Tidal flats and salt marshes
Along most coasts, fine-grained sediments are winnowed away by wave and current action, and landforms are rocky, or composed of sand or gravel. However, there are substantial sections of coast that are dominated by muddy sediments, either in sheltered locations where low-energy marine processes dominate, or where the supply of silt and clay-sized sediment is so large that there is a positive sediment budget. These muddy coasts are distinctive for several reasons. Firstly, fine sediment behaves differently from sand and gravel; it takes a long time to settle, but interactions between grains, such as flocculation, accelerate deposition and promote the cohesion of mud once deposited, meaning it requires significantly higher energy to resuspend. Secondly, these muddy sediments support significant biological activity, including organisms within the sediment (infauna) that bioturbate the sediments, organisms on the surface (benthic epibiota) that form mats and help bind the sediment, and important macrophyte communities in the upper intertidal zone. Thirdly, the accumulation of mud provides a sedimentary record of the gradual accretion that has occurred on these coastlines, providing the opportunity for palaeoenvironmental reconstruction of past habitats and the way in which they have responded to altered boundary conditions, such as sea-level change (see Chapter 1).