A Preliminary Report on the Geomorphology ofKathmandu Valley, Nepal

著者 YONECHI Fumio雑誌名 The science reports of the Tohoku University.

7th series, Geography巻 23号 2ページ 153-161発行年 1973-12URL http://hdl.handle.net/10097/44971

A Preliminary Report on the Geomorphology of

Kathmandu Valley, Nepal

Fumio YONECHI

I Introduction

In January-March 1970, the Yamagata Research team to Nepal and India

led by Prof. M. Nagai made an investigation tour through the piedmont area of

the Himalayas. This paper is concerned with the results of geographical study made by the present writer as one of the members of the team.

The Kathmandu Valley is not only the heart land of Nepal, but also the most

developed area in all the Himalayas. Its prosperity is rather dependent on its

physical setting. The Kathmandu Valley is a basin and it has a mild climate and fertile land. But, there are few reports about physical geography of this basin, and there are many problems to be solved.

This is a preliminary report about the geomorphology of this basin, and the writer appreciates frank criticism.

II Geology and River System

To the south of the main Himalayas, the Midland Region extends as far as the lower mountain area. The Kathmandu Valley, the only big basin in Nepal is located in the Midland Region. It is one of the tectonic basins of the sub-Hima-

layas, similar to the Valley of Kashmir and to many other Duns, or synclinal troughs enclosed between two contiguous anticlinal flexures (Wadia 1953).

The Kathmandu Valley is an amphitheater in shape about 24 km across, around the headwaters of the Bagmati. Most of rivers in Nepal rise in the main Himalayas and the Tibetan Plateau. They cut deep gorges through the Midland

Region. But the Bagmati River is an exception rising in the Midland itself and having higher valley floor.

Because of these distinctive features of the Bagmati River, the Kathmandu Valley is plateau-like. It is about 1300-1400 meters above sea level in altitude. The cross-sections of this basin are shown in Fig. 1.

The drainage pattern of the Kathmandu Valley is the most typical instance of centripetal system (Holmes 1965). The Bagmati River has many tributaries

from every direction, Vishnumati from north, Manohara and upper Bagmati from

northeast, Hanumati from east, Godavari from southeast, small tributaries from

154

w

201;

1000

F. YONECHI

E

2000

1000

0

N S

Kathmandu K Valley

Fig. 1 N-S and E-W sections of Kathmandu Valley (After Government of Nepal; 1969)

Kathmandu ...... _ China Valley

NePal • AAP -.

Mt.Shivapuri" 2688m I

ndia Z-

___.

Bay of

. L. Bengal Mt.Jamacho''''-'• 171

:) 1961m A-,,

,,..2 ''-'

• ''

le /IXV,-t.;,c;&it^

•. 11111110600 +-emim •2400

)Mt.Nagarkot

•

i# . Hanumati R. , 1487M Ai 411411k Mt.Chandiagiri Qo 4.

% , '...

';". 0 3K M',,,z,

4c. 4..,, . Mt.Phulchoki 4:e' • 2718m

Fig. 2 The Drainage Pattern of the Kathmandu Valley (Dot line showing influence of geological Structure)

X: Sampling Site for C-14 Dating K: Kathmandu P: Patan B: Bhadgaon C: Chobar Gorge

west and Nakhu from south (Fig. 2). The last mentioned is the only river in

Nepal that flows from south to north in direction. But, if examined in detail, it is regarded as a kind of rectangular river pattern

Geomorphology of Xathmandu Valley, Nepal 155

........i..... , .=..,., y, - •:.,,m'vi-s.,%-- : v 4,. 1 t.•.1-,....,=,—. maw .......- _,....,....^.----- ..._j'I-d=707.14-11

w ..h..,-....^,..„,

..,..

'- -- - "= — = '''''',-"71^',1=4.10:-

.......,..km,, ..,:---- ... arzi.,- .........m. - II= — =:.:.- - .11 6 ...._-__,.. 5....4 } 7 )1.,"1121%._„2,4:,r. 18 0 3K M',1-7

4^41,..., 1 19 11

1.11;:

Fig. 3 Geological Map of the Kathmandu Valley modified after Nantiyal and Sherma; 1961 (from Government of Nepal; 1969)

1: basal boulder bed 2: granites and migmatites 3: greenish and pinkish crystalline limestone, shales and quartzites 4: phyllites, schistose sandstone with limestone and

quartzites 5: darkgrey limestone marble, purple and greenish shales 6: miceous hemitite, bivalve shales, sandstone and limestone 7: lacustrine (terrace deposits) 8: fan and talus

deposits 9: alluvial deposits

in this basin adjusted to the geological map by Nautiyal and Sherma (Government

of Nepal 1969) in Fig. 3.

III Radio-carbon Dating of Lacustrine Deposits

It is well known that most of sediments in the basin are lacustrine and peat layer is exposed at many places of the basin.

According to legend, a former lake called "Nagahrad" was drained by Manjushri, a Buddist missionary. Then the bottom of the lake was dried up.

(K, Lall 1966, N.B. Thapa and D.P. Thapa 1969) But the deposits of the former lake were identified as Pleistocene by the

presense of fossils. (Government of Nepal 1969) In order to confirm the age, the writer sampled peaty clay from lacustrine at

the road cutting near Khajahl Village (January 23rd 1970, 27°45'N Lat. 85°20'E

Long.), which was dated 33,200 1:12g years, B.P. by radiocarbon measurement carried out by K. Kigoshi of Gakushuin University, Tokyo, (Gak-2875).

The pollen analysis on the same sample by 0. Yamagata and Y. Yamanoi

(Yamaki), will be reported later.

156

Level of Sampling —

Road

;• Sand

Paddy Field

Fig. 4 Section os Pat Sampling Site for 1

(The height of cliff

bed

Gravel

Clay & F. Dry Field

oftheBagmati. DurbarSquare(asql

locatedonthissurfac Thissurfacewas

partsoftheKathman pickedupfromtheci thattheLakeNegal

al stageundera

r, Sand

:: Sand ..••••••.

•

.•

;i:c; o•0

MEI •*.Sand

•••••••

YONECHI

IV

& Sand

Peat

Fig.4Sectionos PaanSurface at the C-14 Dating

is ca. 14 m.) SamplingSitefor (Theheightofcliff

bedoftheBagmati."Patan"isthename DurbarSquare(asquareatthefrontof

locatedonthissurface. Thissurfacewasthelacustrineterrace

partsoftheKathmanduValley.Theafore pickedupfromthecuttingofthisterrace. thattheLakeNegahradwasformedat

glacialstageunderamoistclimate. Atthesouthernpartofthebasin,the sl

At thenorthernmarginofthebasin, alluvi ThePatanIISurface;Thisisthe tet

in thelakereduction.Theupperpartof 14 are locatedonthissurface.Itis30 mete]

TheKathmanduIandIISurfaces; T;

basin.Thelowerpartofthecapital cit depositsarefinesand,siltandclay,and sei

oftheBagmati."Patan"isthename ofthesecondcityinthisbasin.Patan )arSquare(asquareatthefrontof oldpalace)andTribhuvanAirportare

edonthissurface. Thissurfacewasthelacustrineterrace oftheLakeNagahrad,atthecentral

oftheKathmanduValley.Theaforesaidsampleforradiocarbondatingwas upfromthecuttingofthisterrace.ThewritersupposesontheC-14data

theLakeNegahradwasformedattheGattweigerinterstadialinWU= alstageundera moist climate. At the southern part of the basin, the surface has the feature of dissected deltas.

he northern margin of the basin, alluvial fans and talus cover the surface. The Patan II Surface; This is the terrace formed at a short standing time

te lake reduction. The upper part of Kathmandu and the main part of Patan ocated on this surface. It is 30 meters above the river bed of the Bagmati.

The Kathmandu I and II Surfaces; These surfaces are lower terraces in this The lower part of the capital city is located on these surfaces. Their

sits are fine sand, silt and clay, and seem to be secondary deposits carried by

IV Geomorphological Surfaces

The basin floor of the Kathmandu Valley is divided into 5 surfaces as

follows;

1. Kirtipur Surface 2. Patan I Surface

3. Patan II Surface 4. Kathmandu I Surface

5. Kathmandu II Surface There is no distinct relation bet-

ween the surfaces and terrace culture.

The Kirtipur Surface; The flat tops of hills distributing in the Kathmandu Valley, about 130 meters

above lowland, are named "Kirtipur

Surface" by the writer, after the name of an old town on the hill. This surface appears as an eroded

surface. The Patan I Surface; The most

typical terrace surface in the basin about 35-40 meters above the river

f the second city in this basin. Patan d palace) and Tribhuvan Airport are

)f the Lake Nagahrad, at the central lid sample for radiocarbon dating was The writer supposes on the C-14 data

Gattweiger interstadial in Wiirm

Geomorphology of Kathmandu Valley, Nepal 157

Relative height

m 30

20

10

0

Pn PI

31.0.15,C1,13,11.14

K K II

Fig. 5 Schematic Cross-section of the Terraces in

PI: Patan I Surface PII

Kathmandu II Surface

1: fan and talus deposits

: Patan II Surface

2: sand and gr.;

1 Eic 2 4

ra. 5 the Kathmandu Valley

KI: Kathmandu I Surface KII:

sand and gravel 3: peat 4: sand 5 : clay

dissecting rivers from the Patan Surfaces. They are about 8 meters (Kathmandu I) and 2 meters (Kathmandu II) above the river bed of the Bagmati.

V Relationships between the former lake and the Chobar Gorge

The Kathmandu Valley is divided into four drainage basins with three ridges

or spurs of ENE-WSW direction as follows; 1 Northern Sub-basin

(Gokarna Ridge) 2 Boudha-Bhadgaon Lowland

(Balaju-Katunje Ridge) 3 Kathmandu-Patan Lowland

(Kirtipur Ridge) 4 Southern Sub-basin

Since most part of Balaju-Katunje Ridge is submerged under the lacustrine deposits, the Boudha-Bhadgaon Lowland and the Kathmandu-Patan Lowland

make up the central part of this basin. Passing through the Kirtipur Ridge, the Bagmati River leaves a deep gorge,

called the Chobar Gorge, for about 500 meters long. There is the legend about the draining by Manjusuri. Why is the Chobar Gorge so deep and narrow as

shown in Photo. 5-6 ? The rocks along the Chobar Gorge are composed of limestone. The writer

observed some karst landscapes on the hill beside this gorge. Although the

phenomena are small in scale, they are clearly karrens (1.0-1.5 meters above the

ground) and dolines (3.0-4.0 meters in depth).

Judging from such a topogaphy, the Chobar Hill had resisted the erosion of river and dammed up the water of lake in northern side of the hill. Gradually,

158 F. YONECHI

under the hill, the groundwater had made a kind of karst tunnel (K. Nishimura

1964), and a part of water had been drained through this tunnel. At last it collapsed and the deep gorge was remained which drained all lake water.

The writer cannot neglect the possibility that the tectonic movements such as the uplift of the Kirtipur Ridge or the depression of the basin center might have occurred in late Quaternary.

VI Conclusion

1 The Kathmandu Valley is a tectonic basin.

2 One of ENE-WSW ridges composing of limestone dammed up the former lake "Nagahrad".

3 There are karst landscapes on the ridge (the Chobar Hill).

4 The peat from the lacustrine terrace deposits of Lake Nagahrad is dated about 30,000-40,000 years before.

The writer would like to extend thanks to Professor M. Nagai of Tohoku-

gakuin Univ., Professor T. Seino of Tokiwamatsu Joshi Bijutsu College, Mr. R. Tanaka of Yamagata Shimbun Co. and Mr. T. Konno of Yamagata Hoso Co. for their help during his investigation tour.

The writer is grateful to Professor K. Kigoshi of Gakushuin Univ. for his

radiocarbon measurement and to Mr. I. Kaneko and Mr. E. Nakamura of the Ministry of Agriculture and Forestry for their advices about the transport of

sample into Japan. And finally the writer wish to acknowledge to Professor U. M. Malla of

Tribhuvan Univ., Mr. A. Shionome of O.T.C.A., Mr. N. Matsuzawa of Japanese Embassy to Nepal and Mr. D.B. Gurung for their help during the stay in Kath-mandu.

References

Gansser, A. (1964) : Geology of the Himalayas. Interscience Publishers, London Hagen, T. (1961) : Nepal; The Kingdom in the Himalaya. Kummerly Frey, Berne

Hashimoto, S. (1973): Geology of the Nepal Himalayas. Saikon Publishing Co., Tokyo His Majestys Government of Nepal (1966): The Physical development Plan for the

Kathmandu Valley. His Majesty's Government of Nepal, Kathmandu Holmes, A. (1965): Principles of Physical Geology (New Ed.). Thomas Nelson and

Sons, London, (translated into Japanese by H. Takeuchi (1969), Univ. of Tokyo Press, Tokyo)

Karan, P.P. (1960): Nepal; A Cultural and Physical Geography. Univ. of Kentucky Press, Lexington

and Jenkins, W.M. (1963): The Himalayan Kingdoms. D. Van Nostrand Co., Princeton

Lall, K. (19661: Lore and Legend of Nepal (2nd Ed.). Patna Pustak Bhandar, Kathmandu

Geomorphology of Kathmandu Valley, Nepal 159

Nakata, T. (1972): Geomorphic History and Crustal Movement of the Foot-Hills of the Himalayas. Sci. Repts. Tohoku Univ., 7th Ser. (Geogr.), 22 39-177

Nepali, G.S. (1965) : The Newars. United Asia Publications, Bombay Nishimura, K. (1964): Karst-tunnel. Ann. Tohoku Geogr. Assoc 16 149

Shreshtha, S.H. (1968) : Modern Geography of Nepal. Educational Enterprise, Kath- mandu

Thapa, N.B. and Thapa, D.P. (1969): Geography of Nepal. Orient Longmans, New Delhi

Wadia, D.N. (1953): Geology of India (3rd Ed.). Mcmillan and Company, London

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