Quaternary glaciation in the Nepal Himalaya

Quaternary Glaciations- Extent and Chronology, Part III Editors J. Ehlers and P.L. Gibbard 9 2004 Elsevier B.V. All fights reserved

Quaternary glaciation in the Nepal Himalaya Monique Fort

DYNMIRIS (Dynamique des milieux et risques), UMR PRODIG 8586, UFR GHSS, Case 7001, Centre de G~ographie Physique, Universit~ Paris 7- Denis Diderot, 2 Place Jussieu, F. 75 251 PARIS Cedex 05, France E-mail." fort@paris 7.jussieu.fr

that cut perpendicularly across the Himalayan massif. In the latter case the glacial drainage forms are of catastrophic character (characterised by a very steep topographic gradient). In these situations glacial deposits can easily be confused with similar sediments produced by the large scale, extensive mass-movement processes (debris flows, rockfalls and landslides; see below). Isolated massifs, separated from the highest massifs and the processes imposed by the proximity to steep rock walls (avalanches, mass movements) were occupied through the Quaternary by small ice caps. These places hold the most important evidence for understanding Quaternary glacial fluctuations. However, they have as yet received little attention, because they are far from the major communication routes and too close to the Tibetan border.

Knowledge of the extent of Quaternary glaciers in the Himalayan mountains of Nepal based on the accumulated evidence, still remains very fragmentary. However, Nepal occupies a significant central position in the Himalayan mountain chain, with 8 peaks over 8000 m high and with extensive regions over 6500 m a.s.1. Access to the country is quite variable. Several areas of the highest massifs provide the highest mountain climbing and walking stations in the world and are well served by a dense network of footpaths and resting places. However, to gain access to some valleys north of the High Chain, requires several days walk and special expensive permits to enter. This is also the case in western Nepal, which is far from the tourist routes and therefore access demands special arrangements. This explains why certain regions are effectively terra incognita from the perspective of Quaternary glaciations. The overview presented here is therefore incomplete. Another characteristic of the Himalayas in Nepal is the topographic contrast between the eastern and western parts of the countr)', a contrast that conditions both the glaciation today and in the past. To the east (the massifs of the Kangchenjunga, Everest, the Langtang, the Ganesh Himal and the Manaslu), the chain is sufficiently wide (> 150 km), representative of both the western and eastern parts of the Himalaya (Lahul, Garhwal, Ladakh, Karakoram). Here there are deep (c. 4000 m), well developed valleys partially occupied by glacial spreads deposited by glacial tongues that descended from vast glacial accumulation zones. However, in contrast the central and west of the Nepal Himalaya (the massifs of Annapurna and Dhaulagiri) show an asymmetry, intimately reflecting the strong tectonics. This gives rise to a marked climatic contrast between the south slopes, which receive heavy precipitation from the monsoons, and the arid, continental northern slopes (better developed in Nepal than at the frontier which occurs north of the main mountain crest). This climatic contrast is also reflected in the glaciations. A model was previously proposed by the author (Fort, 1995) that shows the different possible glaciation scenarios, which are controlled by the topography and the position of study sites in the Himalaya chain. It also includes the limits used for the reconstruction of past glaciations (fig. 1). In general, the separation of the individual stages of glaciation does not cause problems in the internal valleys within the chain where the deposits are well preserved, the climate is less severe and the longitudinal profiles of the rivers are shallower. The same cannot be said for the major valleys

General problems for the identification of chronology of ancient glacial stages Most of the results presented here were obtained by detailed fieldwork, especially geomorphological mapping, conducted under difficult conditions, including as a lack of topographic maps and aerial photographs, even though conditions have become easier over the last 10 years. The task of locating a site which is a prerequisite for the establishment of a tentative chronology, is very arduous in Nepal and in the rest of the Himalayas. This is because of the poor preservation of the glacial materials. The mountain chain uplifting constantly at a rate of millimetres per year (Bilham et al., 1997), forces the rivers to incise their beds and this causes a destabilisation of the valley slopes and their detrital cover which gives rise to mass movements. This mode of slope movement is even more intense on the steep slopes in the highest parts of the chain, where the paraglacial material is remobilised and glacial material is recycled as debris fans, rockfalls etc. This explains the extreme complexity of deposits that are preserved on the valley sides and bottoms. This complexity, in turn, results in a great diversity of sedimentary facies, which are very heterogenous petrographically. There is also great lateral mixture within the deposits that results in identification difficulties and the differing interpretations between various authors. The complexity of sedimentary facies associated with the sediment bodies generally allows the reconstruction of local palaeotopography, the slope systems, as well as the 261

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Fig. 1. Schematic cross section through the Greater Himalaya, showing recent and Late Pleistocene glaciation: a key to assess older glacial evidence and dating (after Fort, 1995). 1: recent glaciers; 2 avalanche-fed recent glaciers; 3 ." extent of Late Pleistocene glaciers; 4 : older glacial remains (either perched or buried) ," 5 avalanche tracks and Late Pleistocene avalanche-fed glaciers. 6 ." main types of glaciation features. A : isolated massifs on the south side, the climatically most sensitive areas, also allowing the assessment offormer glacial limits ; B : glacial tongues in very deeply-dissected valleys, derived from avalanche inputs, reconstructed ELAs abnormally low ; C : avalanche-fed glacier: the lowest altitudes are controlled by the steep topography ; D : high altitude glacial features and low dissection on north slopes, with dryness as a limiting factor," E : isolated massifs on the north side, also good indicators for glaciation (same as A) ; E : corresponds to upthrown, faulted-block (horst) ; F : perched remains of former glaciation (continuous uplift of the Greater Himalaya), to be found above the present valley bottoms; G : buried older glacial remains (continuous uplift causing more recent glaciation to override the limits o f the older ones) ; H : perched remains of glaciation (same as F, but on north slope), to be found on the deglaciated interfluves.

directions and concentrations of local slides, valley blockages etc. (including palaeolakes). However, the continuous occurrence of diamicton makes facies interpretation very difficult (Derbyshire et al., 1984; Hewitt, 1988; Owen & Derbyshire, 1989; Hewitt 1999; Fort, 2000). It is possible to confuse moraine deposits (tills), avalanche deposits and debris cones. In all three cases there is matrix dominance, the rock fragments show a great size range and their edges show clear signs of wear. However, some distinctive characters allow to differentiate rockfalls from other deposits; the clasts are very angular, the proportion of matrix is slightly reduced and often the deposits are overconsolidated. They may even show shear planes (Weidinger & Schramm, 1995; Schramm et al., 1998). Another characteristic feature is their petrographic composition. In general, tills contain petrographic assemblages reflecting the rocks of their tributary valleys and glacial basins. By contrast, rockfall deposits are composed of a more limited, often monolithological assemblage. These distinctions are often difficult to establish in eastern Nepal where crystalline rocks (gneiss and granites) predominate that can be easily confused but they can be distinguished in the Annapurna and Dhaulagiri massifs. Here the highest peaks are composed of calcareous metasediments or quartzites overlying the gneiss which occurs in the lower part of the slopes (Fort, 2000). The origin and nature of the material can therefore be clearly identified. The other great problem is the establishment of a reliable chronology. For the most part, following mapping

of landforms and deposits, studies rely on relative chronology, based on the freshness of the landforms, their degree of weathering (Schmidt hammer test), the thickness of the weathering rind, the development of superficial microforms, the type and depth of soils, the amount of vegetational development, the presence and thickness of a loess cover. It has not always been possible to find adequate material for radiocarbon dating, because of the arid climate and the instability of ground surfaces. For several years technical progress has made it possible to test new dating methods, e.g. cosmogenic radionuclides and Optically Stimulated Luminescence (OSL) techniques. The results of these analyses must, however, be interpreted with caution because the samples require a very good previous knowledge of the landscape, since there can be many sources of error or ambiguity (see the excellent discussion in Tsukamoto et al., 2002). Before presenting the state of present knowledge, it is useful to consider the general palaeoclimatic context of the region. The goal of the investigation of Quatemary glaciations in the Himalayas is to demonstrate the relationship between the glaciation phases and the strengthening of the Indian Monsoon (Gillepsie & Molnar, 1995). One of the debates centers on the age of the maximum glacial advances during the last glaciation stage (Benn & Owen, 2000; Owen et al., 2002). Does the Last Glacial Maximum (LGM) correspond to Marine Isotope Stages (MIS) 2, 3 or 4? Is there evidence throughout the Himalayas for a general glacial advance after the beginning

Nepal

263 Fig. 2. Glacier extension in each advancing stage in the Ghunsa Khola drainage, Kangchenjunga Himal (after Asahi & Watanabe, 2000). LIA : Little Ice Age ; YD : Younger Dryas ; LGM : Last Glacial Maximum ; MIS4 : Marine Isotope Stage 3 or 4. Note that new OSL dates obtained by the Same team (Tsakamoto et al., 2002) give a 20-2 1 ka age to the Gyabla Stage.

Fig. 3. Khumbu Valley, upstream of Pheriche (down valley view). The morainic ridge o f Pheriche can be seen to the left (4150 m), a third of the way up. In the left background, the Tsuro glacier, descended down the Ama Dablarn (6856 m), displays a frontal rampart that has been breached out by the outburst of a subglacial water pocket that occurred in the early 1970 "s.

264

Monique Fort

Fig. 4. Geomorphic map showing the main glaciers, moraines and OSL sampling locations for the Lhotse Nup terminus, the Lhotse moraine, the Dingbache and Periche area, and the Khumbu Glacier study area (after Richards et al., 2001). The outer morainic ridges indicate the extent of glaciation during the Last Glacial Maximum.

of the Holocene? In this regard, Nepal occupies a particularly important transitional position, between the region influenced by the western winds and that influenced by the monsoon. Thus the eastern half of the country is subjected to the full force of the Indian Monsoon, which brings abundant precipitation in summer and snow falls at high altitude in addition to the winterly snow. On the other hand, the western part of the chain receives winter precipitation from the west, during which time the monsoonal flow cools the imposing wall of the Annapurna and Dhaulagiri massif (rain-shadow effect). For this reason, the palaeoclimatology of the western and eastern Himalayas cannot be treated precisely the same. It is exactly in central Nepal that the transition, reinforced by the narrowness of the chain and the limited size of the glacial drainage basins makes correlations between the glacial stages more difficult. Regional sequences" the state of knowledge The massifs of eastern Nepal The western slope of the Kangchenjunga massif (8586 m a.s.l.)

Kuhle (1990) was the first to map the glacial morphology of the Ghunsa khola and Shimbuwa khola valleys that are situated on the western slope of the Kangchenjunga massif Meiners (1999) completed the work using the same

terminology as that developed for the Kali Gandaki valley (West Nepal) by Kuhle (1982), with minor modifications. She identified eight glacial stages, with the maximal glacial extension at Thuma (890m a.s.1.), downstream from the confluence of the Ghunsa khola with the Tamur fiver. This interpretation was rejected by Asahi & Watanabe (2000), who identified five glacial stages and placed the maximum ice advance limit at Gyabla (2730 m a.s.1.). They attributed this event to MIS 3 or 4 on the basis of morphological criteria and relative chronology (Fig. 2). However, the OSL dates obtained by Tsukamoto et al. (2002) indicate an age of 20-21 ka for the Gyabla Stage. Their OSL dates suggest that the less extensive phases are of Holocene (8-10 ka and 5-6 ka respectively) and Neoglacial age (Little Ice Age). The Khumbu Himal.

The Khumbu Himal represents the southern slope of the Everest massif (Fig. 3). This relatively accessible area is dominated by three peaks over 8000 m a.s.l., the Mount Everest-Sagarmatha (8848 m a.s.1.), the Lhotse (8501 m a.s.1.) and the Cho Oyu (8153 m a.s.1.). From this area numerous observations of modem (Higuchi, 1976) and past glaciers have been reported (Heuberger, 1956; Iwata, 1976; Fushimi, 1978; Miiller, 1980; Williams, 1983; Heuberger & Weingartner, 1985). There is disagreement concerning the maximum extent of the glaciers in the Dudh Kosi valley (cf. synthesis by Fort, 1995, Owen et al., 2000 and Asahi & Watanabe, 2001). Iwata (1976) considered that the glaciers

265

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This interpretation has been conf'Lrmed by radiocarbon dates by van Williams (1983) and Rtithlisberger & Geyh (1986). More recently, this chronology has been strengthened by OSL dating (Richards et al., 2001); the Periche Stage representing the LGM (c. 18-25 ka), the Chukung Stage representing the Neoglacial (c. 10 ka), and the Lobuche Stage the late Holocene (c. 1-2 ka) (Fig. 4). The dates strongly support the view that the glacial tongues of the Tyangboche Stage advanced beyond the limits of MIS 2 in this region.

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descended to Tyangboche (3700 m a.s.l.), whilst Fushimi (1978) thinks that they reached Ghat (2780 m a.s.1.), or even Lukhla (2200 m a.s.1.). This hypothesis is not accepted by Iwata (1984) and Heuberger & Weingartner (1985). The latter authors interpret the Lukhla terrace as slope-fall material. Heuberger & Weingartner (1985) nevertheless think that glaciers might have descended as far as Surkhe (1580 m a.s.1.), where they have identified strongly altered, but undated, old tills. Studies on the Last Glacial Maximum (LGM) are mostly concentrated in the Imja khola valley where some radiocarbon dates have been determined. Fushimi (1978) and Iwata (1976, 1984a) have recognised three ice-marginal positions of Little Ice Age (Lobuche Stage), Neoglacial (Thukla Stage) and LGM age (Periche Stage). An additional Tyangboche Stage is thought to represent a glacial event (MIS 3 or 4?) prior to the Last Glaciation.

An isolated massif continues to the south-east of the Khumbu massif; the Shiptong col area (some 5000 m a.s.1.). This high ground around the col shows all the characteristics of an area formerly covered by a small ice cap (Yagi & Minaki, 1991): ice-scoured bedrock, roches moutonn~es, glacial rock bars/knobs (verrous) and basins (ombilics), and morainic deposits. These authors recognised four gla-cial stillstand positions in this area (Yagi & Minaki, 1991; Fig. 5). The two most recent stages, the Mathi Kopi (4200 m a.s.1.) and the Kopi La Stages (4150 m a.s.1.) are characterised by distinct, poorly-vegetated morainic ridges which Yagi & Minaki (1991) consider are of Neoglacial (Holocene) age. The Thulo Pokhari Stage corresponds to a more extensive advance phase. It is characterised by widespread erosional landforms and by frontal moraines at 3600 m a.s.1. This stage has been correlated to the LGM (c. 18 ka), because the sediments which occupy one of the glacial depressions formed by this event are dated at 11,000 BP (Minaki & Suzuki, 1989). A still older ice-marginal position (the Numbukk Stage) occurs at 3100 m a.s.l, and is thought to represent an advance that pre-dates the last glaciation.

The Rolwaling valley and the Shorung Himal massif To the west of the Khumbu Himal, the Rolwaling valley (>6700 m) and the Shorung Himal (6959 m) occur. These strongly glaciated mountains have only been briefly examined so far (Meiners, 1999; van Williams, 1983). In the valleys that descend from the Shorung Himal, several stages of glaciation have been identified by van Williams (1983). These several systems of intersecting morainic ridges probably represent the Little Ice Age and the late Neoglacial Yuligolcha Stages. Three to four kilometres downvalley, another morainic system, the Tamba Stage, is well preserved. A f'mal morainic assemblage is found over 10 km downvalley from the modem glaciers, preserved on the sides of the Dudh Khunda (van Williams 1983) and Basa Drangka valleys (3400 m a.s.1.; Fort, unpublished). These forms are poorly preserved and soil formation in the material is already considerably advanced. Comparison with the landforms observed close the Khumbu suggest that they date from the LGM. In the Rolwaling valley, Meiners

266

Monique Fort

(1999) considers that during the late Pleniglacial glaciers descended as far as the confluence with the Bhote Kosi, i.e. 900 rn lower than present.

Langtang Massif North of Kathmandu, the northern part of the Langtang valley is dominated by the southen flank of the Xixabangma (8027 m a.s.1.) and the Langtang Lirung (7239 m a.s.1.) mountains. This valley, together with the Khumbu, is the most studied but these studies have provided very confusing results concerning the number of glacial stillstand positions: for example, Heuberger et al. (1985) have identified four, Ono (1986) found five and Shiraiwa & Watanabe (1991) have identified six ice marginal positions

(see also discussion in Owen et al., 2000). As in the Khumbu valley, the altitude of the glacial maximum is much discussed. Usselmann (1980) proposed 2400 m a.s.l., whilst Ono (1986) and Shiraiwa & Watanabe (1991) agreed on 2600 m (Lama Stage). That is the altitude below which the valley morphology changes (the development of gorges). The Lama Stage is thought to represent an ice advance preceding the last glaciation. According to Shiraiwa & Watanabe (1991) and Shiraiwa (1993), the Gora Tabela Stage (3200 m a.s.l.), immediately upvalley from the Lama Stage moraines, probably represents the LGM limit (Fig. 6). The same authors have shown that the stages inside the LGM position, generally referred to the Late-glacial, are in fact more recent and appear to be of Neoglacial age on the basis of radiocarbon dates: the Langtang Stage 3650-2850 BP, the Lirung Stage 2980-550

Nepal

267 (Fig. 9). These moraines are followed downvalley by other morainic systems (Samdo Stage, 3700 m a.s.1.; Fort, 1979), separated from the modem glaciers by an average distance of 2 km (Fig. 10). These moraines probably represent a Holocene-age stillstand. Further still downvalley are more impressive morainic accumulations up to 400 m thick. These deposits comprise subglacial till overlying meltout till facies, passing down-valley into spreads of glaciofluvial sediments. These mo-raines almost certainly represent a major stillstand position (Lho Stage, 3500 m a.s.1.; Fort, 1979) that may be the LGM (MIS 2?) (Fig. 11). In the same area other morainic-ridge elements can be found perched about 400 m above the floor of the Buri Gandaki valley. They can be followed downvalley as far as Prok and Gap, where they pass into a frontal morainic system. These deposits possibly represent a stage that preceded the widespread Last Glaciation (MIS 3 or 4?), or possibly an even older event. Further south the glacial tongue that descends from the amphitheatre formed by Peak 29 and the Himalchuli may according to Jacobsen (1990) have extended downstream beyond the confluence of the Buri

Fig. 8. View from the Larkya Pass (5200 m), down valley. The three glaciers, descended from the Peri Himal (6892 m), are converging near the foot of the west face of Manaslu Himal, near the so-called Bimtang area (3630 In).

BP (B~iumler et al., 1996). The least extensive phases, theYala I and II Stages, represent the Littl~ Ice Age. Complementary studies, undertaken on soil development in the morainic deposits, have confirmed the recent character of the Langtang and Lirung Stages.

Manaslu Massif

The Quaternary glaciation of the Manaslu massif (8156 m a.s.1.) and the neighbouring peaks (Peak 29, 7870 m a.s.1. and Himalchuli, 7892 m a.s.1.) still remains poorly understood. These regions have only been superficially investigated. Geomorphologically, they are essentially surrounded by north- and east-facing slopes (Fort, 1979; Jacobsen, 1990) that are today characterised by large glacial tongues extending into the deep tributary valleys of the Buff Gandaki (Figs 7 and 8). In the absence of an absolute chronology, it is only possible to present a morphological overview. In the upper part of the Buff Gandaki, parallel to the recent glacial tongues, a system of morainic crests is found, probably of Little Ice Age or Late Holocene age

Fig. 9. Manaslu Himal, Upper Buri Gandaki Valley, upstream o f Sama Gompa. The Lake in the foreground developed behind Holocene moraines built by a glacial tongue from the east face of Manaslu. The ridge (left background) corresponds to a lateral moraine, attributed to the Last Glacial Maximum. Beyond the Buri Gandaki valley, an un-named glacial tongue from the west side of the Ruku (Nupri) Himal is fringed with Little Ice Age moraines.

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Monique Fort

Fig. 10. Manaslu Himal, Upper Buri Gandaki Valley. From the north slopes of the Ruku (Nupri) Himal, the Samdo glacier and its set of frontal moraines, belonging to Holocene (foreground) and Little Ice Age (middle ground), as displayed around Samdo village (3850 m a.s.l.)

Fig. 11. Manaslu Himal. Down valley view of the Buri Gandaki valley, from a rocky ridge overlooMng Sama village. The forested hills correspond to the frontal morainic System of Lho (3200 m a.s.l.), built up during the Last Glacial Maximum advance of the Pung Gyen glacier, from the east face of the Manaslu (8163 m a.s.l.). These deposits, over 300 m-thick, display complex sedimentary facies that suggest a long glacial stand, occasionally affected by a succession of small advances and retreats.

Gandaki with the Nyak (1580 m a.s.1.). However, the 'morainic stage' of Philim-Setibas mapped by Jacobsen (1990), is actually an enormous landslide.

The massifs of eastern Nepal Annapurna Massif The Annapuma massif stretches from east to west with a series of peaks (Lamjung Himal 6983 m a.s.1.; Annapuma

II, 7937 m a.s.1.; Annapuma IV, 7524 m a.s.1.; Annapuma III, 7555 m a.s.1.; Annapurna I, 8076 m a.s.1.) and provides a very abrupt southern face. The north face is equally steep, thus it is very likely that glaciers developed here along the line of the crests, were steep and fell in deeply-crevassed ice falls. The north slope is discussed first, then the southern slope before continuing to the western slope, the history of which is difficult to separate from that of the Dhaulagiri massif both of which border onto the Kali Gandaki valley.

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269

Fig. 12. Upper Marsyang-di, up-valley view, The Manang lake developed behind the frontal morainic system of the Gangapurna Glacier (7485 m). The morainic ridges have been radiocarbon dated (Holocene and Little Ice Age; Rrthlisberger & Geyh, 1985). During these stages, the valley was dammed and a lake and fanterraces developed upstream (middle right of the photograph). The gullied and forested landforms in the middle ground correspond to rock avalanche diamictites. Glaciated slopes of the 'Grande Barriere' are visible on the background.

Fig. 13. Upper Marsyangdi valley, upvalley view. Major landforms of the valley bottom are controlled by the very thick deposits of a catastrophic rockfall (c. 12 km 3) derived from the northern flank of the Annapurnas (on the left). The rockfall material (coarse and over-consoli-dated diamictites) is recognisable by its specific shapes (pilars and deep gullies), as displayed in the fore-ground, where Braga village is situated. After the rock avalanche event, the glaciers invaded the Marsynagdi valley again and carved out the diamictites, during a stage that probably represents the Last Glacial Maximum advance. The light grey spot in the middle corresponds to the fini-Holocene, frontal moraine of Manang. In the background, the glaciated ridge of the 'Grande Barriere '.

The high valley of the Marsyangdi

The foot of the northern slope of the Annapuma comprises the high valley of the Marsyangdi, which is part of the arid Himalaya region. The results obtained from this valley are at present only preliminary (Hagen, 1969; Bordet et al., 1976; Fort, 1977 in Fort, 1993). In contrast to the late Holocene, the modem glaciers do not reach the trunk valley floor anymore. Opposite the village of Manang (3600 m a.s.1.), at the exit of the Gangapurna glacier (7455 m a.s.1.), the very well preserved Little Ice Age end moraine (15501875 A.D.) is found. It fits between three other moraine ridges, all dating from the late Holocene (dated respectively at 1200 BP, 2350-3000 BP and 2350-4600 BP; R6thlisber-

ger & Geyh, 1986) (Fig. 12). Other older morainic remains are also preserved, but they must not be confused with the giant rockfall deposits, over 200 m thick, that effectively seal off the valley (Fort, 1993; Fig. 13). This deposit consists entirely of limestone clasts in a pulverised and partly recemented matrix. This partly covers the moraine ridges that can be followed downvaUey to Bangba village, where the valley bottom is blocked by a frontal moraine rampart (Fig. 14). At the mouth of the valley where a tributary glacial tongue descended from Annapurna II, another frontal morainic system can be seen between the villages of Pisang and Bhratang, apparently relating to the same stage. The Bangba and Pisang Stages probably represent the maximal advance of glaciers during the Last Glaciation. At the

270 Fig. 14. Late Quaternary evolution of the Upper Marsyangdi (Manang) Valley. Note the contrasts between both flanks of the valley, and the complex relation between debris of the giant rockfall and more recent glacial deposits.

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The southern slope of the Annapurnas On the southern slope, observation is more difficult because the valleys on Annapuma are very steep and greatly incised in gorges. Fort (1986, 1987) has proposed a scheme according to which the glaciers descended to very low altitudes in a catastrophic fashion (Ghandrung, 1520 m a.s.l.; Ghachok 1130 m a.s.1.; Taprang 1200m a.s.1.) (cf. Owen et al., 2000). In particular, mapping in the Seti khola valley that drains the Pokhara basin has shown that there are several accumulations that can be interpreted as morainic ramparts (Fig. 15). However, today the author thinks that this original interpretation was wrong and that the diamicton deposits instead represent several very large landslips. These landslips temporarily blocked the valley, the collapse of which dam caused the catastrophic flooding and enplacement of over 4 km 3 of debris throughout the Pokhara basin (Fort, 1987). The glacial advance stages should be expected at higher altitudes, upvalley of Siklis (Madi Khola) and Chumro (Modi Khola). In the context of the unstable and deeply-incised slopes of that area, a reliable chronology cannot be established for the deposits that remain.

Taunja-Namun Massif By contrast, in this region there are interfluves that are sufficiently high to have been overrun by local ice. This is

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the situation on the Taunja-Namun massif (5208 m a.s.l.), on the southem promontory of the Lamjung Himal (6983 m a.s.l.), east of Annapuma II. The morphological configuration of this massif can be compared to that of the Shiptong col to the SE of Khumbu Himal. There also well developed erosional landforms (roches moutonn~es, ombilics and verrous) and glacial accumulations (moraine ridges), as far as where the summit crests exceed an altitude of 4300 m a.s.1. (Fig. 16). They are considered as the lowest limit of glaciation (Fort, 1988). These morainic deposits are found to 3500-3600 m a.s.1, that probably mark the maximum ex-tent of ice during the Last Glaciation, but those landforms are poorly preserved. Other morainic ridge systems are spread between 3900 m and 4100 m a.s.l. (Fig. 17). Considering the altitude and position of this massif, which strongly favours trapping of moist air masses, it is possible that these moraines could be the product of Lateglacial-age glaciation. Or it may indeed be of Holocene age if it is shown that during this time the ice accumulation could have been reinforced by the Indian Monsoon which supplies frequent snow falls to this high ground.

The Kali Gandaki valley, between the western slope of Annapurna (8076 in a.s.l.) and the eastern slope of Dhaulagiri (8172 in a.s.l.). The Kali Gandaki valley, the bottom of which lies at 1800 to 2400 m a.s.l., is the deepest gorge in the world. The Dhaulagiri stands directly above the valley so that Annapuma remains difficult to see, hidden by the Nilgiris massif (South Nilgiri, 6839 m a.s.1.; North Nilgiri, 7061 m

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Fig. 15. Quaternary glaciation south of the Annapurna III, along the Seti khola valley (after Fort, 1995). (A) Patches of diamictons, formerly interpreted as till; now considered as remnants of several large landslides 9 1 9Present glaciers 92 9Landslide material (age 5 yr2

10 yr4; Fort, 1987) 93 " Older, probably Holocene landslide material 9 4: Oldest, brecciated landslide material. B): Former, now revised, reconstruction of the Last Glacial Maximum in the Seti Khola valley (shaded)," in fact, the glaciers probably did not expand further downstream than the Seti/Mo'ba confluence. Stippled area: present glaciers.

a.s.1.), which dominates the southem side of the Kali Gandaki. In spite of the extreme altitudes reached by these mountains, modem glaciation is poorly developed. This is because of the deeply-incised slopes, the very small potential accumulation zones and the marked aridity which increases from south to north. The lower limit of the modem glaciers varies between 4800 m a.s.1. (east face of the Dhaulagiri), 4200 m a.s.1. (north face of the North Nilgiri) and 3800 m a.s.1. (west face of the South Nilgiri) (Fig. 18). The reconstruction of Quaternary glaciations poses a problem. As a consequence of the strong topographic gradient and the continuous occurrence of diamicton accumulations, which range from moraine material to mega-slips of remani~ glacial material or huge rockfalls (rock-slides, rock-avalanches). Moreover, the pre-

dominance of sedimentary bedrock, and in particular carbonates, has so far prevented the use of numerical dating methods. This is illustrated by a recent investigation (Fort, 2000), that emphasises the limitations, particularly concerning glacial chronology, that affect this most complex area.

The tributary valleys of the Karl Gandaki

The tributary valleys of the Kali Gandaki are the easiest to study (Fig. 19A); here the moraine ridges are generally sufficiently well preserved to allow the reconstruction of the glacial advances in the Muktinath and Thini valleys on the left bank and in Syang and Tukuche valleys on the fight bank (Fort, 1995). In the Muktinath valley (Jhong khola),

272

Monique Fort

Fig. 16. Tauja-Namun Himal (5208 m). lnfilled former glacially eroded basin, fringed by frontal morainic ridges (right background) and by subglacial till (note the fluting). The altitude 0800 m a.s.l.) and the pattern of the subglacial deposits suggest that glaciers reached even lower altitudes. This is a good evidence for abundant ice flux, characteristic of these 'isolated' massifs and of their prominent situation south of the main Greater Himalayan Ridge. Nowadays, their altitude is too low to be glaciated and their evolution is controlled by cryo-nival processes.

Fig. 17. The western part of the TaunjaNamun Himal and ridge overlooking the Madi khola valley (beyond the ridge). Glaciated peaks in the background are Macchapuchare, 4993 m a.s.l. (left), and slopes of Annapurna 111, 7555 m (righO. On the rocky ledge (foreground), morainic ridges are well preserved at about 4000 m a.s.l. This outer morainic system probably represents a quite extensive advance during the Last Glacial Maximum, yet the sharp break of slope suggests the glaciers may have descended even lower during an earlier stage.

Fig. 18. West face of Nilgiri Himal. Martse morainic ridges observed from the opposite, right bank of the Kali Gandaki. A series of glacial cirques have developed below this ridge not exceeding 5200 m a.s.l. The small glacial tongues issued from these cirques have built a series of nested morainic ridges, the outermost one likely to represent the Last Glacial Maximum.

Nepal several stages have been recognised (Iwata, 1984b; Fort, 1980, 1985): the Thorung III, II, I Stages and the Dzong Stage. The latter represents the maximal extent of the Last Glacia-tion (LGM). At present, there is no basis for a correlation with one of the MIS. The other tributary valleys show simi-lar sequences. The Shokang and Langpoghyun, that drain the north slope of the North Nilgiri (7061 m a.s.1.), are more complex (Fig. 20). In their upvalley areas they contain morainic ramparts very close to the glaciated rock wall. Downvalley, these valleys are infilled by over300 m thick alluvialfan-like spreads that contain mixed assemblages of glacial and avalanche debris (debris flow facies). They are typical of this environment where deeplyincised glaciers occur. At the mouths of these valleys in the Kali Gandaki valley, the remains of two 'glacio-avalanche' diamictons are preserved, the Syang and Jomosom formations, which were catastrophically emplaced respectively betbre and after the Marpha lacustrine episode (i.e. before MIS 4 and in the late Holocene; cf. Fort, 2000).

East face of the Dhaulagiri

The reconstruction of the extent of glaciers that descended the eastern face of the Dhaulagiri remains problematic. In the Larjung amphitheatre, only thick accumulations of debris flows are preserved, on which small morainic ridges represent the traces of regenerated, hanging glaciers ('firnkessel' glaciers) below the rock wall. On the left bank of the Kali Gandaki, facing the Larjung, the slopes are covered by brecciated diamictons that might represent the tills that formed during the steep descent of the glacier east of the Dhaulagiri. This may have dammed the valley and blocked the drainage (Lake Marpha; Fort, 1980). If this hypothesis is correct, this glacial advance pre-dates the first Marpha lacustrine sediments (dated to 79 + 11 ka-~, Baade et al., 1998) and could have been of MIS 5 age. On the same basis, the hanging glacier at Chokopani (Fig. 21), nourished from the western face of North Nilgiri, could represent a more recent stage than the Last Glaciation. It was regenerated below and therefore blocked the Kali Gandaki valley (Fort, 1995), as suggested by the morainic ridges preserved north of Tukuche, several tens of metres above the fiver (Fig. 19B).

South-western face of the South Nilgiri

Further downvalley, the glacier on the south-western face of the South Nilgiri (6839 m a.s.1.) descended as far as Taglung (possibly during the LGM) and before as far as Porchedanda (prior to the Last Glacial stage?) (Fort, 2000). The possibility of a glacial advance down to Ghasa is no longer excluded, but traces of this advance are far from clear; the area has been strongly remodelled by mass movements. During the Last Glaciation the glacial system from the north face of the Annapuma no longer expanded into the Miristi Khola gorges to form the principal tributary

273 on the left .bank of the Kali Gandaki (Fig. 19A). Downvalley from Ghasa, the deposits perched on the flanks of the Kali Gandaki are not glacial deposits, as had been suggested by Kuhle (1982), but land slip or slide deposits, or catastrophically-emplaced debris flows (represented by the Miristi Khola confluence terraces). Overall, the extreme complexity of the high basin drained by the Kali Gandaki and its tributaries results from the predominance of catastrophic deposition. This results in a lack not only of preserved evidence of the evolution of slopes (cf. the Dhampu rockfall; Fort, 2000) but also of evidence of glacial extent, perched between 4000-6000 rn above the valley bottom. Apart from the OSL dates on the Marpha lacustrine sediments (Baade et al., 1998) andseveral radiocarbon dates obtained from the Holocene stages in the Muktinath valley (Iwata, 1982, 1984b), no reliable chronology is currently available. One therefore must rely on assumptions based on a relative chronology based on the relationships of the sedimentary bodies. Finally, despite these chronological weaknesses, the glacial reconstructions based on the preserved sedimentary sequences suggests that the region was subjected to very limited glaciation, in spite of its proximity to the very high peaks. This conclusion may be partially explained by the difficulty in determining the strength of the glacial tongues, the traces of which are rapidly destroyed on the deeplyincised slopes. The latter did not provide sufficiently extensive snow accumulation zones. This is further enhanced by the position of the two massifs and their northern slopes in the Himalayan setting which provides a relatively restricted summer snow supply, because of the rapid weakening of the Indian Monsoonal flow, giving rise to a strong latitudinal aridity gradient. On the other hand, the Kali Gandaki valley could, and still can, be subjected to other catastrophic geomorphological phenomena (e.g. rock slides, rockfalls, floods and earthquakes; cf. Fort et al., 1982; Fort, 1996, 2000). The phenomena would readily cause the destruction of glacial evidence by erosion or modification and thus make it very difficult to reconstruct the glacial history of the region today.

Discussion and conclusion

The foregoing discussion of the glaciations that occurred in the Nepal Himalaya show there considerable work remains to be done. Firstly, the inventory of the products of past glaciations is far from complete; enthT.eregions remain to be discovered and mapped. This is a crucial aspect, but it is made more difficult because much of the country lacks basic documentation, such as topographic maps and aerial photographs. Moreover, access to certain regions is limited, and there are also problems of internal security within the country. Secondly, great chronological difficulties also exist. New numerical dating methods have only been applied to relatively easily accessible sites and on relatively favourable materials. From this point of view, the very dry

274

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Fig. 19 (B). Detailed map of the Chokopani catchment (west face o f North Nilgiri) (after Fort, 1995). Along the axis o f the west face of North Nilgiri, which is still occupied by an avalanche-fed glacial tongue, Late Pleistocene glaciers flowed directly down the Chokopani gorge across the Kali Gandaki valley bottom (4 km distance); they deposited the frontal moraines that are now emerging as hills above the middle terrace level (debris-flow material) of Tukuche. Along the WSW-oriented spur branching out o f the West Nilgiri peak, three small formerly glaciated cirques are well preserved on the north side. They are externally bounded by a series o f arcuate frontal moraines resting upon a rocky ledge (3900 m a.s.l.) that overlooks the Chokopani gorge. Tthe outer morainic crests are related to the Last Glacial Maximum. No glacial evidence can be found where the spur descends below 4200 m; it is therefore considerd that this altitude is a good approximation o f the local glaciation limit during the LGM. Key: 1 : Present glaciers ; 2 : till attributed to the Last Glacial Maximum; 3 : + 150 m and + 100 m debris-flow terraces ; 4 : +400 m fans and terraces ; 5 : Kali Gandala" flood plain ; 6 : glacial cirques ; 7 : morainic ridges ; 8 : terrace edge ; 9 : gorges ; 10 : reconstructed L G M advance of the Chokopani glacier, descending from the North Nilgiri.

northern Himalayan regions, with their abundant deposits, derived from sedimentary rocks, offer less favourable conditions. In other respects, these dating methods and the interpretation of their results require some precaution. It is therefore preferable to apply multiple methods (OSL, cosmogenic radionuclide, 14C) at each site to establish regionally-reliable correlations. Nevertheless, the results obtained so far suggest that without doubt the LGM in Nepal occurred not during MIS 2 but rather in MIS 3. Thirdly, the mapping of past glaciation in general shows that glaciers were never very extensive: The reconstructed length of the glacial tongues is of the order of 15-20 km, and is really very short when considering the height of the peaks surrounding the glacial basins. This can be explained by three factors: The very steep topographic and climatic gradients on the slopes, which, particularly at lower (subtropical) altitudes, often prevent the preservation and advance of ice which thaws before it is able to flow further downvalley. This is particularly true for the southern valleys, orientated north-south, such as the Ghunsa Khola-Tamur, Buff

Gandaki, Seti Khola and Kali Gandaki. Only the internal valleys that run parallel to the mountain ranges (Haut Khumbu, Rolwaling, Langtang and Marsyangdi) allow the development of longer ice tongues. The position of Nepal within the Himalayan chain is also significant. A position far enough to the east and south to have been controlled throughout the Quaternary, as today, by the abundance and seasonality of high altitude pecipitation. In the situation where the snow supply is for the most part controlled by the flow of the Indian Monsoon, where accumulation occurs mostly in summer, with freezing phenomena only occurring from the beginning of autumn, the effect of such climatic conditions is to reduce the volume of glacial ice. This explains the contrasts of glaciation styles between eastern Nepal and the Karakoram, the latter being characterised by summer dryness. The division of the high mountains, which makes the Nepal Himalayas and the Pakistani Himalaya-Karakoram opposites. The Himalayan chain is narrowest in its central, Nepalese part. As has already been discussed, it is the Annapuma and Dhaulagiri formations where this

276

Monique Fort

Fig. 20. (82.37.32). North Nilgiri Peak (7061 m). Below this very steep north face, two sets of morainic ridges are well preserved." the younger one (near the snow limiO and the older one, underlain by thick, gullied, deposiis. These latter are in fact composed of a series of coarse, diamictic beds, either unlayered (tills) or crudely layered (glacial debrisflow), alternating with well layered and sorted beds (pro-glacial gravels), a succession of sedimentary facies quite representa-tive of this very high-energy type of glacial environment.

Fig. 21. (74.12.32). West face of North Nilgiri Peak (7061 m a.s.l.). Avalanche-fed glacier, suspended on a rocky ledge (3800 m a.s.l.) perched 1300 m above the Kali Gandatd valley bottom.

narrowness is most evident. However, compared to the western Indo-Pakistani ranges, where the high plateaux such as the Deosai, provided vast glacial accumulation

zones situated well above the lower glaciation limit during the Quaternary cold stages, even the Everest or Kangchenjunga massifs appear to have been less favourable to supply powerful glacial tongues: the highest altitimde localities (>6000 m) are not the most suitable in this respect. There the air is thinner and can only contain reduced quantities of water vapour. In fact, it is the 'small massifs', that occur to the south of the highest ranges, with their moderate slopes, large areas of which occur at altitudes between 4000 and 5500 m a.s.l., cf. the Shiptong La and Taunja-Namun massifs, and also the Gosainkund (south of the Langtang), the southern flank of the Ganesh Himal and the Gorkha Himal massif (south of the Himalchuli; not described here), that were the most extensively glaciated by small ice caps. The latter are indicated by very clear morphological evidence that is the best preserved since it is protected from later erosion by its perched, interfluve position. To conclude, it is hoped that these descriptions will encourage further field investigation (reconnaissance, mapping, site description and application of modem methods). The complex interactions between the Himalayan glacier advances and the forcing that influences the ice masses, especially the climatic forcing (Indian Monsoon from the south, westerly, 'Mediterranean' air flow), but also the seismotectonic and topographic movements (ice falls and catastrophic avalanches), are still far from being completely understood. In particular, it has to be considered that the debris cover of a glacier, which is an expression of the dominant slope instability in the glacial accumulation zone, plays a major role that effects glacial activity by reducing and/or slowing the ablation of a glacial tongue. It is therefore necessary more than ever to work at the different scales, not only at that of a whole mountain chain, but also at the regional and local scales, to establish the hierarchical relationships between the different factors that control the global ice masses. These factors operate concurrently, rather than against each other. Understanding

Nepal their role and their individual importance will allow the presentation of a model of glaciation in these mountains, especially for the Last Glaciation. However, this model should not be 'monolithic' but should integrate all the palaeogeographical details, such as that presented herein and to explain the situation of the present-day Himalaya. Translated by Philip Gibbard.

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