The banks of the Limpopo River

Abstract

The curious fate that befell the Elephant’s Child on the slimy banks of the Limpopo River in Rudyard Kipling’s story reminds us that muddy sediments are as remarkable in their properties and behaviour as they are commonplace in a wide range of sedimentary environments. What is a muddy sediment? It must be one containing enough clay minerals to render it cohesive. In practice, the majority of muddy sediments are dominated by particles smaller in size than sand (Table 2.2). River and tidal muds contain numerous small fragments of rock-forming minerals other than the clays, and have an average particle size typically in the silt grade. It is only in caves, the oceans and deep lakes that true clays with an average particle size less than 2µm are to be found. Try chewing pieces of clay from these environments; the harsh gritty feel of the river and tidal deposits will soon convince you of their relative coarseness. Some muddy sediments even contain significant amounts of sand-sized and coarser debris, for example, many glacial tills and debris-flow deposits, and texturally may be neither silts nor clays.