On the formation of cracking clay soils (Vertisols) in West Bengal

During various soil survey programmes of West Bengal in the past soils of extensive areas with shrink-swell characteristics were classified as “Vertic” intergrades at the subgroup level of classification. The occurrence of Vertisols was identified in 2004 during a GEFSOC project programme in Chunchura of Hooghly district of West Bengal and such soils are described in this paper. The Chunchura soils have fulfilled all the necessary requirements viz. wide cracks, clay content (>30%), COLE (>0.06) and slickensides to classify them to Vertisols. Other properties including micromorphological and mineralogical properties are similar to Vertisols occurring in Deccan Trap basaltic areas. The soil clays of Chunchura are dominated by low-charge dioctahedral smectites similar to that obtained for Vertisol clays of the Deccan Trap area. Analysis of geomorphic and tectonic history of this part of the Bengal Basin indicates that the huge amount of smectitic parent material in the old fluvial/deltaic plains including the Damodar deltaic plain existing in the Vertisols area of Chunchura, were brought down as alluvium from the weathering Rajmahal Trap Basalt by the then east flowing rivers before they were shifted to the south in the late Holocene period.