Late Quaternary History of the Ganga Plain
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The sediments coming from the Himalaya undergo significant chemical weathering in the alluvial plain where much Na, Ca, Sr, K, Mg, is removed, while As, Cr, Ni and Th are enriched. In suitable conditions smectite and kaolinite are formed. These sediments are partly eroded and transported to the delta region and beyond.
The Ganga Plain consists of a large number of river systems, originating in different parts of the Himalaya, Peninsular Craton, and alluvial plain formed at different times during the late Pleistocene-Holocene Surface runoff and groundwater are the main sources of water. It is difficult to make meaningful scheme of classification of channels. Based on the Sediment-Water transfer patterns, the channels are classified into three broad categories. The river systems are in different stages of evolution. The active perennial rivers are incised within the alluvium, and show avulsion on a scale of 104 -105years, some channels, part of the ancient anastomosing network, act as flood channels and show avulsion on a scale of 101- 102 years.
The Upland interfluve Surface (T2) exhibits a number of microgeomorphic features which have helped in the reconstruction of the history of the Ganga Plain during the last ∼20 ka BP 20- 13 ka BP - new drainage of low density, 13-8 ka BP - extension of drainage system, high rainfall, 8-6 ka BP - channel disruption due to Base-Level rise and tectonics and formation of large lakes which retained most Water-Sediment of T2-Surface, 6-4 ka BP - dry climate, increased terrigenous clastic sediments in lakes, 4-2 ka BP - increased siltation of the lakes, moderate rainfall, 2-0 ka BP - fragmentation and drying of lakes, strong anthropogenic influence. The basin exhibits active tectonism, which is mostly in the form of contractional system in the prominent zone, and prominent extensional system in the southern part of the basin One of the most significant events in Ganga Plain history is tectonics of 8-5 ka BP, which produced areas of centripetal drainages and changed the fluvial landscape into a landscape of ponds and lakes with few high grounds. It was helped by the climate change from humid to dry around 5 ka BP.
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