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.
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.
Plant growth, waterlogging, and other processes that 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.
Detrital flint clays consist of kaolinite-rich sediments eroded and transported from uplands deeply weathered under tropical climates and redeposited within the coastal plains, in which coal-bearing strata accumulated.
In the case of tonsteins found within coal, the formation of flint clays resulted from the alternation of glass comprising volcanic ash by acidic waters after it accumulated as thin beds within peat swamps or mires.
Detailed studies of ganisters, which occur either as seatearths or elsewhere within coal-bearing strata, have found them to be ancient paleosols, which are equivalent in both physical characteristics and origin to modern silica-cemented soils, called silcretes.