Namcha Barwa Research Articles

Susceptibility assessment of glacier-related debris flow on the southeastern Tibetan Plateau using different hybrid machine learning models

The southeastern Tibetan Plateau (SETP) is a construction area of several key infrastructure projects in China, such as the Sichuan-Tibet Railway and hydropower developments, which has historically faced the threat of glacier-related debris flows. However, a robust assessment of such debris flow susceptibility is a challenge due to the complex and variable climate, terrain and glacial environment. In this study, we used the hybrid models that combine statistical techniques (certainty factors, CF) with machine learning methods (logistic regression, LR; random forest, RF; extreme gradient boosting, XGBoost) to more accurately identify debris flow susceptible (DFS) areas. Topography, geology, and hydrological factors including glaciers and snow cover were used in these models to assess the DFS. Results show that 21 % to 42 % of the study area is very high susceptible to debris flows, particularly from Ranwu to Bomi and around Namcha Barwa. The hybrid models effectively enhance the accuracy of the DFS assessments. The CF-RF model showed the greatest improvement, with an 8.4 % increase in accuracy compared to the single model, the DFS spatial distribution of which aligns closely with field survey results. The glacial area ratio and annual snowmelt positively impact DFS accuracy, ranking 2nd and 9th in the factor importance, respectively. The results of this research could provide valuable assistance and guidance in mitigating glacier-related debris flow hazards.

3-D flow velocity time-series analysis from SAR-derived datasets towards maritime glaciers in the Namcha Barwa, Southeastern Tibetan Plateau

In reaction to a warming climate in the Southeastern Tibetan Plateau (SETP), the direction and intensity of surface flow in maritime glaciers undergo adjustments, causing glacial debris flow and ice avalanches. The safe operation of mega transportation projects across the region has been seriously threatened, e.g., the Sichuan-Tibet transportation corridor. Consequently, the measurement in three dimensions (3-D) consistently merits attention in the surface flow evolution for this glaciated region. In this study, we first mapped the 3-D and long-term surface flow of the glacier ensemble in the Namcha Barwa during 2019–2022 using the space-borne Synthetic Aperture Radar (SAR) technique, specifically applying the pixel offset tracking (POT) method to obtain deformation estimation of SAR-derived data. Subsequently, 3-D glacier velocities were acquired from the Multidimensional Small Baseline Subset (MSBAS) method conducted with the above datasets. Results show that two fast-moving glaciers were detected in the glacier ensemble, namely Dem Glacier and Namjagbrawa Glacier. Dem Glacier mainly flows eastward horizontally with a velocity of approximately 120 m/yr. And the inconspicuous vertical changes reveals that the glacier has sufficient winter supply. Although the detectable proportion of Namjagbrawa Glacier is relatively small, SAR results can perform well in limited pixels verified through optical results. It flows swifter over roughly −150 m/yr in the east, north, and vertical directions due to the vast elevation differences. However, other glaciers in the ensemble generally maintained slow flow, with multidimensional velocities below 20 m/yr. All glaciers’ flow velocities and the local climate in temporal are parallelism in the variations, which exhibit summer-accelerated and winter-slowed seasonal patterns. There were no surge signs during the study phase, which does not preclude the possibility of future emergence. The maritime glaciers in the SETP should be therefore multidimensional and continuous monitored to implement early warning and forecasting for mega transportation projects.

The mechanisms of high mobility of a glacial debris flow using the Pudasaini-Mergili multi-phase modeling

Glacial debris flows with high mobility and long run-out distance have posed a growing threat to human life and critical infrastructure under climate change. The dynamics and mobility of glacial debris flows are related to many physical factors, among which initial ice/water content, phase transition, and entrainment might be the most important. A typical glacial debris flow occurred on 10 September 2020 near Namcha Barwa Peak in Tibet Plateau and travelled about 10 km under no precipitation condition. The major unsolved mechanical questions for the high mobility of this glacial debris flow include: What is the primary water source of this glacial debris flow? How do the water and ice affect the dynamics of the glacial debris flow? Which physical factor dominates the high mobility characteristics of the glacial debris flow? To answer these questions and deepen our understanding, we adopted the Pudasaini and Mergili (2019) advanced multi-phase mass flow model implemented in the computational tool (r.avaflow) to back-calculate this event. Through well-designed numerical experiments, we revealed that the initial water content is the major water source of this glacial debris flow event. We analyzed the impact of the initial ice/water fraction, phase transition, and entrainment on the mobility of the glacial debris flow. Results show that the initial ice/water content and entrainment are critical to the mobility of the glacial debris flow in the initial and later transport stage, respectively, while the phase transition only plays a minor role. Our study promotes mechanical understanding of the high mobility of this kind of glacial debris flow events.

Initiation mechanisms and dynamics of a debris flow originated from debris-ice mixture slope failure in southeast Tibet, China

Glacial debris flows are a common catastrophic hazard in alpine regions due to glacier recession and permafrost thawing. They are frequently triggered by heavy rainfall and outburst floods. To date, studies on debris flows originated from debris-ice mixture slope failures triggered by glacier melting in low-intensity rainfall conditions are still limited. In this study, a typical glacial debris flow that occurred in September 2020 on the west side of Namcha Barwa in southeast Tibet, China, is taken as a benchmark to reveal the initiation mechanisms and dynamics of debris-ice landslide-debris flow hazard chains. The flow discharge, erosion, entrainment, deposition, and dynamic process of the debris flow are evidenced by both field investigations and numerical simulations with a single-phase cell-based simulation program, Erosion–Deposition Debris Flow Analysis (EDDA) (Chen and Zhang, 2015) and a multi-phase flow model r.avaflow (Pudasaini and Mergili, 2019). This hazard chain was originated from a debris-ice mixture slope failure with a volume of 1.14× 106 m3 in a temperate area under little rainfall. The mobile mixture entrained saturated moraines and water in the ravine channel and rapidly transformed into a viscous debris flow, traveling about 9.78 km and entraining 50% more materials along the path. The peak discharge and frontal flow velocity at the outlet of the ravine reached 4700 m3/s and 11.4 m/s, respectively. About 3.75× 105 m3 of debris deposited on the riverbed and partially blocked the Yarlung Tsangpo, causing severe damage to local infrastructures. This study provides a scientific basis for understanding the evolution process of a debris-ice landslide-debris flow hazard chain and for minimizing its adverse impact on key infrastructures in the region.

Research on the geological route selection of Sichuan-Tibet Railway passing through Eastern Himalayan syntaxis Areas

The eastern Himalayan syntaxis (EHS) area is characterized by complex topography due to the strong collision between the Indian and Eurasian Plates, developing deep fractures, active faults, and prominent high in-situ stresses. The designed Sichuan-Tibet railway passes through this area and connects Bome and Nyingchi on the east and west sides of the syntaxis. We encountered the following dilemma on railway route selection due to the large investment scale and the linear distribution of the railway: 1) the investment might skyrocket if the route runs outside the arc-shaped structure of the Namcha Barwa syntaxis in the Northwestern part of the EHS (detour syntaxis scheme); 2) the dynamic geological risk may increase if the route directly runs through the arc-shaped structure and Yarlung Zangbo Suture zone (crossing syntaxis scheme). Combined with a detailed field investigation, we comprehensively analyzed the regional deep structure, Holocene active faults and earthquakes, and regional in-situ stress and crustal deformation. The crustal stability in the Namcha Barwa syntaxis is extremely poor, having an uncontrollable risk. Concurrently, under topographic control, the tunnel’s bury depth of the crossing syntaxis route scheme is extremely deep, and the risk of rockburst, large deformation of soft rock, and water inrush is extremely high, severely affecting the construction period and potentially causing a significant increase in investment due to construction period delay. Therefore, the Sichuan-Tibet railway project is recommended to avoid crossing the Namcha Barwa syntaxis as much as possible and conduct detailed scheme research based on the bias of the detour syntaxis route scheme.

Insights into the geohazards triggered by the 2017 Ms 6.9 Nyingchi earthquake in the east Himalayan syntaxis, China

On 2017-11-18, an Ms 6.9 (Mw 6.4) magnitude earthquake struck the NBTN (Namcha Barwa tectonic node), located in the EHS (east Himalayan syntaxis) on the south Tibetan Plateau, China, with an epicenter at 29.87°N and 95.02°E and a focal depth of approximately 10 km. Multiple remote sensing interpretations and field investigations revealed that at least 1816 landslides and 3 dammed lakes were triggered and formed by the Nyingchi earthquake, and the landslide-affected area was approximately 527 km2. Most geohazards are concentrated in the southeast section of the epicenter, and compared to the outer section (503 landslides), the inner section (1313 landslides) exhibits a notable concentration. The total landslide area is 32.7 km2 and follows an exponential distribution, namely, Y = 0.023*e[-(Log(x)-3.06)^2/0.2738]. The estimated landslide volume is (14.096 + 3.023/-1.844)*106 m3. Rock avalanches are the most common failure pattern in the landslide-affected area, and the largest landslide area reaches 3.6*106 m2. >60% of the landslides occur at elevations ranging from 2600 to 3800 m and at distances to the river midline ranging from 400 to 2400 m, and this range coincides with the upper-top section of the slope. Landslides occurred most frequently in schist and gneiss (1313 landslides, 72.3%). Among the 3 dammed lakes, two dammed lakes were directly triggered by the Nyingchi event, and one dammed lake was formed by an active debris flow gully, namely, the Sedongpu gully. Moreover, numerous dangerous deposits remain in the main flow stream, and special attention should be given to these deposits. Finally, the AHP (analytic hierarchy process)-CF (certainty factor) method is adopted to establish a landslide susceptibility map for further risk assessment.

Spatiotemporal changes of glacier and seasonal snow fluctuations over the Namcha Barwa–Gyala Peri massif using object-based classification from Landsat time series

The glacier environment in the Namcha Barwa–Gyala Peri (NBGP) massif is regarded as one of the most sensitive areas to climate change, yet the change estimates remain inadequate due to the limited knowledge of the complex debris-covered glacier environment and the lack of available remote sensing data caused by continuously cloudy weather. To examine the changes of the complex glacial environment, a multiple hierarchical object- and rule-based classification (MHORC) scheme is proposed by combining Landsat time series images and topographic data. Object-based image analysis (OBIA) is introduced due to its capabilities of handling data contextually and hierarchically and using multi-source data jointly so that multiple hierarchical rule-based classification can be developed to distinguish the objects with similar spectrum such as debris-covered glaciers and their surrounding terrain. All the available Landsat images from 1987 to 2019 are filtered by the cloud cover condition (less than 10%) and accordingly used in the MHORC to obtain spatial and temporal information of the glacier and snow over the NBGP massif. The images collected in the snow-free season are used to estimate the glacier extent state and the historical changes, and those in the snow-covered are used to estimate the snow cover fluctuation using harmonic analysis. The results show that the NBGP contains 65 glaciers (≥0.1 km2) with a total area of 462.7 km2, among which valley glaciers account for 21.5% in number but cover an area of 354.2 km2, more than 76% of the total area. An accelerating reduction trend of the glacier area is confirmed as the area change velocity is −0.62 km2 yr−1 between 1999 and 2003 and reach −2.95 km2 yr−1 between 2003 and 2015. The seasonal snow cover area shows a reduction in the secular trend and an increase in fluctuation amplitude. Both glacier and seasonal snow display heterogeneity in the spatial and temporal patterns. Such heterogeneous dynamics might be related to the trade-off between water vapor supply and regional thermal conditions. This MHORC developed in this study demonstrates its effectiveness in delineating the debris-covered glacier environment and the potential of using Landsat time series to track glacial environment evolution. Besides, the analysis of seasonal snow fluctuations provides a novel perspective to re-recognize the glacial environment dynamics, and the knowledge of glacier extent changes and season snow fluctuations over the NBGP massif can improve the understanding of environmental effects from climate change.

Glacial advances and stability of the moraine dam on Mount Namcha Barwa since the Last Glacial Maximum, eastern Himalayan syntaxis

The Tibetan Plateau and Indian Plain exhibit the largest altitudinal difference between two geographically contiguous areas on Earth, but the eastern margin of the Plateau has not experienced headward erosion of the Yarlung Zangbo River (YZR) since ~2.5 Ma. In the past, some scientists have argued that moraine dams on Mount Namcha Barwa (in the Eastern Himalayan syntaxis) frequently blocked the YZR, thus keeping the Plateau's margin intact at this point. In this study, three sets of moraines were identified in the Pai Valley alongside Namcha Barwa, and were dated using optically stimulated luminescence (OSL) and 10Be exposure dating. The resultant ages would suggest that three glaciations occurred at ~22.2, 4.7 and 1.3 ka, respectively. Comparison of OSL and 10Be ages for glacial advances with local paleoclimatic proxies would suggest that temperature was the dominant factor controlling glacial change. Because of the paleoglacial advance frequency, coupling with rapid fluvial incision, moraine dams were cut through by the YZR. Thus, any moraine dams that lost their efficacy in impacting upon knickpoint migration. Instead, similarities between rock uplift and fluvial incision, rather than moraine dams, might have been the main control impeding headward erosion of the YZR, thereby keeping this part of the eastern Tibetan Plateau margin intact.

Seismic Sequences of the 2017 Mainling Mw 6.5 Earthquake: Imaging the Seismogenic Fault by Aftershock Analysis

In the southeastern part of Tibet, an earthquake with a local magnitude of 6.9 occurred in the prefecture of Mainling on 18 November 2017. The mainshock and more than 900 aftershocks were recorded by a local seismic network comprising seven three-component seismic stations. In this study, both HypoDD location of aftershocks and focal mechanism inversion of moderate events were performed in order to accurately identify the pattern of active faults. The result reveals that the mainshock has a thrust source mechanism located at a depth of 14 km beneath the NE flank of the Namcha Barwa–Gyala Peri (NB-GP) massif. The aftershock sequences are caused mainly by two determined faults, one of which is the seismogenic fault stretching with a SE–NW trend parallel to the GP ridge and with a high NE-oriented dipping angle, and the other is activated by the mainshock and displays features of a SSE-NNW trend and SW-dipping, inferring the adjustment of stress in the focal area. The source parameters of the mainshock and the selected aftershocks show the reverse property of the seismogenic fault and its adjunct fault, thus inferring the backlash and uplift of the NB-GP massif, especially GP, for adjusting the uneven extrusion from the eastern Himalayan syntaxis to the adjacent Lhasa block. Furthermore, it is deduced that the rupture energy of the mainshock and aftershocks was limited by the surrounding rigid rock mass with high seismic velocity, such as the Lhasa block in the north and east, and Namcha Barwa complex in the south, and other aftershocks appearing at the NW top of GP and the SE side of Yarlung Tsangpo Big Bend reflect the strong squeezing effect of the NB-GP massif to its northeastern geological mass.

Magnetic Fabric and Petrofabric of Amphibolites from the Namcha Barwa Complex, Eastern Himalaya

The magnetic fabric and petrofabric are often used as tectonic indicators of geological and geodynamic processes that a rock has experienced such as growth, deformation and metamorphism. This study presents the low field anisotropy of magnetic susceptibility (AMS) and the crystallographic preferred orientation (CPO) of constituent minerals in amphibolites from the Namcha Barwa Complex in the eastern Himalayan Syntaxis, Tibet. The bulk magnetic susceptibility varies significantly from 7.29×10−4 to 3.31×10−2 SI, with the Jelinek’s anisotropy values (Pj) ranges from 1.094 to 1.463. The maximum susceptibility is approximately parallel to the lineation while the minimum susceptibility is subnormal to the foliation plane. Electron backscatter diffraction (EBSD) analyses show pronounced CPOs of amphibole in all samples, with a preferred alignment of the [001] axes along the lineation and the [100] axes spreading along a girdle normal to the lineation. Numerical simulations and comparison with laboratory measurements suggest that the magnetic anisotropy of amphibolite is largely controlled by the CPOs of amphibole. If present, the well oriented iron-titanium oxides such as ilmenite along rock foliation and lineation could increase the susceptibility and the anisotropy of a rock. Our results show a strong correlation between the magnetic anisotropy and the petrofabric of amphibolite, which could provide constraint for the interpretation of strong magnetic anomalies observed in the tectonic syntaxes of Tibet.

Mio-Pliocene piracy, relict landscape and drainage reorganization in the Namcha Barwa syntaxis zone of eastern Himalaya

The presence of unique elevated low relief relict landscape in the transient Dibang catchment, at the orographic edge of Tibet-Himalaya in the tectonically active Namcha Barwa syntaxial zone, is modelled to understand evolving regional landscape, drainage reorganization and tectonics. This elevated low relief landscape represents a Mio-Pliocene abandoned paleo-channel of the Yarlung river, which was captured by the headward eroding Siang river owing to >600 m base level advantage. The river capture caused isolation of the Dibang river, which evolved as a transient parched catchment since 3–6 Ma after loss of ~17 times drainage area and 4–17 times discharge. The drainage area and discharge gained by the Siang river triggered enormous incision causing aneurysm leading to the accelerated growth of the Tsangpo gorge and affected regional tectonics. This paleo-drainage reorganization is reflected in the Mio-Pliocene sedimentation pattern in the southern Tibet-Himalaya and foreland basins.

Dynamic processes of 2018 Sedongpu landslide in Namcha Barwa–Gyala Peri massif revealed by broadband seismic records

At 22:48 pm, October 16, 2018, a large glacial mudslide occurred upstream of the Sedongpu gully on the southern mountainside of Gyala Peri in Mainling County, Tibet. To investigate its dynamical features, the force-time history of it was inverted using the broadband recording of six stations in the Namcha Barwa network. Preliminary results of the landslide force-time history indicated that the mass of the Sedongpu landslide is about one million tonne. The velocity and displacement curves from integration implied that the mass center of the Sedongpu event slipped down for a distance of 5100 m with an average velocity of 10 m/s (peaked at about 25 m/s). The fitted displacement curve (using the motion equation) revealed that the movement of the Sedongpu landslide experienced four processes. In the first 150 s, the stability at the upper part of Sedongpu glacier broke down, and the detached glacial deposits gradually moved downward. The subsequent 70 s is the stage of acceleration for the debris flow with the velocity increased and the acceleration peaked. After encountering the leading edge of the glacial deposits, the debris flow decelerated for about 80 s as the slope of the path declined and the canyon narrowed. In the final 180 s, the debris flow might have been blocked by the leading edge of the deposit or a large topographical bend; hence, it stopped eventually. The entire process lasted for 480 s. We proposed that the surface wave generated by an earlier earthquake (about 10 min before the landslide event) in Nagqu, Tibet, played a decisive role in triggering the severe event on the basis of the prolonged impact from rainfall and regional fault activity.

Landslides triggered by the Ms 6.9 Nyingchi earthquake, China (18 November 2017): analysis of the spatial distribution and occurrence factors

On 18 November 2017, a magnitude Ms. 6.9 (Mw 6.4) earthquake struck Nyingchi, Tibet Autonomous Region, China, which is located in the famous Grand Canyon region of the Yarlung Zangbo River in the eastern Himalayas, Tibet. The Nyingchi event was a thrusting event, with a focal depth of 12 km at 29.87° N and 95.02° E. According to emergency investigations and remote sensing, the Nyingchi event triggered at least 1820 co-seismic landslides. The landslides mainly occurred in the Grand Canyon region within an area of 527 km2 (the inner area of Namcha Barwa tectonic node). The landslide distribution characterises an obvious hanging wall effect and is classified as “small concentration region and large landslide distribution area”. The failure patterns mainly consist of rock falls, rock avalanches, and deposit failures. One co-seismic landslide partially blocked the Yarlung Zangbo River, and the barrier lake remained. Different influencing factors, such as the seismic fault, river, slope aspect, slope angle, rocks, and elevation, have different influences on landslide occurrences, and the co-seismic landslides in the hanging wall area and footwall area present obviously different characteristics. Additionally, the post-earthquake effect impacted the recent Sedongpu landslide.

Seismic anisotropy inferred from direct <i>S</i>-wave-derived splitting measurements and its geodynamic implications beneath southeastern Tibetan Plateau

Abstract. The present study deals with detecting seismic anisotropy parameters beneath southeastern Tibet near Namcha Barwa Mountain using the splitting of direct S waves. We employ the reference station technique to remove the effects of source-side anisotropy. Seismic anisotropy parameters, splitting time delays, and fast polarization directions are estimated through analyses of a total of 501 splitting measurements obtained from direct S waves from 25 earthquakes ( ≥ 5.5 magnitude) that were recorded at 42 stations of the Namcha Barwa seismic network. We observe a large variation in time delays ranging from 0.64 to 1.68 s, but in most cases, it is more than 1 s, which suggests a highly anisotropic lithospheric mantle in the region. A comparison between direct S- and SKS-derived splitting parameters shows a close similarity, although some discrepancies exist where null or negligible anisotropy has been reported earlier using SKS. The seismic stations with hitherto null or negligible anisotropy are now supplemented with new measurements with clear anisotropic signatures. Our analyses indicate a sharp change in lateral variations of fast polarization directions (FPDs) from consistent SSW–ENE or W–E to NW–SE direction at the southeastern edge of Tibet. Comparison of the FPDs with Global Positioning System (GPS) measurements, absolute plate motion (APM) directions, and surface geological features indicates that the observed anisotropy and hence inferred deformation patterns are not only due to asthenospheric dynamics but are a combination of lithospheric deformation and sub-lithospheric (asthenospheric) mantle dynamics. Direct S-wave-based station-averaged splitting measurements with increased back-azimuths tend to fill the coverage gaps left in SKS measurements.

Seismotectonics at the terminal ends of the Himalayan Arc

The Himalayan arc has an arcuate E–W trending geometry with reversal of trend at the terminal ends – Nanga-Parbat (western) syntaxis and Namcha-Barwa (eastern) syntaxis. Both ends are characterized by an actively deformed uplifted dome with its flanks bounded by active shear zones/faults that cause the majority of the seismicity. Compiled map data and seismo-geological depth sections around these two syntaxial zones have brought out active crustal structure and seismotectonic setup. The Nanga-Parbat syntaxis exhibits upward bending and subsequent thickening of the Indian plate with the cluster of seismicity along the NNE–SSW trending Raikhot fault/Diamer shear in its western margin and a comparatively less active Rupal–Chichi shear zone of N–S trend with diffused seismicity towards the east. The 2005 Kashmir earthquake is spawned due to interaction of the Main Boundary thrust and the Muzaffarabad fault. The Namcha–Barwa syntaxis displays a fault-bounded upliftment and thickening of the Indian plate where Canyon thrust marks the boundary between the Indian and Eurasian plates. The occurrence of the 1950 Assam earthquake in the vicinity of the eastern syntaxis is attributed to a regional right lateral strike-slip motion on the causative fault plane. The seismicity in the syntaxes is primarily controlled by strike-slip faults/shear zones along the flanks of popup antiforms.

Preliminary assessment of large mammals in the Namcha Barwa region of south-eastern Tibet

In 1994 the authors made two rapid assessments of wildlife in the remote Namcha Barwa region, south-eastern Tibet, China. They conducted interviews, examined and recorded trophies in the possession of local Menba, Luoba and Kangba Tibetans and made opportunistic field observations. Despite the extensive forests in this extremely remote region, wildlife populations appeared to be low except in inaccessible areas along county borders. Frequent reports of tiger predation on cattle, horses and mules in glaciated valleys on the south slope of the Himalayas may indicate a shortage of natural prey. Uncontrolled hunting, human population growth and expedient economic development threaten the survival of large mammals in the region.

Silurian acanthomorphitae acritarch from the Shiala Formation, Tethys Garhwal Himalaya, India

DOI = 10.3126/hjs.v5i7.1331 Himalayan Journal of Sciences Vol.5(7) (Special Issue) 2008 p.150-1

Peaks of the Assam Himalaya

FROM the mountaineer's point of view nothing is known of the Himalaya east of Chomolhari, and not very much is known of the topography. For 180 miles the range forms the northern boundary of Bhutan, and for another 250 that of Assam; but it should be noted that though the Himalaya may be assumed to mark the Assam-Tibet frontier, the boundary has never been delimited. The whole range from the Tista valley in Sikkim to Namcha Barwa (25,545 feet) and beyond is called by Burrard and Hayden the Assam Himalaya; it would be less confusing to confine the name to the mountains lying north of Assam. Where the range passes from Bhutan into Assam there is a wide breach formed by the upper eastern basin of the Manas, and immediately east of this gap is a group of high peaks which cover about 50 miles on an E.N.E. alignment. These I hoped to explore this year. The major peaks have been fixed trigonometrically from stations in the Assam valley. During their great explo? ration of the Tsangpo gorge in 1911 Bailey and Morshead travelled and surveyed along the Tibetan side of the range. In 1935 and 1938 Kingdon Ward skirted the western end and visited the valley of the Gorjo Chu, which, for reasons that will be apparent, is the only practicable approach to the range. In 1934 and 1936 Messrs. Sheriff and Ludlow covered much the same ground, but the peaks, passes, and glaciers of this region remain unvisited. Two routes into Tibet traverse the gap formed by the eastern tributaries of the Manas. The more eastern route skirts the Assam Himalaya, crosses the Gorjo Chu valley, and passes within 10 miles of Gori Chen (21,450 feet), thus forming the obvious line of approach from the south. It would be simpler if an approach could be made from Tibet, where there are villages from which food could be got and where the effects of the monsoon would be less felt; but the good will of Lhasa for British mountaineering parties is given principally to Mount Everest expeditions. On the south side of the range, except for the narrow strip of country between these two routes, the foothills of Assam are the home of savage tribes: Miji Akas, Silung Abors, and Daflas. Known as the Balipara Frontier Tract, this unadministered territory stretches east to the Subansiri and beyond. The eastern route to Tibet lies on the edge of this hostile country and it is not possible to leave the route until near the Himalaya, where the country becomes too high for these forest-dwelling savages. A Political Officer stationed at Charduar, some 20 miles north of the Brahmaputra, is in charge of the Frontier Tract.