoured locations. Correct assessment of land formingprocesses and chronology of events during the early part of the Quaternary is difficult,especially because of a sharp erosional contact between the Quaternary and theunderlying Tertiary/pre-Tertiary deposits, shallowness of the Quaternary sedimentthickness at many places, paucity of good exposures in the areas of deep sequences,and problems of dating the old sediments through currently available techniques.Reliable chronology of events, on the basis of radiocarbon dating of organic materialsand luminescence dating of quartz and feldspar in the sediments (Singhvi andKrbetschek, 1996), is available for the late Quaternary period only. The number ofsites explored systematically for such purposes is, however, limited. Therefore, thespatio-temporal resolution of the derived chronology of events is coarse, and needsimprovement.

So far the majority of older surviving Quaternary deposits have been found to bea mixture of sand and gravel. Signatures like cross-bedded gravel-and-pebble-richsediments over many of the Tertiary/ pre-Tertiary sequences in the western part ofthe desert suggest a wetter climate during the transition from Tertiary to Quaternaryperiod, with high intensity rainfall that was responsible for such high energy fluvialdeposition. Wadhawan (1988), Raghav (1992), Dhir et al. (1994), Wadhawan andKumar (1996), and Rakshit and Sundaram (1998) have described typical Quaternarylitho-stratigraphy from different parts of the desert and its eastern fringe. A summaryis provided in Singhvi and Kar (1992). At many places the lowermost conglomeratesconsist of boulders and gravels of quartzite and other metamorphics of Aravalliprovenance, as also granites, gneisses and sedimentaries from within the desert.However, considerable discordance was found in the sedimentation pattern, whichprovoked Wadhawan and Sural (1992) to suggest that the sedimentation took place

Quaternary Geomorphic Processes and Landform Development in the Thar Desert of Rajasthan 227

in four different neotectonically disturbed linear sub-basins, named as ‘Shahgarh-Kishangarh’, ‘Sanchor-Shergarh-Dechu’, ‘Merta-Degana-Jayal-Didwana’ and‘Bikaner-Churu-Ganganagar’. They opined that a series of NE-SW trending horstand graben structures was responsible for controlling the origin, configuration anddevelopment of these sub-basins of Quaternary deposition. These supposed sub-basinsare, however, different from those which controlled the pre-Quaternary depositionalpattern in the region, and which were described by Dasgupta and Chandra (1978).

Although the early Quaternary deposits at many places were found to containfine to medium sand of possible aeolian provenance, the paucity of purely aeolianstrata within or below the conglomerate deposits, and a sharp contact with theunderlying Pre-Cambrian or Tertiary beds still intrigue researchers about the subaerialprocesses which preceded the early Quaternary fluvial activity. Only on rare occasionsthe contact with basement is in the form of aeolian sand deposits, as for examplenear Osian (60 km N of Jodhpur) where the Pre-Cambrian Jodhpur Sandstone ofMarwar Supergroup is followed upwards by a slightly lithified, fine aeolian sand,and then by the calcreted sand and gravel. It is possible that aeolian deposits ofearlier dry phases in the desert were mostly reworked and mixed with fluvial sandand gravel during a prolonged and intense wet phase sometime during the early partof the Quaternary, which resulted in obliteration of older sand units, except at fewfavoured niches where one might come across aeolian deposits beneath the ‘basalconglomerate’.

A number of recent studies on the simulation of palaeomonsoon rainfall distributionand wind fields have enriched our knowledge on the possible process efficiency inthe past, as well as on the relationship of Indian summer monsoon with earth’s orbitalchanges and other forcing mechanisms. The simulation of monsoon variability forthe last 150 thousand years (or kyr) (Prell and Kutzbach, 1987), and othercontemporary studies encompassing the last major glacial and interglacial periods(Bryson, 1989; Sirocko, 1991; Sirocko et al., 1993; Overpeck et al., 1996; de Nobletet al., 1996; Zonneveld et al., 1997) are notable in this regard. Simulation studies ofPrell and Kutzbach (1987) suggest that the monsoon rainfall was perhaps less thanthe present-day average during the following periods (in kyr before present): 131-150, 108-118, 87-98, 60-75, 14-50. The intervening periods possibly witnessed higherthan the present-day average rainfall. Studies of oceanic cores from near Oman byZonneveld et al. (1997) reveal extremely weak SW monsoon between 17.2 and 14.7calendar kyr. Strong SW monsoon occurred between 14.7 and 11.8 calendar kyr,with several pulses of low monsoon (e.g., between 12.6 and 12.4 calendar kyr, perhapsresponding to the Younger Dryas cooling). Between the last glacial maximum (LGM)and the early Holocene, two abrupt peaks of increased monsoon activity were noticedby Overpeck et al. (1996). One was between 15.4 and 13.9 calendar kyr BP andanother was between 13.4 and 10.4 calendar kyr. They also noticed that during theHolocene period maximum monsoon intensity lagged peak insolation forcing byabout 3 calendar kyr. Sirocko et al. (1993) noticed a similar lagged response of theSW monsoon to the insolation forcing. Provided these phases are identifiable in the


sedimentary sequences in the desert, the results will help much in linking the regionalclimate dynamics with the past geomorphic processes, their spatial domains andcontribution to sedimentation pattern. For example, during the Holocene ClimaticOptimum (6 kyr) and the last major Pleistocene interglacial (126 kyr) the simulatedrainfall intensity and rainy days in south Asia was found to be significantly higherthan at present (de Noblet et al., 1996). By implication, the periods had a very effectivefluvial activity, and the imprints should be widely noticed in the stratigraphy.

During the periods when rainfall was significantly lower than at present, ariditywas widespread and the desert expanded beyond its present limit. The more severearidity took place at about 20 kyr (LGM) and ~137 kyr, when the rainfall amountswere at least 20 per cent less than the present average (Prell and Kutzbach, 1987).The dry phases were also the periods of much subdued fluvial activities when theaeolian processes were dominant. Conventionally, the efficiency of aeolian processesis thought to be synonymous with the intensity of aridity. This follows from theargument that in a desert, the decreasing rainfall generally creates conditionsfavourable for sand reactivation. The strength and duration of wind are usuallyunderplayed; often these are thought to be common in the desert. New data on thetiming of late Quaternary sand reactivation and dune formation in the Thar (Chawlaet al., 1992; Kar et al., 1998a, 2004; Thomas et al., 1999), as well as interpretation ofdata from ocean cores on the strength of SW monsoon wind in the past (Sirocko etal., 1993), now suggest that the conventional views need serious reconsideration.

Palynological evidence from a number of saline lakes in the Thar suggests that ahyperarid condition prevailed in the region from about the LGM. It continued up to~13 kyr when the summer monsoon picked up. During the Holocene the mean annualprecipitation was about twice the present value in 7.5–6.0 kyr, but it showed decreasingtendency from about 5 kyr (Singh et al., 1990). From ~3 kyr the region experienceda fluctuating climate between wet and dry phases, the notable dry phase being between0.6 and 0.3 kyr, corresponding to the Little Ice Age. Analysis of rainfall data for thelast 100 years at meteorological stations within the desert and its eastern fringe suggestsa trend towards slight increase in the margin areas (Pant and Hingane, 1988). At thesame time, rainfall in the humid eastern part of the country shows a declining trend.Such a scenario was also simulated for the past by de Noblet et al. (1996). Broadlythe stratigraphic records from the desert provide numerous evidence of climatic eventssketched above, especially as fluvial, aeolian and lacustrine deposits. We shall discussin the following sections how the geomorphic process variation and landformdevelopment in the desert are related to climate change and other forcing like tectonicactivities.

Major fluvial landformsMost landforms in the Thar are polygenetic in nature, but it is possible to classifythem according to the processes which dominated for a fairly long period. The majorfluvial landform sequence is hills and uplands - rocky/gravelly pediments (andpavements) - buried pediments (colluvial plains) - flat older alluvial plains - younger


alluvial plains - river beds. The hill slopes in the arid areas have usually a generalpaucity of debris. Concave and straight segments dominate over the convex segment.Lithological and structural variations are faithfully replicated in the slopeconfigurations, as noticed in the cuestas and mesas, as well as in the shapes of thesummits in hills formed of rhyolite, granite and other rocks. The pediments at thebase of hills usually have a slope of less than 4º, where the debris character is highlyinfluenced by local lithology. Usually the pediment slope is concave with less than2( slope, but the joint-controlled weathering and erosion along the granite pedimentsproduce a multi-convex profile. The piedmont angle is more pronounced on sandstoneand granite, but the least on rhyolite. The long history of pediplanation in thesoutheastern Thar, dating back to the pre-Tertiary era, has left the remnants of manydissected gravelly pediments without any trace of an adjacent hill. The desertpavements occur in the very dry western part of the Thar, mostly in the Bap-Phalodiand Pokaran-Chandan-Devikot-Rajmathai tract, where the surface is characterisedby closely packed gravels and pebbles. Another notable area of occurrence is nearJayal, which yielded a rich hoard of palaeolithic tools (Misra et al., 1980). Thesepavements have a broadly convex outline, with rills and gullies along their margins.Down the slope from pediments and pavements, the buried pediments/ colluvial plainsare composed of heterogeneous sediments. The thickness is more than a few metersnear the Aravallis, but it decreases gradually westward where it varies from 30 to 60cm. The flat older alluvial plains occur further downslope and are usually characterisedby zones of illuviated soft nodular kankar, or gypsum at 20 to 200 cm depth withinthe sand and alluvium. The younger alluvial plains occur downslope of the olderalluvial plains, especially along the major ephemeral channels, including the Luniand its tributaries in the southern Thar and the Ghaggar in northern Thar.

In the non-sandy areas of the very dry western part the above landform assemblageis rarely found because of the meager rainfall (


stream networks, many of which have become extinct now. Satellite images andaerial photographs show buried courses of several former streams which used toflow through the desert at different time during the early and middle Quaternaryperiods. Notable among these were the two presently extinct Himalayan streams, theSaraswati and the Drishadvati. Earlier, many scholars during the last one hundredyears identified the Saraswati as the present dry bed of the Ghaggar along the northernmargin of the Thar in Haryana and Rajasthan (Oldham, 1893; Ali, 1941). The Raini,the Wahinda and the Nara in Pakistan were identified as some of the shifted coursesof that river. A dry stream bed between Hisar, Nohar and Bhadra was identified asthe course of the Drishadvati, which is mentioned in ancient literature as a majortributary of the Saraswati.

Satellite remote sensing by Ghose et al. (1979), Kar and Ghose (1984) and Kar(1986, 1993a) suggested that the early courses of these two Himalayan rivers wereroughly through (1) the vicinity of Rajgarh, Hadyal, Ratangarh and the present misfitvalley of the Jojri; (2) Nohar, Surjansar and Samrau; and (3) Sirsa, Lunkaransar andeast of Bikaner. Subsequently, the Saraswati began to flow roughly through Nohar,Anupgarh, Sakhi (in India), Khangarh (in Pakistan), Ghantial, west of Shahgarh (inthe extremely western part of Jaisalmer district in India) and then the lower coursesof the Nara (in Pakistan). Further shifts took the Saraswati through the Raini and theWahinda and the Hakra-Nara segment in Pakistan. Finally the river ceased to floweven through that course and met the Sutlej (mentioned as the Satadru in the earlyIndian literature) via Anupgarh to the west of Ahmadpur East (in Pakistan). TheDrishadvati also gradually shifted northwestward, ultimately occupying the Narnaul-Hansi-Hisar-Bhadra-Nohar course to meet the Saraswati near Rangmahal. Vast alluvialplains were built by these streams, but their survival depended to a large extent onthe contribution of the Sutlej in the sub-Himalayan plains, neotectonism and thefluctuating climate. The Luni system, originating from the Aravalli, once drainedinto this Himalayan system. The direction of shifts in the Saraswati-Drishadvati riversystem was roughly from east to west, so the above-mentioned northeast-southwestoriented courses could be the gradually shifted courses of the Saraswati system overa period of time. The configuration of a mapped palaeochannel between Tanot andGhantiyalji in the difficult dune country in western part of Jaisalmer district (Fig. 2)was established through geophysical depth soundings and a potable aquifer waslocated (Kar and Shukla, 1993; Fig. 3). Subsequent field campaigns showed upstreamextension of this potable aquifer between Tanot and Kishangarh (Singh et al., 1994).

In the southern part of the Thar a number of palaeochannels of the Luni drainagesystem have been identified (Ghose, 1964; Ghosh, 1977; Pal, 1991; Kar, 1994, 1999a).Interpretation of a synthetic aperture radar imagery revealed some southwest-flowingto south-southwest-flowing palaeovalleys of the Luni in the alluvial plains betweenJodhpur and Pali (Kar, 1999b; Fig. 4). The westernmost palaeovalley from nearKankani cuts across the present westward flow direction of two major tributaries ofthe Luni, the Guhiya and the Bandi, implying that the present channels are partlysuperimposed on the ancestral south-southwest-flowing channel of the Luni. The


existence of the above palaeovalleys suggests that the Luni formerly used to flowthrough a number of easterly courses in the middle part of its present basin. Thepoints of deflection from the present course were near Malkosni, Mortauka andKankani. These possibly represent the successive shifted courses of the Luni. Thetrend of the SSW-oriented palaeovalley from Kankani to Sonai Lakha and beyondsuggests that further south the river used to follow the tract between a set of twomajor NE-SW lineaments through Mandawas-Vayad (to the south of Garwara-Jetpur),and flow through the vicinity of Vayad, Nilkanth, Bhavrani and Balwara to the presentcourse of the Jawai. Two lineaments tend to confine the SW-flowing course of theJawai further south. In other words, this south-flowing early course of the Luni waspartly structurally controlled. It also follows that the present course of the Luni fromKankani to Tilwara (where the river takes a sharp southward turn) and from there tothe present confluence with the Jawai, is a recent one. It is likely that some otherstreams whose courses were subsequently occupied and modified by the Luni thendrained the area. In the vicinity of Tilwara the south-flowing Lik river from Pokaranupland and beyond had a major contribution in the past (Kar, 1988a). Considering

Figure 2. Palaeochannels of the Saraswati river in western part of Jaisalmer district


Figure 3. Tanot-Ghantiyal geo-electric cross section

Figure 4. Present and palaeodrainage systems in the Luni-Bandi interfluves to the west of Bilara-Pali ( based oninterpretation of radar imagery and field survey)


that the straight line distance between the present south-flowing course of the Luniat Tilwara and the earlier SSW-flowing course at Kankani is more than 100 km, wemay assume that either a very large flood or a major tectonic event provided thenecessary energy to deflect the course of the Luni clockwise by about 40° from nearKankani, although validation of these premises will require further research.

While several large tributaries of the Luni, originating from the Aravallis, used toflow for long distances and maintained their courses to the trunk stream, most of thesmaller streams originating from the hills and rocky uplands within the desert usedto flow for shorter distances. In the present


downstream and is sustained by seasonal flow through several smaller dry valleysfrom the Aravallis. There are other evidences of such inversion of relief due tolithification of channel bed material in the Thar. In the sandy plain between Jodhpur,Ratkuriya and Osian the channel gravel of former small ephemeral streams havebeen cemented as hard lithic calcretes through ground water mineralisation, and thesecalcretes are now left as higher surfaces due to selective erosion. Aeolian sedimentsoccur both above and below many of these calcreted channel deposits. A number ofother conglomerate beds in the Luni basin, especially between Pali and Sojat, andbetween Tilwara and Sindari bear testimony to early Quaternary fluvial episodes.One of these episodes was dated to ~80 kyr (Jain et al., 1999). It is likely that muchof the alluviation took place before 40 kyr. Considering that periods of significantlyhigher rainfall during the last 150 kyr were approximately 120–130 kyr, 100–108kyr and 75–85 kyr (as deduced from Prell and Kutzbach, 1987), stream activitiesduring those periods might have been very high and of sustained nature to leave theirimprints on the landscape.

The transition from the Pleistocene to the Holocene was marked by increasedprecipitation. Palynological studies on lake sediments (Singh, 1971; Singh et al.,1974, 1990) suggest that the climate became gradually wetter from around 10 kyr.As we have mentioned earlier, the mean annual precipitation in 7.5–6.0 kyr wasabout twice the present value. The surviving streams of the period might havecontributed significantly then to the process of alluviation. Ground water recharge tothe buried palaeochannels was also increased. Geyh and Ploethner (1995) dated theground water samples from one of the pre-Ghaggar Saraswati palaeochannels in theCholistan desert of Pakistan and found that the last recharge of fresh ground watertook place between 12.9 and 4.7 kyr (i.e. before pre-Harappan), with a break between8 and 7 kyr BP. It is interesting to note that this and other recent dating of materialssuggest that the Harappan and the pre-Harappan civilisations might have flourishedunder a decreasing rainfall regime. Studies by Singh et al. (1996), Rao and Kulkarni(1997) and Nair et al. (1999) on ground water along the palaeochannels of theSaraswati, which were identified earlier in the extreme southwestern part of Jaisalmerdistrict (Ghose et al., 1979; Kar, 1986), confirm that the recharge to the aquifer didnot take place from local rainfall. The rainfall in the desert showed a decreasingtendency from about 5 kyr (Singh et al., 1990).

Tectonic activities during the Quaternary period played a significant role in theevolution of stream networks. Interpretation of satellite imagery of the Luni-Jawaiplains indicated the presence of a number of lineaments across the plains. Strikingrelationship has been noticed between many of the NNE-SSW lineaments and drainagefeatures like sudden branching and widening of channel bed (e.g. the Somesar, theSukri, the Jawai and the Sagi), disappearance and reappearance of channels (e.g. theMithri), and angular bends in the stream courses (e.g. the Sukri, the Ungti, the Bandi,the Jawai, etc.). Additionally, sudden incision of stream beds and formation of tributarygullies, or terracing along many streams like the Somesar, the Chhali, the Sukri, theJawai, the Bandi and the Sagi, especially on the upstream side of the major lineaments,


provide evidence for neotectonic movements. At least two terraces can be identifiedalong many of the above streams, as well as along the Luni to the south of Sindari.The fall from the upper to the lower terrace is often between 1.5 and 2 m, while thatfrom the lower terrace to the stream bed is between 1.0 and 1.5 m. Maximum terracerelief of 10 to 12 m has been noticed between Saila and Daman along the Jawai river(Kar, 1984, 1988b). Downstream of any major lineament the streams become braidedand show signs of shifting till the next lineament across them favours downcuttingand terracing (e.g. Bandi, Sukri, Somesar, Jawai).

If we consider the areas separated by the major NNE-SSW lineaments as individualfault blocks, the pattern suggests that recent tectonic movements uplifted some ofthe blocks, effecting local base level changes along the streams, which cross themand helped in downcutting and terrace formation on the uplifted blocks. Unable tokeep pace with the downcutting of the trunk streams the smaller tributaries on thoseblocks responded by the formation of gully networks. The adjacent blocks mighthave experienced relative down-faulting, so the streams on them responded by braidingand wider shifting of channels. Streams like the Ungti and the Mithri, which couldnot maintain their courses over these differently moving blocks were obliterated inparts and provided a segmented look. Since the amplitude of terrace relief and depthof stream dissection tend to increase from NNE to SSW, it may not be inappropriateto suggest that the lineaments represent some hinge faults which close to the northeast.The pattern of fluvial landform assemblages also suggest that the movements produceda horst and graben sequence across the Luni-Jawai plains, and possibly step faultingtowards the west, the deepest segment lying especially between the lower reaches ofthe Jawai and the Luni in the west (roughly between Bhadwi, Jhab and Guda), andits linear extension further northeast through Balwara, Raithal, Motisara and Sanwarla(Kar, 1988b). Recent studies of subsurface lithofacies and Bouguer anomaly profilesconfirm such possibilities (Bajpai et al., 1998).

The satellite images taken after the flood of 1990 in Luni basin provide supportiveevidence for neotectonically-controlled current channel processes. One of the bestevidences was found in the Balwara-Debawas-Sanwarla sector where the ephemeralstreams flowing eastward from the Siwana hills (e.g. the Luniwala and other smallerstreams) and those flowing westward from the Aravallis (e.g. the Mithri and itsdistributary networks), were suddenly lost in their own microfans along a majorlineament, in spite of the fact that the streams were in spate (Kar, 1994). All themajor streams near the confluence of the Luni with the Jawai behaved abnormallywhile crossing a major NNE-SSW lineament and its subsidiaries. For example, thefloodwater of the Sagi was divided through a number of channels while crossing amajor NNE-SSW lineament near Dhani Goliya. Further north, the Bandi water wassimilarly divided when it negotiated another parallel NNE-SSW lineament. The Jawaiwidened its course near Pantheri where the river encountered this lineament.Downstream, near Dadal, the Jawai is characterised by anastomosing channel pattern,after it encounters another NNE-SSW lineament. The other notable controls werefound in the case of the Luni to the south of Guda, where some of the former courses


of the stream were partially revived and followed some of the major E-W lineamentsthrough the vicinity of Punjaberi-Narsana-Kawatra, Piprala-Sewari, and Arniyali-Jhab-Dhani Goliya (Fig. 5). It revealed that the Luni once used to flow through amore westerly course in the area, and the course was partly controlled by E-W tectoniclineaments. A few other shifted courses of the Luni in Guda-Gandhav sector wereidentified by Pal (1991) from the signatures left after the flood of 1979.

Satellite image interpretation also suggest that the major south-flowingpalaeochannels of the Saraswati tended to follow some NNE-SSW lineaments, andare cut across by E-W lineaments. Some of these might have experiencedneotectonism. Evidence of recent tectonic movements has also been found from theeastern fringe of the desert, especially around Sambhar lake and in the Kantli riverbasin (Dassarma, 1986, 1988).

Aeolian landform developmentWhenever the aeolian processes dominated over the fluvial processes during the dryperiods, a set of new landforms was created over the existing fluvial landforms,especially as sand sheets, sandy hummocks and sand dunes. This happened not onlyin the Thar, but also in its northern fringe in Punjab and Haryana, southern fringe inGujarat alluvial plains, as well as in the eastern fringe beyond the Aravalli hill ranges.Parts of the colluvial plains and older alluvial plains were transformed into sandyundulating plains. Sand dunes and interdune plains are the other major aeolianlandforms. The saline depressions are the results of a complex interplay between thefluvial and the aeolian processes.

Presently significant aeolian erosion of the rocky/gravelly tract occurs in the verydry Pokaran-Jaisalmer-Ramgarh area, producing new materials for the aeoliandeposition. Some of the classic examples of fluting and grooving on hard rocks arenoticed in the limestone terrain around Jaisalmer where the saltating, wind-drivenquartz particles grind the softer limestone surfaces during sand storms and generatefiner particles. Ventifacts abound on the rhyolite, sandstone and limestone pedimentsand on the gravelly pavements. Over the millennia enormous quantity of medium tofine sand and silt has been produced from the hill-pediment-depositional plaincomplexes of the region through such aeolian erosion and through fluvial activities.The material has been recycled several times to construct the aeolian landforms.

The sand dunes of the Thar can be broadly classified into two groups: the old andthe new (Pandey et al., 1964; Singh, 1982). Most high sand dunes in the Thar are theold dunes. These include the presently stabilised and vegetated linear, parabolic,transverse, star, network and major obstacle dunes (Fig. 6). The average height ofthese dunes varies between 15 and 30 m. The parabolic dunes cover the maximumarea of the dune-covered landscape. The average windward, flank and leeward slopesare 2° to 4° , 8° to 12° and 22° to 24° , respectively. The dunes occur in chains and athree to four tier arrangement is noticed almost everywhere. Hair pin parabolics andchevron pattern occur in the western fringe of the field, where 3–4 km long arms join


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at a sharp angle. Towards the northeast the arms become shorter (0.5–1.0 km long)and the sharp junction at nose is replaced by a curved one. The upper middle andcrestal parts of the dunes account for about 37 per cent of the total sand volume inthe dune chains (Kar et al., 1998b; Fig. 7). Further northeast the parabolics disintegrateinto two major kinds of network dunes: (a) compound hooked dunes with transverseforms at right, and (b) large parabolic forms with network within (Kar, 1993b). Thecoalescing of forms has so long been explained by either joining of the individualsby transverse ridge formation (Singh, 1982), or through movement and overriding ofindividuals at different rates and times (Wasson et al., 1983). Our studies in theinterdune plains where new bedforms are currently forming suggest that sandaccumulation downwind of a parabolic chain can occur as isolated depositional lobeswhich subsequently tend to align themselves in the form of two near parallel sandstreaks. A major zone of accumulation occurs further downwind, astride the path ofthe wind. It captures more sand through deflation in between the pair of streaks andalso through the supply from the dune chain upwind. Gradually the nosal part of thedune is formed here. The sand streaks do also grow simultaneously and form thedune arms. In other words, the parabolic bedform can develop here without the needof migration of sets of dunes and in a manner quite different from the growth sequenceof the coastal parabolic dunes (Kar, 1993b). Another implication of the finding isthat the interdune plains in the parabolic dunefield, when subjected to sand drift,may witness within a few years’ time, new parabolic bedforms, even though thesecould be of lower height. Since the nosal part of the parabolic dune is a zone of netsand deposition and since the sand surcharged wind that blows at a relatively higherspeed through the constricted corridor between the two arms of the parabolic dune,the wind is expected to decelerate and drop its load once it is out of the constrictedcorridor. Hence, it is most unlikely that these vegetated inland parabolic dunes, whensubjected to vigorous erosion, will lead to the truncation of the nose altogether, leavinga pair of linear sand ridges. Instead, the eroded sand load will be dropped in front ofthe nose, either as another curved segment, or as low linear arms of another newparabolic bedform (Kar, 1996a).

The linear dunes occur mainly in the western part of the Thar, especially inJaisalmer region, and are oriented in the direction of the wind. The dunes were earlierthought to have originated from parabolic dunes (Verstappen, 1968; Singh, 1982),but studies confirm their development from streams of barchans in the high windenergy zone to the west of 150 mm isohyet, and from lee vortices behind majorobstructions, or along the major stream valleys through funnelling effect (Kar, 1987,1990a). The dunes are characterised by a broad convex summit. The length variesfrom more than 10 km in the extreme west of the field to 1–2 km in the east. The hightransverse dunes occur mainly to the west of Bikaner and were formed astride thepath of sand-laden wind. The slopes of the leeward, flank and windward sides of thedunes are 22° , 10° –12° and 3° –4° , respectively. The average spacing betweentransverse dune chains is 300 to 800 m. Simple and compound obstacle dunes wereformed on the windward and leeward sides of hills. The dunes are highly dissected


by rills and gullies. A number of fossil dunes occur along the wetter eastern marginof the Thar, especially between Rohtak, Sultanpur, Bandikui and Lalsot in the northeastand Idar-Langhnaj in the south. These areas now receive more than 550 mm of averageannual rainfall, but were affected by aridity during the last glacial period. The dunesdefine the maximum eastern limit of the past aeolian activities during the dry phases(Goudie et al., 1973).

In contrast to the old dunes a number of new dunes are being formed presently inthe desert. Under the natural set up the dunes are formed in high wind energy regimeof the west, especially as 1 to 8 m high barchans and 20 to 40 m high megabarchanoids.These are crescentic in shape. Smaller barchans move fast. Near Pokaran the barchansmove at an average speed of 30 m per year (Kar, 1998). The brink line of the dune is

Figure 7. Form characteristics of a parabolic dune, Thar desert (based on GPS measurements)


usually located downwind of the crest. When the dune is in its formative stages thecrest line and the brink are almost at the same place, but as the barchan capturesmore sand from the upwind direction, the distance between the brink line and thecrest increases. The simple barchanoids are first noticed near the 120 contour ofwind erosion index, where they are usually 1–3 m high and are formed in dry channelsor in other flat sandy terrain with a thick deposit of loose sand. In the western part thebarchanoids are 15–20 m high, but are not completely devoid of vegetation. Eachdune consists of a high arcuate segment in its upwind part which is partly vegetated;a field of disjointed chains of low barchanoids which occur downwind of the arcuatesegment and across the path of the dominant wind in the direction of wind. Thecharacteristic horns of the barchans are missing in these dunes (Fig. 8). Compoundmegabarchanoids occur in areas where the index value is 480 or more (i.e. extremelyhigh category). The pattern and development of bedforms within these fields of largecrescentic dunes suggest that these are the zones where sand is collected from thesurroundings by wind, processed and then released along linear strips, either alongthe crests of the old linear dunes, or as narrow zones of barchans (Kar, 1990a). Furtherdownwind the barchan strips themselves are coalesced along their path of movementto ultimately grow into linear dunes.

In the eastern and northern parts of the desert wind strength is not sufficient nowto form these dunes under natural conditions. However, at places where the naturalstability of the old aeolian landforms have been disturbed by human activities,especially around the settlements and in deep-ploughed agricultural fields, localisedcolonies of barchans can be noticed. In Ganganagar-Hanumangarh-Rawatsar areasuch low barchans are numerous in the alluvial plains. Their formation is related todestruction of the erstwhile low network dunes, followed by land levelling for irrigatedagriculture. The dunes mostly occur at the sites of the old, low dunes in the sandyplains, which are at a slightly higher elevation than the adjoining alluvial plains.Mobility of the dunes is very less. Sandy undulating plains are characterised by sandsheets of 50 to 300 cm thickness, as well as low sand streaks and shrub coppicedunes (nebkhas). The height of such hummocks seldom exceeds 5 m. The averageslope of the sandy undulating plains varies from 1° to 3° . Like the crescentic dunesthe low sandy undulations and loose sand sheets are of recent origin. In many casesthese are associated with high human activities.

Past aeolian processesFor better appreciation of the aeolian processes in the past, we shall first provide ashort overview of the current processes and their manifestations on the sandy terrain.Presently there is a distinct rainfall gradient in the Thar from east to west. Theefficiency of aeolian processes increases with decreasing rainfall from east to west,as well as with the increasing wind speed in that direction. The strength of the summermonsoon wind (SW wind) between March and July counts, rather than the feeblewinter wind (NE wind). In normal years, aeolian processes are most efficient between


March and June, or early July. The maximum aeolian activity and development ofnew aeolian bedforms without human interventions takes place mostly in the westernpart of the desert (Kar, 1993b, 1998). Wind erodibility pattern (based on sedimentcharacter and vegetation) and human pressures determine the areas of localisedacceleration to aeolian processes in the eastern fringe areas (Kar, 1996a).

Given the past spatial patterns of monsoon distribution in the region, as derivedfrom different simulations, it is likely that the pattern of rainfall gradient seen atpresent, was also present during similar climatic phases in the past. With the changein climate the isohyets were shifted across the longitudes, but the pattern of rainfallvariation remained almost the same across the desert. Wind speed, especially the SWmonsoon wind speed, also pulsated with the monsoon. Therefore, the areas underdifferent categories of wind erosion fluctuated over the time, but the broad pattern

Figure 8. Morphology of a recently forming barkhanoid chain to the northeast of Ramgarh(based on aerial photo interpretation)


remained almost the same. Aeolian stratigraphic records suggest that winter windswere not a major factor of dune building in the past. Additionally, prolonged dryweather periods, a possible reduction in vegetation type and cover, and a higheralbedo favoured increased aeolian activity.

Earlier studies suggested that the major dry phases favoured aeolian processes,and implied that aridity was a major factor of dune building. More recent studies,involving luminescence dating of the sediments, reveal that aeolian processes werepronounced for shorter time windows during the dry phases, and that maximum sandaccretion took place during the periods when monsoon wind started building up(Chawla et al., 1992; Kar et al., 1998a). LGM was possibly the time of much lessaeolian activity, because the wind strength was very less effective. Studies on oceaniccores suggest that the periods of sustained higher rainfall might have lagged theinception of higher wind speed by years or centuries. This is analogous to the synopticconditions, which we notice in the desert at present. The SW wind peaks up fromMarch onward, and the maximum speed in the central and western parts is obtainedin June-July. The rain starts by the end of June, or early July. The wind falls fromAugust, and by September the monsoon recedes. The wind, therefore, gets a smallperiod between March and early July to move the sand and build the dunes. Moresand shifting takes place in the years when the summer wind is relatively strongerand is preceded by a series of drought years, which reduce the vegetation cover.Possibly the pattern remained so in the past also; only the relative strength of SWwind changed. If we consider such a scenario at century and millennium scales in thepast, the wind got fewer periods to build up the landforms. The opportunities occurredespecially during the transition from a dry climatic phase to the better monsoons andduring the periods of drought within those better monsoon phases. Such periodswere dominated by stronger monsoon wind but rainfall amount was meager, a scenarioanalogous to that of the severe drought of 1984-87 in the Thar. The view that theaeolian processes were more efficient during the periods of stronger SW monsoongets support from the trend of the old dunes and the modern wind (Kar, 1993b).

Most high sand dunes in the Thar were last formed between 11 kyr and 18 kyr. Itis customary to suggest that linear, parabolic and transverse dune types are the vestigesof a past dry climate, but as we have shown in the preceding section, these dunes areforming even now and under different geomorphic settings. If the present is a key tothe past then the mode of formation of the present dunes should provide clues to thepast processes. Systematic study of a number of deep aeolian sand-dominatedsedimentary sequences across the desert and its wetter fringe and their luminescencedating are now helping to reconstruct the past aeolian histories. The growth of onelee side linear dune, behind a quartzite hill near Didwana, has been sequentiallydated near its keel from about 200 kyr to about 3 kyr BP (Wasson et al., 1983; Dhiret al., 1994). Several aeolian units were also dated to around 100–130, 65–75 and30–55 kyr. The time brackets are approximate, and mark the periods of enhancedsand mobilisation (Dhir et al., 2010). The last 20 kyr saw at least three major periodsof sand accretion (Singhvi and Kar, 1992, 2004; Kar et al., 1998a, 2001; Thomas et


al., 1999). The last major sand accretion took place from 2 to 0.5 kyr in the west. Inthe east it took place from 2 to ~0.7 kyr, especially at some favoured locations. Mostparts of the eastern fringe area was, however, almost free from sand accretion after~3 kyr (Singhvi et al., 1994). Studies on a transverse dune in the less than 200 mmrainfall zone revealed that the dune crest advanced by 0.3 to 0.9 m per year duringthe period 2 kyr–0.5 kyr, but now the rates of dune mobility and sand accretion are atleast four-fold higher than the geological rates, possibly because of high human andcattle pressures (Kar et al., 1998a).

Formation of saline depressionsThe saline depressions with ephemeral lakes (or the ranns) are a significant componentof the landscape assemblage in the Thar desert. Some of the major ranns occur nearDidwana, Tal Chhapar, Pachpadra, Thob, Bap, Kanod, Jamsar and Lunkaransar. Theranns are characterised by alternate layers of silt-clay and sand dominated layers, aswell as by gypsum-rich layers (Singh et al., 1974; Rai, 1990; Kajale and Deotare,1993). Studies in the Bap rann suggest that sedimentation began there during theterminal Pleistocene (Deotare et al., 1998), but the other ranns, like those at Sambhar,Didwana and Pachpadra, may hold older sediments also.

Early hypotheses on their formation like aeolian transport of salts from the Rannof Kachchh (Holland and Christie, 1909) or recession of the Tethys sea (Godbole,1952), have been disproved by later studies (Ramesh et al., 1993). Aggarwal (1957)first emphasised a riverine connection for the lakes at Pachpadra and Didwana. Ghose(1964) suggested salt deposition at the confluence of streams as the cause of theirformation. Recent studies (Kar, 1990b) revealed that many lakes in the eastern partof the desert lie at the wind shadow zone of major topographical barriers like hillswith associated linear dunes, which favoured strong deflation and created the hollowedbasins (e.g. Degana, Didwana, Tal Chhapar, Parihara). In some cases, like that atDegana, courses of former streams which used to flow away from the deflation zone,were obstructed by the linear dunes which were advancing from the flanks of thehill, and were guided to the basin. Such events initially create a fresh water lake, butwith time it turns into a saline rann (Fig. 9). Similar process-form interaction wasnoticed near Jaidu in western Thar. Blockage of ephemeral stream valleys byadvancing sand dunes, followed by accumulation of water and salt, are also importantfactors of rann formation. In the Jaldhari-Deunga tract near Mohangarh the JaisalmerLimestone beds, dipping at 2° to 5° northwards, are now being etched and groovedcontinuously by the sand-surcharged SW wind. This created a number of small, wind-aligned depressions along the dip slope, separated by micro-escarpments (Fig. 10).Enlargement of the depressions and breaching of the micro-escarpments will createan elongated depression in the rocky topography, parallel to the Kanodwala Rann,and then a new rann. It will be interesting to know if the evolution of the MithaRann, the Khara Rann, the Kanodwala Rann and the Kharariwala Rann in the vicinitywas partly controlled by such wind erosion. The ranns are partly oriented in the


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direction of the dominant SW wind, although a dry valley in the rocky terrain alsolinks these. Erosion along zones of weakness along a supposedly SW-NE trendinglineament and possible groundwater-related weathering could be the associatedprocesses of rann formation here. A number of other factors, including tectonicdisturbances, were also responsible for the formation of many other ranns. Tectonismappears to have played a crucial role in the development of Sambhar lake, while acomplex process involving disruption of fluvial regime by neotectonic activities, aswell as aeolian activities might have aided the formation of a number of smallerranns, like at Sanwarla, Nilkanth, Nosra and Daman in Luni-Jawai plains (Kar, 1988b,1995).

Other processesA slow process of rock weathering is continuing throughout the geological periods,and is preparing material for the subaerial erosional processes. Forms like weatheringpits, honeycombs, spheroidal features, tafonis and alveolar caverns in sandstone,granite and rhyolite are related to moisture availability that, surprisingly, is adequateeven in the low rainfall areas of the desert. Desert varnish occurs in the averageannual rainfall belt of 180–250 mm, and is possibly related to some microbial activities.Case hardening and box weathering features in sandstone and limestone are noticedin the less than 250 mm rainfall zone. These latter features, although common onsmaller outcrops, have also been noticed at giant, landscape scales in Jaisalmer-Ramgarh area. At places it has led to the formation of ferricreted surfaces ofconsiderable size. Splitting of boulders, granular disintegration and other forms ofmechanical weathering are common in the drier parts of the desert. In the gravellypavements to the southwest of Pokaran gravels of 2 to 30 cm length abound on thesurface and occur in a matrix of fine sand and silt-sised particles. Larger boulders arealso occasionally encountered. Numerous cleaved gravels/ boulders and theaerodynamically formed single or multi-faceted ventifacts on their surfaces beartestimony to a long period of insolation weathering and wind erosion, with littledown-slope transportation. The profiles usually show a near-surface concentrationof the gravel and other coarse particles, followed down the profile by sand and silt-rich sediments, and then the parent conglomeratic material. Such near-surfaceconcentration of coarser fragments has been noticed in many other deserts, includingthe deserts of Arabia and Australia. While a part of it can be explained as a laggravel, related to the winnowing of the finer top sediments, the lack of a gradationalcontact between the surface gravels and the parent formation below suggests thatsome other processes like long cycles of wetting and drying of the fine sedimentsand consequent forcing upwards of the gravels through desiccation cracks could beinvolved. In some areas the large gravels tend to be sorted along polygonal cracks.This mechanism is akin to the formation of stony gilgai topography in the Australiandesert (Mabbut, 1977).

Carbonate enrichment of sediments, especially in the subsurface, is a major process.


It produces a variable thickness of calcic horizon with powdery to nodular carbonates(kankar). Such pedogenic carbonates are common in the semi-arid areas also, butbecome less frequent with higher rainfall. Surface induration of carbonates as thinpatinas occur over the rocky pediments, which seal the joints and fractures, andenhance run-off. A thicker induration of calcrete, partly covered by later aeoliansand sheets, is widespread in the western part of the desert in Jaisalmer-Barmer areawhere the vast pedeplains on rhyolite, sandstone and limestone have such induration.Stratigraphic records from the desert show thick calcretes involving sand, alluviumand conglomerate. Some of these are groundwater calcretes, but some others couldhave a complicated history (Dhir, 1995; Achyuthan and Rajaguru, 1998). Withinduration and hardening the deposits put resistance to subsequent erosion, ascompared to the surrounding sandy plains. Ultimately the process creates an inversionof relief. Typical examples of such inversion of relief due to calcretisation of fluvialdeposits are noticed in Kherapa-Anwana-Nandiyan Khurd area to the east of Jodhpur,where a number of 2–5 m thick, massive cream to buff coloured calcrete bands insinuous pattern define the small palaeochannels which used to drain earlier into aSSW-flowing ephemeral stream, the Jojri.

Man as an agent of landform developmentDuring the last half of the Twentieth Century Thar desert has witnessed tremendousrise in population and the uses of land and its resources, especially for agriculture.There has been a four-fold increase in human population between the 1961 and the1991 census. The present density is 84 km–2. There has also been a significant rise inthe livestock population. To cater to the needs of the increasing human and livestockpopulations, pressure on the existing resources of the land is increasing. During thelast two decades use of tractor for ploughing on dune slopes and other sandy/shallowmarginal lands have spread even into the dry and more windy western part where thewind erosion index is more than 120 (very high). Overgrazing and other forms ofvegetation destruction are continuing. Ground water is being exploited on a widerscale, without much concern for the quality of the water. A giant irrigation scheme,the Indira Gandhi Nahar Project (IGNP), has come up in the western part of thedesert. It has transformed the agricultural scenario in the region, but unregulatedflood irrigation, land levelling in the dune landscape in the hope of good supply ofcanal water, and other forms of mismanagement of land are continuing. As we havenoted earlier, the landforms in the region are in a very fragile state due to the prevailingclimate and terrain characteristics. Consequently, such activities are accelerating thenatural geomorphic processes to such a degree that the human-induced rates ofmobility of sediments from the fluvial and aeolian landforms are more than the normalrates, and there are more spread of waterlogging and salinity-alkalinity (Kar, 1996b).For example, many of the erstwhile ‘stabilised’ old dunes have become reactivatedand have a thick cover of recent sand, which is advancing. New smaller dunes,especially barchans, are also forming in most parts of the sandy plains where none


existed earlier. Gully erosion is increasing in the sandy plains of Sikar area. In thecanal-dominated Suratgarh-Rawatsar-Hanumangarh-Pilibangan tract waterloggingand salinity-alkalinity have engulfed large areas, especially along a maze ofpalaeochannels. Some of these trends can be halted or reversed if the engineeringstructures and associated land use alternatives are suggested on the basis of a soundunderstanding of land-forming processes and vulnerability of the terrain to inducedpressures.

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