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INTRODUCTION

The Ganga basin, an important constituent of the Himalayanforeland, formed as a consequence of the India-Asia collisionprocesses that were initiated in the Palaeogene. The Ganga isthe axial river of the basin, and originates in the Himalayanorogen, being joined by a number of major Himalayantributaries including the the Yamuna, Ramganga, Ghaghra,Gandak, Kosi, and Tista before draining into the Bay of Bengal(Fig. 1). The mighty Brahmaputra also meets the Ganges andforms a major deltaic depocenter in the Bengal basin. A numberof tributaries join the Ganga system from the Indian cratonand Deccan Basalt terrain such as the Chambal, Betwa andKen (which join the Yamuna), Son, and Punpun. Since theinception of the basin and due to the continuing rise of theHimalaya, the rivers draining the Ganga basin have formed alarge conduit of sediment transfer from the Himalaya to theGanges delta. Although the bulk of the sediments aretransferred to the Ganges delta, considerable sediment hasaccumulated in the Ganga plains to generate a thick fill offluvial sediments which may be up to several kilometers thickin various parts.

The study of the Quaternary sedimentary fill of the Gangabasin, including sedimentary architecture, and the process-

Himalayan Geology, Vol. 26 (1), 2005, pp. 223-240, Printed in India

Late Quaternary geology and alluvial stratigraphy of the Ganga basin

R. SINHA1, S. K. TANDON2, M. R. GIBLING3, P. S. BHATTACHARJEE1 & A. S. DASGUPTA1

1Engineering Geosciences Division, Indian Institute of Technology Kanpur 208018, India2Department of Geology, University of Delhi, Delhi 110007

3Department of Earth Sciences, Dalhousie University, Halifax, Nova Scotia, B3H 3J5, Canada

Abstract: The Ganga basin in the Himalayan foreland is a part of the world’s largest area of modern alluvial sedimentation

and supports a population of over 200 million people. The Ganga plain deposits not only provide a modern analogue for

the ancient fluvial sequences of the Himalayan foreland basin but they also provide one of the most significant continental

records for understanding the interplay of climate, tectonics and eustatic changes in generating thick sedimentary fills in

a monsoon-dominated foreland system. Given the large dimensions of the Ganga basin and the lack of an integrated

approach, the available data are fragmentary, and several important questions regarding the sedimentary architecture and

the process-form relationships of the parent rivers remain unanswered. This paper reviews the available information on

the near-surface Late Quaternary stratigraphy of the upper, middle and lower Ganga basin, and reports new results based

on studies of river cliff sections and shallow boreholes down to ~50 m depth. We record significant variation in

sedimentary architecture between the upper and middle Ganga plains in the western and eastern parts of the basin,

respectively. This variation reflects geomorphic diversity (linked especially to precipitation gradients) and tectonic

history in the frontal orogenic areas which, in turn, impact sediment supply into the basin. Many of the alluvial

sequences in the upper Ganga basin are interrupted by discontinuities that probably reflect Quaternary climatic

fluctuations. In addition to controlling the long-term accumulation rate, tectonics may have had some local influence, and

the effects of eustatic changes were almost certainly limited to the deltaic region of the lower Ganga plains, extending to

a maximum of 300-400 km landward.

form relationships of rivers which deposited the fill have gainedenormous importance during the last several decades. Thegeological perspective of such studies has been to developmodern analogues for ancient fluvial sequences in theHimalayan foothills and below the modern plains, as well asto understand the interplay of climate, tectonics and eustaticchanges in generating this thick sedimentary fill. Given thelarge dimensions of the Ganga basin, the available data arefragmentary and limited. To assess our understanding, a fewsignificant questions may be considered:a) For how long have the present-day large rivers, for

example the Ganga and Yamuna, been in their presentposition?

b) How wide are the present-day river valleys in the Gangabasin and how thick are their valley fills?

c) What are the sedimentation patterns of these depositsin the valleys and on interfluves? Discontinuities havebeen recognized in the interfluve stratigraphy but howextensive are they and what sort of time gaps do theyrepresent?

d) How have these river systems and basins been affectedby Quaternary climate change and monsoonalfluctuations, active tectonics and earthquakes, and sea-level change?

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The scarcity of basin-wide data and the lack of integratedgeomorphologic, stratigraphic, sedimentological, mineralogicaland geochemical approaches have precluded comprehensiveanswers to these questions. Most of the previous studies(Kumar & Singh 1978, Singh et al. 1997, 1999a,b, Sinha et al.2002, Srivastava et al. 2003a,b) in the Ganga basin haveinvolved the study of near-surface sediments from exposedsections or shallow auger holes. These studies are from widelyseparated sections, and no detailed correlation schemes couldbe evolved. A process-based understanding has been lackingbecause the evolution of the present-day landscape and itsgeomorphic diversity have not been integrated in such studies.This paper reviews the present state of knowledge of theQuaternary geology and alluvial stratigraphy of the Gangabasin and presents our recent research in parts of the plains.The latter research has involved an integrated approach ofsurface geomorphology, near surface stratigraphy, and thestudy of drill cores from shallow boreholes coupled withdetailed sedimentological and geochronological analysis.

GEOLOGICAL EVOLUTION OF THE GANGA BASIN

The Ganga basin is an active foreland basin having an east-west elongated shape. The basin formed in response to the

Fig. 1. Major drainage and rainfall distribution across the Upper (UGP), Middle (MGP) and Lower (LGP) Ganga plains (compiled from Singh1994).

uplift of Himalaya after the collision of Indian and Asian plates(Dewey & Bird 1970). The Ganga plain foreland basin showsall the major components of a foreland basin system (DeCelles& Giles 1996), namely an orogen (the Himalaya), deformedforeland basin deposits adjacent to the orogen (Siwalik Hills),a depositional basin (Ganga Plain) and peripheral cratonicbulge (Bundelkhand Plateau) (cf. Singh 1996). This phase ofthe foreland basin’s history possibly originated in the EarlyMiocene. In the early phase of the foreland basin (EarlyMiocene), the foreland basin had small dimensions, withcomparatively minor subsidence (France-Lanord et al. 1983).The foreland basin was more completely established in theMiddle Miocene, after considerable lithosphere flexure andsubsidence of the basin. During the Middle Miocene to MiddlePleistocene (deposition of Lower to Upper Siwalik Group), thenorthern part of the Ganga plain was uplifted and thrustbasinwards, and the Ganga basin shifted southward(cratonwards) in response to thrust loading in the orogen(Singh 1996). Covey (1986) applied the term “under-filled basin”to the Ganga foreland basin at this time, which represents atopographic low between the thrust belt (Himalaya) and theperipheral bulge. The under-filled condition developed due toan efficient transport system for the sediment supplied, which

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removed the bulk of the sediment to the Ganga delta andBengal fan.

Burbank (1992) suggested that the Ganga foreland basinhas been dominated by transverse river systems since thePliocene due to erosionally-driven uplift (symmetricsubsidence of the foreland), whereas the Indus foreland basinis dominated by longitudinal river systems due to tectonically-driven uplift (asymmetric subsidence of the foreland). LargePlio-Pleistocene sediment fluxes combined with lessasymmetric subsidence and uplift of the proximal foreland ledto the progradation of the transverse drainage systems thatdisplaced the Ganga River to the edge of the foreland basin.The present day river position is consistent with erosion-driven uplift in the adjacent Himalaya. Further, the sedimentaccumulation rates generally exceeded the subsidence ratesof the foreland throughout the history of the Ganga basin.

SUB-SURFACE GEOLOGY AND TECTONICFRAMEWORK

Geophysical studies in the Ganga basin have included gravity,seismic and aeromagnetic surveys coupled with some deep

drilling carried out by ONGC for oil exploration. These datahave been used by several workers to interpret the basementstructure and sub-surface geology (Rao 1973, Sastri et al.1971, Karunakaran & Ranga Rao 1979, Raiverman et al. 1983,Lyon-Caen & Molnar 1985). The thickness of the alluvium is~6 km near the foothill zone and decreases gradually towardsthe south (Rao 1973). Geophysical surveys show that themetamorphic basement exhibits a number of ridges and basins(Fig. 2). The Ganga basin is characterized by three subsurfaceridges, i.e. Delhi-Hardwar ridge in the west, Faizabad ridge inthe middle, and Monghyr-Saharsa ridge in the east (Rao 1973,Parkash & Kumar 1991). There are two important depressionsin this area, namely the Gandak and the Sarda deep. Theforeland sediments rest on these basement ridges. In the areabetween the Delhi-Hardwar ridge and the Faizabad ridge, thesediments rest on Late Proterozoic unmetamorphosedsediments, which are part of the Vindhyan basin in the southand the Krol basin in the north. East of the Monhgyr-Saharsaridge, the foreland sediments lie on a thick succession ofGondwana rocks.

The Ganga basin is traversed by several transverse andoblique subsurface faults (Agarwal 1977, Dasgupta et al. 1987,

Fig. 2. Subsurface geology and tectonic framework of the upper and middle Ganga basin; numbers indicate the major sub-surface faults identifiedby geophysical surveys (based on Sastri et al. 1971, Rao 1973, GSI 2000). 1- Great Boundary Fault, 2- Delhi-Moradabad Fault, 3-LucknowFault, 4- west Patna Fault, 5- East Patna Fault, 6-Monghyr-Saharasa Ridge Fault, 7- Malda-Kishanganj Fault.

Dehradun

Satpura Massif

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Valdiya 1976) and the seismic data have shown that many ofthese faults are neotectonically active as evidenced by recentseismic activities (major earthquakes in 1833, 1906, 1934 and1987), with the possibility of large earthquakes in the nearfuture (Bilham 1995). The present models of Himalayanseismotectonics predict westward decrease in crustalshortening and uplift rate along the Himalayan Frontal Thrust(HFT) (Peltzer & Saucier 1996, Wesnousky et al. 1999) basedon the higher crustal shortening rate and average Holoceneuplift rate in Nepal (≅ 20mm/a and 15 mm/a respectively) andlower rates in Dehradun (11.9±3.1 mm/a and 6.9±1.8 mm/arespectively 10-12mm/yr). These longitudinal and transversefaults along with the basement configuration of the Gangaplains have long been considered to influence the fluvialprocesses and sedimentation (Raiverman et al. 1983, Parkash& Kumar 1991, Agrawal & Bhoj 1992, Pant & Sharma 1993,Ghosh 1994, Parkash et al. 2000). Thus, the foreland basin isnot a simple linear zone dominated solely by Himalayandynamics but represents the complex interaction of thrustloading with zones of varied subsidence rate and major, activefaults that localize deformation and influence thegeomorphology of the plains.

RIVER SYSTEMS AND MAJOR FLUVIALPROCESSES IN THE GANGA PLAINS

The east-west trending Ganga plains are the surfaceexpression of the Himalayan foreland basin and are drainedby a number of N-S trending river systems in a varied climaticsetting (Fig. 1). The normal annual rainfall in the Ganga plainsvaries from 60 cm to more than 160 cm (Fig. 1). In general, thewestern part of the Ganga Plains receives less rainfall (from60-140 cm) in comparison to the eastern parts (90- >160 cm).Further, the northern part of the plains area receives higherrainfall than the southern part. The temperature in the Gangaplains varies from 5° to 25°C in winter and from 20°C to morethan 40°C during summer.

The primacy of three distinct types of fluvial systems inthe evolution of the Ganga plains has been recognized, eachcharacterized by different source area characteristics, viz.mountain-fed, foothills-fed and plains-fed (Sinha & Friend1994). Mountain-fed rivers such as the Ganga, Gandak andKosi are generally multi-channel, braided systems,characterised by discharge and sediment loads that are manytimes higher than those of the single-channel, sinuousfoothills-fed and plains-fed river systems. They also transfera large quantity of sediments from their high relief catchmentsto the plains and consequently form large depositional areas(megafans). The foothills-fed (e.g. Baghmati, Rapti) and plains-fed (e.g. Burhi Gandak, Gomti) rivers derive their sediments

from the foothills and from within the plains, and a largeproportion of this material is re-deposited in the plains afterlocal reworking.

In a geomorphic perspective, each of these fluvial systemshas characteristic geomorphic ‘units’ such as channel belt,floodplain, dissected plain and piedmont plain. These unitsthemselves consist of different geomorphic ‘elements’, forexample, active/inactive channels, lakes/swamps, gullies andeolian features. Apart from developing a hierarchical sense toour descriptions, this approach has allowed us to examinegeomorphic heterogeneity at various spatial levels. Our earlierwork has noted significant geomorphic diversity across theplains manifested as variability of fluvial processes, spatialdistribution of different geomorphic units and frequency ofgeomorphic elements (Sinha et al. 2002, Jain & Sinha 2003a,Sinha et al. submitted). This consequently characterizes theplains to be dominantly aggradational or degradational. Suchgeomorphic diversity is attributed to differences in streampower of the rivers draining the plains and sediment supplyfrom the catchment areas (Sinha et al. submitted) which, inturn, are controlled by varied climatic and tectonic settings.

The alluvial rivers in the Ganga plains occupy narrowvalleys which are separated by large interfluves. Tandonet al. (submitted) have shown that the processes controllingthe valley formation and filling in the Gangetic plains areextremely variable in space. Near the Himalayan front, bothtectonic and climatic factors have been responsible for valleyformation and incison. In contrast, the strong incised valleysin the western and southern plains have mainly been controlledby climatic factors since tectonic activity and subsidence havebeen minimal in these regions.

Channel movements through avulsion and cut-offs havebeen recognized in most of the rivers of the Ganga system,albeit with a difference in scale and frequency. In areas southof the Himalayan Front, especially in north Bihar, early studiesof the migration of the Kosi by Geddes (1960) and Mookerjea(1961) were followed by several studies on Kosi channeldynamics and causes of migration (Gole & Chitale 1966,Arogyaswamy 1971, Wells & Dorr 1987, Agarwal & Bhoj 1992).Mohindra et al. (1992) described the migration of the Gandakover its megafan from west to east over a distance of about 80km. Phillip et al. (1989), Phillip & Gupta (1993) and Sinha (1996)reported the migration of several smaller rivers such as BurhiGandak, Baghmati, and Kamla-Balan in the Gandak-Kosiinterfluve. Jain & Sinha (2003b, 2004) have presented one ofthe most comprehensive data sets on the ‘hyperavulsive’Baghmati river over a period of ~250 years and attributed thisavulsive pattern to neotectonic perturbance andsedimentological readjustments.

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The rivers of the Ganga system draining the UP plains arenot as dynamic as the north Bihar rivers, but they do showsome channel movement over a long time period. In the areabetween Bithur and Kanpur Railway Bridge, the main Gangachannel has recorded major movements in the historical period(1857-present) between its left and right bank (Hegde et al.1989). These have been attributed to the highly irregular shapeof the valley in the area. The Ghaghra river in the UP plainshas also shifted in certain places by ~5 km on either side of theactive channel over a 7-year period from 1975-1982. Thismigration has been related to neotectonics in the area (Tangri1992, Srivastava et al. 1994). Chandra (1993) noted a SWdiversion of Rapti river near Baharaich due to aggradation inthe old channel. The Sarda river has been characterised byseveral westward shifts (Tangri 2000).

Many rivers in the Ganga plains, particularly in the easternparts, are prone to flooding and the region is regarded as oneof the worst flood-affected regions in the world (Agrawal &Narain 1996). The plains of north Bihar have the dubiousdistinction of recording the highest number of floods in Indiain the last 30 years (Kale 1997). The overall hydrologicalresponse of the basin depends, apart from the rainfall intensityand duration, upon the geomorphometric characteristics (Jain& Sinha 2003c, d), neotectonics and fluvial processes. Thedynamic behaviour of river channels and frequent avulsionscaused by sedimentological readjustments or other factorsoften divert the flow into a newly formed channel with lowbankfull capacity causing extensive flooding (Sinha 1998, Sinha& Jain 1998, Jain & Sinha 2003e). In contrast, the westernGanga plains do not flood so frequently due to their incisedvalleys and lesser frequency of avulsions.

Earthquakes may exert a significant effect on short-termsediment supply in the Ganga catchment, which is affected bylarge seismic events. In recent time, their effect has been bestdocumented in the Brahmaputra catchment, where the 1950Assam earthquake dislodged 47 billion m3 of material, resultingin temporary dams in rivers and catastrophic dam bursts. Thefollowing monsoon season resulted in unprecedented floodsdue to aggradation of up to 8 m of sediment in the channels,with a wave of downstream aggradation over the next fewdecades (Goswami 1985, Keefer 1999).

ALLUVIAL STRATIGRAPHY

One of the most convenient divisions of the Great Plains ofIndia has been on the basis of geographic and climatic features(Singh 1994). Following these criteria, we focus on Upper,Middle and Lower parts of the Ganga plains (Fig. 1) afterSingh (1994) to discuss the alluvial stratigraphy below this

vast alluvial tract. Although tectonic approaches mightsuggest dividing the basin into proximal (northern) and distal(southern) regions with respect to the Himalayan Front andsubsidence rates, considerable evidence suggests that west-to-east climatic and geomorphic gradients exert an especiallystrong control on the dynamics of the Ganga system in UPand north Bihar. The Lower Ganga plains are essentially thedeltaic areas liable to sea-level effects.

Upper Ganga Plains

In the Upper Ganga Plains, megafan deposits of the Gangaand its tributaries occupy a large part of the basin. Geddes(1960) first showed that the Upper Ganga Plains are made upof cone and inter-cone areas. The Ganga River incises into the150 km long and 100 km wide Yamuna-Ganga plain, generatinga broad valley. The valley walls rise 15-30 m above the GangaRiver along cliff lines, and extensive gullying has exposedthe Ganga plain deposits. These deposits are exposed forabout 100 km in the down current direction and, at places,form sections several hundred meters to kilometers long(Shukla et al. 2001). The modern Ganga and Yamuna rivers aredeeply incised into this surface and are not actively depositingsediments on most parts of the ‘megafan’. The Ganga‘megafan’ deposits can be classified into four zones, depictingproximal to distal facies changes : zone I- gravelly braidedstreams; Zone II- sandy braid plain; Zone III-anabranchingchannel plain; and Zone IV-meandering channels with broadinterfluve areas (Shukla et al. 2001). Paleoflow in the megafandeposits was towards the SW and SE to E with a distal decreasein grain size . The Ganga ‘megafan’ is a relict feature formedduring Late Pleistocene, when coarse grained sediment andhigher sediment and water discharge was available (Singh etal. 1990).

In the Uttar Pradesh plains to the south and east, cliffsalong the Ganga, Yamuna and Sengar rivers provide“windows” into the subsurface. Singh et al. (1997 1999a,b)studied the exposed sections and provided a first-levelstratigraphic framework. Srivastava et al. (2003) providedluminescence chronometry of several sections in the upperGanga plains and discussed the time constraints on thedevelopment of different geomorphic surfaces.

During the last few years, we have focused on the Ganga-Yamuna interfluve between Kanpur and Kalpi (Fig. 3), andhave studied the exposed sections along river banks as wellas in shallow drill cores down to a depth of ~50 m. We haveused an integrated approach involving geomorphic analysisaided by satellite images followed by rigorous stratigraphic

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Fig. 3. False Color Composite of the IRS-LISSIII image showing the Ganga-Yamuna interfluve between Kanpur and Kalpi. Three cliff sections at Bithur,Mawar and Kalpi have been studied and summarized stratigraphic sections are shown in Fig. 5. A fourth section at Kotra lies in the Yamuna-Betwainterfluve, ~50km south of Kalpi (outside this image). Locations of five drill holes (1 through 5) are also shown and the logs at these sites arepresented in Fig. 6.

and sedimentologic analysis of the cliff sections and drill cores.The detailed geomorphic mapping has allowed theidentification of the main geomorphic units,including majoractive channel belts, major active floodplains, minor activechannels and floodplains, minor inactive channels andfloodplains, slightly dissected plains and highly dissectedplains (Sinha et al. 2002, Gibling et al. in press, Sinha et al. inpress-a). The stratigraphic work on the cliff sections (Fig.4) is based on the preparation of photomosaics of cliffs up toseveral kilometers long, from which a detailed tracing of thestrata was prepared. This was followed by stratigraphicmeasurements and graphic facies logging at selected pointsin the cliff sections along the Ganga (at Bithur), Sengar (atMawar), Yamuna (at Kalpi) and Betwa (at Kotra). Fig. 5 showsthe summary logs at these four locations and a brief descriptionpresented below has been extracted from Sinha et al. (2002),Gibling et al. (in press) and Sinha et al. (in press-a). Thesections are tens of kilometers apart, and the extent ofindividual stratigraphic units and bounding surfaces remainsto be determined. They lie along the major rivers, and may notbe representative of strata in the central parts of the interfluves.Accumulation rates in this southerly part of the foreland basin

appear to have been modest over the past 100 ka, in the orderof 0.3 mm/year.

A ~12 m section at Bithur on the Ganga river (Fig. 4) hassix distinct lithostratigraphic units (Fig. 5a). Unit 1 is a floodplaindeposit consisting of mottled clayey silt with abundantcarbonate nodules. Units 2, 4, and 6 have well-developedstratification within dark silty clay and show strongefflorescence coatings of halite. We have interpreted them tobe lacustrine deposits. Units 3 and 5 are composed of paleyellow silt to very fine sand with prominent rhizoconcretions,and have been interpreted as eolian deposits. The ~10 m Mawarsection (Fig. 5b) is located along a smaller, plains-fed river, theSengar. Units 1 and 2 of this section are interpreted as floodplaindeposits consisting of silty clay and clayey silt with mottlesand carbonate nodules. Two channel sand bodies, units 3 and4, are encased within the upper floodplain unit, and the channelfills consist of alluvial gravels interbedded with fine sand andsilt of eolian and lacustrine origin.

The cliff section at Kalpi (Fig. 5c) along the Yamuna riveris one of the thickest sections available in the region and weidentify five lithostratigraphic units here. The basal exposed

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Fig. 5. Summarized stratigraphic logs at (a) Bithur, (b) Mawar, (c) Kalpi, and (d) Kotra sections. Each unit has been interpreted in terms ofdepositional environments (C-Channel, E- eolian, F-Floodplain, G-Gravel-fills, L- Lacustrine).

Fig. 4. Cliff sections in the Ganga-Yamuna plains; (a) Bithur (~12m), (b) Kalpi (~20m), and (c) Kotra (~30m). Numbers indicate the majorlithostratigraphic units at each section (based on Gibling et al. in press; Sinha et al. in press).

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unit 1 is a floodplain unit comprising silty clay with dark mottlesand scattered carbonate nodules and rhizoconcretions. A layerof pink feldspathic sand is present low in the unit, and isdiscussed below. Unit 2 is a thin channel sand body made upof fine to medium sand with a gravel lag deposit at the base.The unit has abundant nodules and rhizoconcretions and iscapped by a cemented sand layer. Unit 3 has been interpretedas a paleosol layer deposited in a swampy environment andconsists of poorly stratified silty clay with sparse gastropodshells and rhizoconcretions. Unit 4 is ~23 m thick and consistspredominantly of mud, representing prolonged floodplainaccumulation with pedogenesis manifested as mottles andcarbonate concretions. Dark layers within this unit andespecially in its topmost part are interpreted as lacustrinedeposits. Distinct gully cutting and filling events are insetlocally in unit 4, and we designate them as unit 5. The gully fillsconsist of steeply dipping silty sand and brown mud withlenses of carbonate gravel.

The Kotra section (Fig. 5d) overlies Pre-Cambrianbasement at the southern margin of the Ganga basin. A~23 msection is exposed along the Betwa river which joins the Yamunariver downstream of Kalpi. The basal gravel unit 1 is probablyvery old and is exposed only locally. The overlying unit 2 is afloodplain mud with minor silt and sand lenses, dark mottles,carbonate nodules and rhizoconcretions. Unit 3 is a gravel-filled channel body with planar and trough cross-sets. Unit 4 is~14 m of floodplain clay with abundant rhizoconcretions andcarbonate gravels.

A common feature at all four localities is the presence ofmud-dominated deposits underlying the modern interfluves atshallow depth. We interpret them as the floodplain deposits ofmajor rivers and plains-fed tributaries. These deposits showevidence of moderate pedogenesis, and are overlain byprominent discontinuities marked by widespread erosion or aradical change in facies. At Bithur, more than 8 m of lacustrineand eolian strata overlie the floodplain deposits. At Mawar,similar floodplain deposits are incised by channels filled withreworked carbonate gravel, lacustrine and eolian strata. Thefloodplain deposits at Kalpi are cut by large gully fills andcapped by possible lacustrine deposits. Further, we record thepresence of discontinuities at deeper levels in the thickerexposed sections, for examplein the Kalpi and Kotra sectionswhere they are marked by extensive, low-relief degradationalsurfaces with gully fills of mud, sand and reworked gravel.Rhizoconcretions are abundant at these levels, suggestingstrong pedogenesis, and some surfaces are cemented withgroundwater carbonate. Thin channel bodies of gravel andsand are encased within thick floodplain muds and representsmall plains-fed rivers. By analogy with near-surface deposits,

the discontinuities are inferred to mark former degradationalsurfaces in the floodplains. Age dates for these sections(Gibling et al. in press) suggest that floodplain aggradation,punctuated by periods of stronger pedogenetic activity andlocal degradation, dominated the southern Gangetic Plainsduring Marine Isotope Stages 3-5 (about 35-100 ka). Thissedimentation pattern changed significantly late in Stage 3,when gullying commenced at Kalpi. The marked change fromfloodplain to eolian and lacustrine strata at Bithur is dated atabout 27 ka (late in Stage 3) and continued through the LastGlacial Maximum of Stage 2. A further episode of floodplaindegradation took place at Mawar at about 9-13 ka (early inStage 1). Although the timing of incision that created themodern river cliffs is poorly constrained, much of the incisionmay have taken place early in Stage 1 (see also Williams &Clarke 1984, 1995). As discussed in Gibling et al. (in press),these dates show a reasonable first-order correlation with themonsoonal precipitation record set out by Prell & Kutzbach(1987). They further suggest that fluctuations in precipitationgoverned river discharge and sediment supply, which in turncontrolled aggradation and degradation within the channelsand adjacent floodplains. Climatically-induced discontinuitiesmay have been accentuated by low subsidence rates in thesouthern part of the foreland basin.

To follow up the work on cliff sections and to establishbetter correlation, we have carried out drilling at severallocations in the Ganga-Yamuna interfluve (see Fig. 4 forlocations of boreholes). Continuous cores were collected andanalysed for understanding the stratigraphic development atall these locations, and we include here some unpublisheddata on petrographic composition and luminescence age dates.

Within the Ganga valley, the Firozpur core (Fig. 6a) ismarked by an alternation of channel and floodplain facies. Theupper and lower channel sands are 9-10 m thick, and areseparated by ~7m of floodplain muds which are moderatelypedogenically altered and bounded at both ends by prominentkankar layers that mark discontinuities. The channel sands aresimilar in petrographic features to modern Ganga sand, andage dates suggest that the Ganga has been close to its presentsite for at least the past 30 ka.

Within the interior of the Ganga – Yamuna interfluve, theIITK core (Fig. 6b) consists mostly of silty-clay with occasionalsilt layers. Kankars and mottles are distributed at all depths,and the core represents a distal floodplain environment thathas been in effect for more than ~100 ka (unpublished data).There is no major change in the facies except for a few thin siltylayers, probably crevasse splay deposits, which are morefrequent in the upper units. The Rania core (Fig. 6c) has 15-20

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Fig. 6. Summarized logs of the drill cores at Firozpur (1), IITK (2), Raina (3), Bhognipur (4) and Kalpi (Yamuna bank) (5); see Figure 3 forlocations of the drill sites. Different lithostratigraphic units have been interpreted in terms of depositional environments (C-Channel,CM-Channel margin, F- Floodplain). CL is marked as the beginning of Cultural Level.

m of floodplain strata underlain by channel deposits. Thechannel body is ~6 m thick and is analogous to present-dayplains-fed rivers such as the Rindi which flows close to thedrill site. The floodplain muds have distinct bands of smaller(2-4 mm diameter) kankars and soft dark mottles which markancient soil horizons. The fine micaceous sandy layers havefewer but larger (3-6 cm diameter) kankars at regular intervals.

Near the southern interfluve margin at Bhognipur, 20 mof floodplain strata with abundant kankars and mottles passto a sand-silt alternation perhaps representing a channel marginfacies. Between 35-43 m depth, we recognize a ~7 m thickcoarse sand with numerous pink feldspar grains and a fewrock fragments. The lower boundary of this sand body ismarked by a ~10 cm thick zone of large (5-8 cm diameter)kankars. This sand (locally called ‘Betwa sand’ where it hasbeen penetrated in water wells) resembles the channel depositsof the present-day Betwa river, which drains the peninsulargranitic catchment. Petrographic examination of frameworkgrains and heavy minerals in this sand shows a typical cratonicsignature and close similarity with modern Betwa sand(unpublished data).

Within the Yamuna valley, the Yamuna bank core shows achannel margin sequence (levee?), about 12 m thick, consistingof interbedded very fine sand and silty clay. The sand layersare micaceous with thin patches of white silt, and petrographicdata suggest a predominant Himalayan source. The silty claylayers have few soft dark mottles but kankars are absent. Thesestrata are underlain by a feldspathic coarse sand unit (basenot reached), much coarser than the present-day channelsands of the Yamuna river and petrographically similar to thesands at the base of the Bhognipur core. The core is cappedby a micaceous fine sand layer with some pink feldspar grainsand a few kankars and rhizocretions, perhaps representing in-channel bar deposits.

The drill data contribute significantly to ourunderstanding of Upper Ganga plains history over the past100 ka. Firstly, the Ganga and Yamuna valley cores suggestthat the Ganga & Yamuna rivers have been close to theirpresent positions for a prolonged period, probably at leasttens of thousands of years. Secondly, the IITK and Rania

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cores in the interfluve mainly penetrated floodplain depositswith only one small channel body, suggesting that majorHimalayan rivers have not occupied this region for perhapsthe past 100 ka. However, mountain-fed rivers may haveflooded large parts of the interfluve during earlier periods ofhigh monsoonal intensity, and the plains-fed rivers themselvesmay have been very active in reworking sediment from theHimalayan Orogen. Finally, the identification of cratonicsediment at depth in the Bhognipur and Yamuna cores suggeststhat wedges of sediment derived from the craton to the southformerly extended much further north in the Kalpi area thanthey do at present (and probably as far north as Kanpur atdeeper levels: Singh & Bajpai 1989). Although we currentlyhave no dates on this core material, a feldspathic sand fromUnit 1 of the Kalpi outcrop section yielded a date of 119 ka, inthe mid part of Stage 5 (Gibling et al. in press). The apparentvitality of cratonic rivers during this period may reflect strongmonsoonal activity in central India, or may reflect such factorsas river capture and changes in course of Himalayan rivers,changing the relative dominance of cratonic and Himalayanrivers. Our data suggest that, at least in the Kalpi area,Himalayan rivers may have moved to a basin-margin position(Burbank 1992) as recently as the past 100 ka.

Sets of calcite veins and deformed channel sands in theKalpi section (Singh et al. 1999, Agarwal et al. 2002, Gibling etal. in press) suggest that the region experienced majorearthquake activity in the past, although this part of the plainsis not currently active seismically.

Middle Ganga plains

Despite several studies in the middle Ganga plains, relativelylittle is known about the Quaternary alluvial history of theregion and the organization of the deposits, in part becausefew natural exposures exist. The channels are not deeplyincised in this area, and exposed bank sediments are those ofthe modern, aggrading floodplain system, rather than that ofearlier Holocene or late Pleistocene sediments. Gohain &Parkash (1990) and Mohindra et al. (1992) used the term‘megafan’ to differentiate the large alluvial fans such as theKosi and Gandak from the small fans of the piedmont zonedeveloped close to the Himalayas. Sub-surface data from theKosi Fan (Singh et al. 1993) show a sheet of gravel and sand>60 m thick capped by a surficial unit of sand and mud, typicallyup to 10 m thick but locally up to 40 m. The authors interpretedthe lower unit as a braided-river deposit and the upper unit asa “megafan sweep” succession, generated by migration ofthe active zone of smaller channels across the fan. No agedates presently constrain the timing of these events.

Shallow alluvial stratigraphy of the Baghmati river plainsin the Kosi-Gandak interfluve reveals the presence of 30-50 m

of mud with thin sand layers (2-3 m) representing crevassedeposits (Sinha 1995, Sinha et al. in press-b). Floodplainaccumulation estimates of 0.7-1.5 mm/year in the north Biharplains over the past ~2400 years (Sinha et al. 1996) imply rapidaggradation during the late Holocene. These rates are muchhigher than those documented for the other near-surface partsof the Ganga plains. Joshi & Bhartiya (1991) estimated a rateof 0.2 mm/year over a period of 104 years based on 14C datesfrom eastern Uttar Pradesh, and Chandra (1993) estimated 0.2-0.05 mm/year over 104 years based on luminescence datesfrom the middle Ganga plains. The eastern UP plains alsoshow mature soils, 3-4 m thick, with well-developed carbonatehorizons, estimated to be as old as 13,500 years BP (Srivastavaet al. 1994).

Sinha et al. (in press-b) analysed borehole records downto about 100 m and showed that the modern anabranchingreach of the Baghmati in north Bihar plains is underlain bythick sand units (typically 10-25 m) and thick mud units (up to25 m), with widths of some channel bodies constrained to lessthan a few kilometers, probably much less. Extrapolation ofnear-surface floodplain accumulation rates to thesemudstones suggests that channels were stably positionedfor tens of thousands of years, allowing thick fine-grainedunits to build up. Repeated reoccupation of drainage lineswith depths comparable to those of the modern channels mayhave promoted the creation of thick channel bodies. Thefloodplain deposits probably include both repeated (seasonal)flood deposits, from floods such as those that inundate theBaghmati plains in most years, and avulsion deposits intofloodplain lakes (tals).

Although outcrop data are sparse, it is probable thatalluvial deposits in these northern areas accumulate relativelyrapidly and continuously on account of rapid subsidence nearthe Himalayan Front, coupled with the high discharge andsediment supply of rivers in this high-precipitation part of thenorthern Indian plains. Discontinuities in the successions arelikely to be uncommon and cryptic, unlike those documentedwithin alluvial successions of the Upper Ganga Plains. Thestrata of this region may be especially relevant to interpretingparts of the Siwalik Group in the foothills north of theHimalayan Front. Much of the Siwalik Group is inferred tohave been deposited on megafans (Friend et al. 2001), and thepresent exposures probably represent the more proximal partsof the plains succession disrupted by the southerly advanceof thrusts.

In the plains south of the Ganga, Williams & Clarke (1984,1995) described alluvial sequences in the Belan and Sonvalleys ranging in age from Middle Pleistocene to Holocene.An interesting comparison of these sequences with those in

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the upper Gangetic plains described by us is the recognitionof a widespread discontinuity predating 10 ka in the form ofreworked aeolian deposits capping the alluvial succession. Ithas been suggested by these authors that the bulk of theseaeolian deposits accumulated during the LGM period whenthe river levels were low.

Lower Ganga plains and deltaic plain

The Lower Ganga Plains of the Indian sub-continent form oneof the most extensive fluvio/deltaic plains of the world andthey conceal the Bengal sedimentary basin. After drainingthrough the alluvial plains of UP and Bihar, the Ganga riverenters the lower plains area and delta region and finally meetsthe sea in the Bay of Bengal. The Brahmaputra river drainingfrom the northeast joins the Ganga, and together theyconstitute the largest delta in the world. The Ganges-Brahmaputra river systems together transport ~1x109 t/yr ofsediments, and this puts them among the world’s largestsediment load carrying systems. The Bengal basin acts as alarge sink for this huge sediment dispersal system, about 80% of which is delivered during the monsoon (Goodbred &Kuehl 2000).

The geomorphology of the Lower Ganga Plain has beenstudied by Niyogi (1975) and Bhattacharya & Banerjee (1979).Three major landforms - Uplands, Old Fluvial/deltaic plainsand young fluvial plains - are identified from the Lower Gangaplains (Singh et al. 1998). Due to reactivation of some basementfaults and tectonic subsidence, the eastern and western sub-units of the tectonic shelf were sites of active transgressionduring the early Pleistocene and around 7-6 ka (Banerjee &Sen 1987, Singh et al. 1998). Uplift of these sub-units atdifferent times triggered regression in the Holocene, whichcontrolled the timing of pedogenesis of the sub-units (Singhet al. 1998).

The modern Bengal Basin comprises ~100,000 km2 oflowland floodplain and delta plain deposits and is boundedby Tertiary highlands. Within the Bengal Basin, the MadhupurTerrace and Barind Tract are uplifted alluvial deposits ofPleistocene age (Morgan & McIntyre 1959, Johnson & Alam1991). The seaward extent of the Ganges-Bramhaputra deltaicdeposits is marked by the base of the subaqueous deltaforesets (Kuehl et al. 1997). A number of stratigraphic andgeochronologic studies in the Bengal Basin have helped todefine the depth and timing of the late Quaternary Ganges-Brahmaputra fluviodeltaic sequences (Banerjee & Sen 1987,Hait et al. 1996, Goodbred & Kuehl 2000a,b).

Stanley & Warne (1994) concluded that the sedimentsupply, not the rate of sea-level rise, controlled the initiation

of the delta development, and was responsible for deltastability under conditions of rapid eustatic rise. The initialdelta formation began at 11 ka B.P when rising sea level led toback-flooding of the lowstand surface and the trapping ofriverine sediments, an event marked by transition from alluvialsands or Pleistocene laterites to overlying mud that containestuarine shells and wood, along with thick valley-fill deposits(Umitsu 1993, Goodbred et al. 2003). The thick estuarinedeposits and the persistence of the intertidal facies indicatethat sediment supply to the delta system has been sufficientto infill accommodation created by the rapid sea-level rise.Sediment discharge has probably varied significantly underdifferent climatic regimes since ~11 ka. Goodbred & Kuehl(2000) estimated a mean load of 2.3x109 t/yr for the period 11ka-7 ka, which is more than twice the present day load of~1x10 9 t/yr. The intensification of the southwest monsoon(Sirocko et al. 1993) supported regionally wetter conditionsand increased river discharge (Cullen 1981). Late QuaternaryBengal Basin sediments show high smectite-kaoliniteconcentration during the early Holocene period (~10-7 ka),and this appears to reflect enhanced chemical weatheringunder warmer and more humid conditions (Heroy et al. 2003).Williams & Clarke (1984) also found evidence for 20-30 m offloodplain incision along the Ganga tributaries during thistime, & Pratt et al. (2002) suggested strong incision inHimalayan valleys during this period. The sediment load wasextremely reduced prior to 15 ka because of the dominance ofthe dry northeastern monsoon, generating lower discharge(Wiedicke et al. 1999). Recently, Chauhan et al. (2004) haveidentified two phases of major fluvial influx in the Bay of Bengalaround 11.5 ka and 9.5 ka, based on temporal variation instable isotopes, mineral magnetism, clay minerals andgranulometric data. These variations have been related tomonsoonal intensification at around 9.5 ka.

Goodbred (2003) presented a comprehensive analysis ofthe response of the Ganges dispersal system to climatechange, and suggested that this system has responded tomulti-millennial scale climate change in a system- wide andcontemporaneous manner, right from the source to the sink.Although this analysis is based on a rather limited dataset, itraises the important issue of whether sediment signals in theGanges system are transported very rapidly from source tosink with little apparent attenuation. There is strong evidencethat large sediment volumes generated by earthquakes moverapidly down the river systems (Goswami 1985). However, thesuggestion by Goodbred (2003) that the entire dispersal systemresponded to climatic changes during the past 60 ka overshort time-scales may be somewhat generalized, keeping inview the varied geological and climatic settings transected bythe system and the geomorphic diversity in the Ganges system

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- from source catchment area to the lower delta plain. Sedimentmay be stored for longer periods in some parts of the plains,and the strong gradients in precipitation from west to eastand north to south across the Ganga plains (Fig. 1) may affectthe nature of the sedimentary response, the response rate,and thresholds for transport conditions. Goodbred’shypothesis is provocative and requires further testing.

In the deltaic region, stratigraphic development has beencontrolled by, apart from sediment supply, active tectonics(Morgan 1970), and the interplay between the two has resultedin unique and differing stratigraphies within the delta system(Goodbred et al. 2003). In the northeast part of the delta, wheretectonic processes are most active, the presence of sub-basinsfavors the dominance of fine grained floodplain deposits(Umitsu 1993). In the western part of the delta, sandy alluvialdeposits dominate the stratigraphy due to fewer tectonicfeatures. Fluvial processes dominate this part of the delta,but channel migration and avulsion tend to erode the finegrained floodplain deposits (Stanley & Hait 2000). In thesouthern delta coastal plain, due to the presence of an estuary,a mix of fine and coarse grained facies with the muddy depositswas preserved during early Holocene sea-level rise. Overall,these individual stratigraphic sequences located in the samedelta system emphasize the importance of local basin factorsin modifying the alluvial architecture. Additional controls areapplied by riverine processes such as avulsions and episodicearthquakes. A long history of delta switching in the Bengalbasin has been related to channel avulsion of the Ganges andBrahmaputra rivers. Heroy et al. (2003) established a sequenceof river switching in the delta sequence using clay mineralogyand heavy mineralogy. The major diagnostic differencebetween the Ganga and Brahmaputra sand fractions turns outto be a low (<1) epidote to garnet (E/G) ratio and relativeabundance of smectite in the Ganges alluvium. Proximity tothe Himalaya and catchment basin tectonics had a more directeffect on the delta system, including controls on river course,avulsion, sediment dispersal and facies preservation (Alam1989, Goodbred et al. 2003).

DISCUSSION

The Quaternary alluvial stratigraphy of the Gangetic plains isextremely varied from its upper reaches in the piedmont zoneto the flat middle part and finally to the lower parts and deltaicregion. The upper reaches of both the Ganga and Yamunariver systems are deeply incised and presently the depositionis confined within the channels. The older stratigraphy in theupper Ganga plains close to the foothills show gravelly andsandy deposits of braided rivers and the deposits fine distally.It has been inferred that the coarser deposits in the upper

plains were deposited in Late Pleistocene at times of higherdischarge and sediment availability (Shukla et al. 2001), whenalluvial fans within Himalayan foothills valleys were also active(Suresh et al. 2002).

Further south, an important element of stratigraphicdevelopment in the interfluve region of the upper Ganga plainsis the recognition of ‘discontinuity-bound’ sequences (Giblinget al. in press). Although a major proportion of the interfluvesequence is made of muddy sediments, the alluvial depositionhas been punctuated by phases of non-deposition as well asby accumulation of non-riverine deposits in the interfluves.At several locations, these levels are marked by strongpedogenic events manifested in calcrete development andimmature soil formation. At other locations, aggradationcontinued in the form of aeolian and lacustrine deposition(Bithur) or gully development and filling (Mawar, Kalpi). Weinfer that these discontinuities in the stratigraphic record ofthe interfluves have formed due to partial to completedetachment of the floodplains from major rivers. Thesediscontinuity-bound sequences reflect repetitive phases offloodplain aggradation and degradation in response to regionalclimatic fluctuations during the last 30-40 ka (Gibling et al. inpress). This interpretation accords well with the present-daygeomorphology of the river systems. Major rivers such as theGanga and Yamuna, and minor rivers such as the Sengar andRindi in the Ganga-Yamuna interfluve, are deeply to moderatelyincised (Fig. 7a) and do not flood during the monsoon periodexcept during exceptional events. On the contrary, most partsof the middle Ganga plains are characterized by a rapidly fillingand aggradational regime. Most rivers in the eastern UP andnorth Bihar plains have no incised banks (Fig. 7b) and therates of floodplain sedimentation are ~1.5 mm/yr (Sinha et al.1996). Major rivers such as the Kosi and the Gandak havecontinuously moved across their valleys and have formedlarge sandy deposits up to 10 mthick in the sub-surface andemuch more where vertically stacked (Singh et al. 1993). Themulti-storied, sandy alluvial architecture in the subsurface ofthese fans is in sharp contrast to the thick muddy deposits inthe interfluve, encasing thin sand bodies representingavulsion deposits.

The contrasting alluvial architecture in the middle Gangaplains may reflect larger sediment supply and a higher rainfallcompared to the upper Ganga plains. A higher sediment supplyin the middle Ganga plains may be a function of, apart from itsgreater proximity to the Himalayan front, a higher crustalshortening rate and a higher average Holocene uplift rate(~20mm/a and 15 mm/a respectively, after Bilham 1995, Peltzer& Saucier 1996, Lave & Avouac 2000) in the hinterlandcompared to lower values of 11.9±3.1 mm/a and 6.9±1.8 mm/a (

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previous work where spatial homogeneity in geomorphicdevelopment, manifested as regional geomorphic surfaces, hasbeen emphasized over vast regions of the Ganga Plains (Singhet al. 1990, Singh 1996, Singh et al. 1999a).

Such spatial inhomogeneity in geomorphic developmenthas significant implications in understanding fluvial responseto climate change. Spatial or geographical differences in fluvialresponse due to global climate change, resulting insimultaneous changes in discharge regimes, are generallyexpected (Blum & Törnqvist 2000). However, our work in theGanga plains shows that significant intra-basinalinhomogeneities may also occur in different ‘domains’ of largeriver basins separated by less than 1000 km due to spatialdifferences in sediment supply and subsidence rate governedby rainfall and tectonics. Such inherent inhomogeneities wouldimply that these areas would also respond differently to anychange in global circulation pattern i.e. they would have a‘differential sensitivity’ (Blum & Törnqvist 2000) to climatechange. If such differential sensitivity has existed over a longtime scale, this must have affected the development of thealluvial architecture below the plains. Data presented in thispaper and elsewhere do suggest that there are markeddifferences in alluvial stratigraphy across the Ganga plains.

A dominant upstream control such as climate and tectonicson the geomorphic development of the Ganga Plains is alsoborne out by our work, in contrast to downstream controlmechanisms such as sea-level changes suggested by earlierworkers (Kumar & Singh 1978, Singh & Bajpai 1989, Singh etal. 1990, Singh 1996, Singh et al. 1999a). Fisk’s (1944) work onthe Mississippi river postulating eustatic control for more than1000 km upstream from the modern shorelines has beencontested by many workers, and Blum and Törnqvist (2000)have presented a summary of developments. New data fromdifferent parts of the world (Schumm 1993, Shanley & McCabe1993, Saucier 1994) suggest that the landward limit of sea levelchanges and its influence on fluvial incision and aggradationcan be very variable, but for low-gradient, high sediment supplysystems such as the Ganga, it may not be more than 300-400km. The upper and middle Ganga Plains are located ~1500 kmand ~800 km respectively from the sea, and therefore, anyinfluence of sea-level related changes in both these regions isquite unlikely. Goodbred (2003) also pointed out that anyadjustment, due to eustatic change, in the Ganga system at adistance of 600-1200 km from the sea is implausible.

The Lower Ganga plains and the deltaic plains present acompletely different scenario. Sea-level rise and fall has greatlyaffected this area, with the creation of well marked valleys andlowstand surfaces. However, an important conclusion fromsome of the recent work (Goodbred 2003, Chauhan et al. 2004)

Fig. 7. (a) The Kalpi cliff along the Yamuna river as an example of‘detached floodplain’; (b) The Gandak river bank in northBihar plains as an example of ‘attached floodplain’.

Peltzer & Saucier 1996, Wesnousky et al. 1999) in the hinterlandof the upper Ganga plains. Although more data on the rates oftectonic activity are needed along the whole Himalayan front,these data are suggestive of a regional west-to-east trend inuplift rates. Our earlier work has also highlighted the fact thatthe rivers draining the middle Ganga plains have much lowerstream power compared to the rivers in the upper Ganga plains(Sinha et al. submitted), and therefore have less energy toforce incision. Low stream power combined with highersediment supply has resulted in less prominent valleys,frequent avulsion of rivers, and the inundation of large areasduring monsoon floods. As a result, a dominantly aggradationalregime is obvious in the surface geomorphology as well as inthe subsurface stratigraphy in the middle Ganga plains. Suchclimatic and tectonic variance between the upper Ganga plainand the middle Ganga plain may have existed over 104 -106 yeartime scales, and this has resulted in marked geomorphic diversityacross the plains. These observations are at variance with

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is that the fluvial influx in the Bengal basin has responded tosystem-wide changes since the inception of the delta around11 ka. Climatic fluctuations have an important role in controllingthe fluvial influx from the ‘source’,in addition to the effect ofsea-level fluctuations. The overall stratigraphic developmentin the delta has also been strongly influenced by tectonics,manifested as delta switching and subsidence.

CONCLUDING REMARKS AND FUTUREPERSPECTIVES

The vast stretch of the Ganga plains, extending over 1500 kmdownstream from the Yamuna river exit at the Himalayan Front,shows extreme variability in terms of climatic parameters,geomorphic setting, hydrological regime and tectonics of thehinterland. This has resulted in variable stratigraphicdevelopment across the plains along an east-west transect.The upper Ganga plains have deeply incised rivers with 15-30m of cliff sections along the river banks. Repeated degradationand aggradation events separated by stratigraphicdiscontinuities are recognized in the alluvial sequences, whichrepresent relatively slow rates of accumulation. The middleGanga plains are characterized by an aggradational regimeand 25-30m thick sand/mud intervals, and have developedapparently without similar discontinuities. Long-termaccumulation rates for this region are not known but wereprobably high in this region just south of the Himalayan Front.

Both the upper and middle Ganga plains have remainedoutside the influence of the sea level fluctuations; climate,and to some extent tectonics, have been the major controllingfactors in stratigraphic development in this region. In contrast,the lower Ganga plain and the deltaic region have beeninfluenced by an interplay of climate, tectonics and eustasythroughout the late Quaternary.

The overview presented in this paper emphasizes the needfor an integrated approach to understand the complexities ofthe geomorphic form and stratigraphic development over thevast tract of the Ganga plains, which extend for more than350,000 km2. We emphasize the need for detailed multi-disciplinary studies on the surface and shallow sub-surfaceof the plains to decouple the responses of the river system toforcing functions. Such studies in different spatial domainshave an important bearing on the understanding of theaccumulation and filling of foreland basin systems in areasdominated by monsoonal regimes and Cenozoic tectonics.

Interdisciplinary, integrated research programmes arebeginning to provide some answers to the four questionsposed in the Introduction, as well as to many other importantquestions about the Ganga plains. However, considerable

research is needed to test more fully the hypotheses suggestedhere and to constrain the complex controls on the alluvialsystems. Many more well described and dated sections andcores are needed, especially in the Middle Ganga Plains whereno age model currently exists. Additionally, much insight willarise from comparing the Ganga plains alluvial record withrecords from elsewhere in the sub-continent, especially thoseof Himalayan valleys, Peninsula India, the Thar Desert, anddeltaic regions (which events are widespread and which localacross these areas with diverse tectonic and geomorphiccontrols?) In particular, millennial-scale climatic changes havebeen well documented in adjacent marine areas (Sirocko et al.1993) but their effects have yet to be established in mostalluvial terrains; documenting successions where alluvium isintercalated with climatically sensitive lacustrine and eolianstrata may provide a way forward. Although much researchhas focused in tectonically active regions, activity on majorfaults is localized and episodic, and a proxy record for tectonicevents through the late Quaternary has proved much moredifficult to define rigorously than proxy records for monsoonalprecipitation (Prell & Kutzbach 1987) and sea-level change(Blum & Törnqvist 2000). Consequently, more research isneeded in areas close to the Himalayan Front (e.g. Mukul2000), especially in key watershed areas where earthquakesmay have a major effect on river dynamics and wheretectonically induced changes in river course may exert a majoreffect on the hydrology of the plains rivers. Finally, muchmore modeling is needed to understand the complex balanceof water and sediment discharge through time in rivers of theGanga plains.

Acknowledgments: We are grateful to the Department of Scienceand Technology for providing grants to RS and SKT under the projectsanction number ESS/23/VES/110/2000. We thank our respectiveinstitutions, IIT Kanpur, Delhi University and Dalhousie Universityfor providing the institutional support. We are thankful to our students,Vikrant Jain and Subir Dutta for their help during the field work anddrilling programme. Our special thanks are due to Dr. K.R. Gupta,Department of Science and Technology for his endless enthusiasm andsupport for the Gangetic plain research and to Dr. B.R. Arora, Director,Wadia Institute of Himalayan Geology for inviting us to write thispaper.

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