Geography of Sylhet

The Sylhet Division with its hills and basins constitute one of the most distinctive regions in Bangladesh. Situated to the north-east of the country the Division lies between 23.59' and 25.13’notrh latitude and 90.54’and 90.30’ east longitude. The physiography of Sylhet comprises mainly of hill soils, encompassing a few large depressions known locally as "beels" which can be mainly classified as oxbow lakes, caused by tectonic subsidence primarily during the earthquake of 1762. It is flanked by the Indian states of the Meghalaya in the north, Assam in the east, Tripura in the south and the Bangladesh districts of Netrokona, Kishoregonj and Bramanbaria in the west. The area covered by the region is 12,569 sq. km. which is about 8 percent of the total area of Bangladesh. 

Geologically, the region is complex having diverse sacrificial geomorphology; high topography of Plio-Miocene age (20 million years B.P) such as Khasi nad Jainta hills and small hillocks along the border. At the center there is a vast low laying flood plain of recent origin with saucer shaped depressions, locally called Haors. Available limestone deposits in different parts of the region suggest that the whole area was under sea in the Oligo-Miocene (30 Million years B.P)

Landforms:

The Sylhet region may be divided four distinct landforms. These are 1. Eroded hills; 2. Alluvial fan; 3. Alluvial plain; 4. Basin plain.

1. Eroded hills: The eroded hills are mainly formed by the hill ranges and hillocks (locally called tila) appearing the north east and south of Greater Sylhet and also round about the Sylhet town. These hill ranges attain a low elevation and have gentle slope. To the north of Sylhet town there are numerous isolated tilas of approximately 70 meters height. The Khasia-Jainta hills, laying outside the international border in India as well as some tilas within the districts of Sunamgonj ad Sylhet lie in an east-west direction. A small part to the tila lands is lying in the north of Tahirpur. To the north-east of Sunamgonj there is an area of scattered hills both west and east are Kashimara river. The Chattak hills to the south-east are continuation of these tilas. Further east, there are two hills, close to Bhologonj. To the east of Piyain river, there is a five mile long hills known after Jaflong. To the south-east there is a continuous hilly area covering Jainatpur and part of Kanairghat. The Jainta series are mainly composed of sandstone and nummulatic limestone and the Surma conglomerates, nummulatic limestone and pebble beds of the Pliocene age (about 10-12 million years B.P) these hills are continuation of the Chittagong hills running in north-south direction. These hills gradually slope down to the Sylhet plain with alternating valleys between them. Form east to west the hill ranges are: 1. The Patharia, 2. Harargaj, 3. Rajkandi-Ita, 4. Bharugaj, 5. Tarap, 6. Raghunandan. The overlying deposits of these hills are Pleistocene clays and sands over a coarse ferruginous sandstone, mottled sandy clays and shales of middle Miocene age.

2. Alluvial Fan: The Alluvial fan, mainly composed of the tipam and Dupi Tila sediments, occupies the narrow strip of discontinuous low foothills starting from Jaflong in the east to the Jamuna in the west. Among these fans, the Jaflong fan attains a maximum height of 61m above sea level with the elevation decreasing westward. Alluvial valleys frequently separate the Alluvial fans.

3. Alluvial Plain: the alluvial plain, primarily formed by the river Surma and the Kushiyara, merges with Meghna to the south. Elevation varies from 3m to 10m above mean sea level.

4. Basin Plain: A large number of swamps have developed within the alluvial palin; some of thses are locally called haor. It is believed that the present saucer shaped depression may be connected with the rise of Madhupur Tract. These saucer shaped, perennial water bodies covering a total area of about 6,000 km are considered the largest single inland depression in the country. Small permanent water bodise within the haors are called beels which occupy the lowest part of the depressions. The minor hilly steans like Manu, Khowai, Jaqdukata, Piyain, Mogra and Mahadao ofrm the dense reainage network of the reaion. The river are primarily responsible ofr devel0oping low floodplains of Sylhet . the floodplains remain deeply flooded ofr about 7-8 months. During the rainy season the haors turn into a vast inland sea, the villages appearing as islands. Occasional high winds during July to September generate large waves which cause considerable damage to homesteads. There are 35 big haors and 475 small haors in Greater Sylhet. The most prominent haors are: 1. Hakaluki haor, 2. Tenguar haor, 3. Santeer haor, 4.Hail haor, 5. Dekar haor.

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