Underclay
Underclay is a seatearth composed of soft, dispersible clay, or other fine-grained sediment, either immediately underlying or forming the floor of a coal seam. Underclay typically contains the fossil roots and exhibits noticeable development soil structures. It often has been noticeably altered due to weathering. Underclays, which occur within Carboniferous coal measures, commonly contain Stigmarian roots. Synonyms for underclay included seat clay, root clay, thill, warrant, coal clay, and warrant clay
Underclays typically show considerable evidence of having been altered by plant activity and soil forming processes and are either in whole or part buried soils, called paleosols. As documented in various detailed studies of underclays, underclays and seat earths typically exhibit features characteristics of soil profile development. Depending on the specific underclay, these soil features can include some combination of pedogenic slickensides, pedogenic ped structures, illuviated clay pore fillings, different types of pedogenic microfabrics, rhizocretions, caliche nodules, root molds, and soil horizons. In the better-developed paleosols, significant alteration of the mineralogy, i.e. leaching and translocation of alkali and alkaline earth elements and the kaolinitization of smectites and hydroxy-interlayer vermiculite, will have occurred. In poorly developed paleosols, as seen in the soil profiles of modern poorly developed soils, called "Inceptisols", of modern river deltas and floodplains, there might not exist any noticeable alteration of the underclay. These studies demonstrate that a paleosol, which is either developed in or comprises underclay, largely reflects the effects of plants and other soil forming processes on the underclay while it formed the ground surface prior to being buried by organic sediments. Plant growth, waterlogging, and other processes, which occurred during the development of a mire or swamp, in which a layer of peat accumulated that later became the overlying coal, modified the paleosol to create an underclay
Read more about this topic: Seatearth