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Alluvium the unconsolidated, loose material brought down by a river and deposited in its bed, floodplain, delta or estuary or in a lake. Alluvium consists of boulder, cobble, gravel, sand,silt, and clay and often contains organic matter that makes it a fertile soil. The term is generally restricted to unlithified, size-sorted fine sediments (sand, silt and clay) of riverine sediments. These are rich in mineral content and so form some of the most fertile soils of highest agricultural value. The most prominent characteristics of alluvium are its stratification, sorting and structure. Sedimentary structures of alluvium may be complex and dependent on the type of river activity.

A distinction between the alluviums of braided and meandering rivers is often drawn. For instance, the alluviums of the river jamuna are more coarse-sandy than those of the river meghna. Again, the size of sediments may depend on its supply source and on the distance travelled. As the loads move downstream with water the river sorts and modifies alluvial materials. Thus fine-grained alluviums are observed in the lower courses and coarser alluvium in the upper courses or close to the supply points.

Most of Bangladesh is made of river alluvium - old and recent. These alluviums are unconsolidated and non-homogeneous in age, texture and mineralogy, and deposited under diverse conditions like estuarine, tidal, piedmont and meander floodplains. Under riverine condition, floodplains are enriched with new alluvium every year. But as the river changes its course, the floodplain remains cut off from new alluvium. Some floodplains of the country received little or no new alluvium for the last few hundred years. The rivers have changed their courses in the past; thus parts of the floodplains are abandoned and reoccupied. As a result, alluviums of different ages are added in different parts of the floodplains. The alluviums derived from the river ganges, brahmaputra and tista are characterised by larger contents of mica (both muscovite and biotite) than those of the northern and eastern hilly rivers. Ganges alluvium is calcareous but other sediments are not.

Brown and grey, coarse to very coarse-grained sand constitute most of the alluvium along the foothills of sylhet, chittagong, chittagong hill tracts, dinajpur and rangpur districts. The colour of the sand changes from yellowish and brownish to grey. Channel sand and clay are continuously developing along all the rivers of Bangladesh, sand being dominant in their upper reaches, clay more prevalent in lower reaches. In the delta the rivers mostly deposit clay and very fine sand. Numerous hill streams deposit boulders and pebbles within the hill ranges and in the plains close to these ranges. The large thickness of the recent alluvial sediments being continuously deposited in the bengal basin suggests a subsiding basin probably maintaining an isostatic equilibrium. Faulting also occurred in the recent alluvium causing diversion of some rivers including the Ganges and the Tista. [Mohd Shamsul Alam]

Old alluvium the sediments occurring on the elevated plains or terraces in the Bengal Basin. The major areas within Bangladesh containing the older alluvium are Madhupur Tract, barind tract and the lalmai hills. Apart from the major units there are occurrences of this alluvium in the Sylhet region. The sediments on these units vary significantly from those occurring on the flood or deltaic plains. They are characterised by intense weathering and easily identified by a typical reddish brown colour. There are a number of different opinions about the origin of these sediments. These areas lie 10 to 30m above the surrounding floodplains and are fault bounded. They stand above the monsoon flood levels and are drained by relatively few small rivers, which have developed distinctive, entrenched, tightly meandering courses. These areas are still tectonically active and are undergoing gradual slow upliftment. Another view suggests that the terraces were simple topographic features developed by intense weathering due to monsoon rains. There is yet another view that relates the origin of the terraces to Pleistocene sea level changes.

The term Old Alluvium does not exist in recent geological literature, but still does in geographical, soil and archaeological literatures. Geologically the sediments exposed on the Madhupur Tract are named as madhupur clay. Similar sediments from the Barind Tract are named as Barind Clay. However, the term Madhupur Clay is generally used to describe the characteristic red brown clay/silty-clay unit occurring on different parts of the country. [Kazi Matinuddin Ahmed]

Recent alluvium a general term for detrital material deposited during a comparatively recent geologic time by rivers or streams or found on alluvial fans, floodplains, etc. Alluvium consists of gravel, sand, silt, and clay and often contains organic matter that makes it a fertile soil. It does not include the subaqueous sediments of lakes and seas. Sand, silt and clay constitute the most dominant rock types of the alluvium. Boulders and pebbles of all sizes and carbonaceous wood intermix with the sand. The sand is mostly composed of quartz with subordinate ferromagnesian minerals and feldspar. As the floodplain slopes away from the base of the piedmont, the sand becomes finer and finer, loses its ferromagnesian minerals and feldspar and grade into very fine sand, silt and clay in the delta. The colour of the sand changes from yellowish and brownish to grey.

The piedmont of the Dupi Tila hill ranges and the adjoining very gently sloping plains contain medium quality white to whitish glass sand almost free of ferromagnesian minerals, mica, and feldspar at shallow depths. Originally yellow and brown in colour, the Dupi Tila sand turns whitish and white due to reduction of iron oxide by constantly flowing acidic water containing organic acids, particularly humic acid derived from the decomposition of vegetal matters. White sand occurs as large and small pockets and lenses from Sylhet district to teknaf in Chittagong district. A few centimetres to several metres thick soil overlies many areas of the tableland and the hilly regions. Brown soil has developed on the tableland due to the weathering of the red clay. The soil, formed on the hills due to the weathering of Tertiary rocks is coloured grey. The Dupi Tila sediments produce brown soil.

corals and seashells consisting mostly of lamellibranchs and gastropods lie on and around st martin's island. Such invertebrates may be present in the sea all along the coast from the north of Chittagong to Teknaf and in the sea around St Martin's Island. [Sifatul Quader Chowdhury]