Seatearth is a British coal mining term that is used in the geological literature. As noted by Jackson, a seatearth is the layer of sedimentary rock underlying a coal seam. Seatearths have also been called seat earth, "seat rock", or "seat stone" in the geologic literature. Depending on its physical characteristics, a number of different names, such as underclay, fireclay, flint clay, and ganister, can be applied to a specific seatearth.

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 fossil roots and exhibits noticeably developed soil structures. It has often been altered by 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 in part buried soils, called paleosols. As documented in various detailed studies, underclays and seatearths typically exhibit features characteristic 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 moulds, 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 an 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 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.

Underclay, which consists of siliceous refractory clay rich in hydrous aluminium silicates, is also called fireclay. Just as not all underclays are fireclays, not all fireclays are underclays. Within Carboniferous and other coal-bearing strata, fireclay quite commonly comprises many underclays. The alteration of sediments by weathering, plants, and other soil processes comprising underclay resulted in the formation of the vast majority of fireclay that comprises underclay.

Another clay associated with coal beds is a smooth, flint-like refractory clay or mudstone composed predominantly of kaolin, called "flint clay". Flint clay breaks with a pronounced conchoidal fracture and resists slaking in water, due to its highly crystalline kaolinite.

Flint clay can be either detrital or authegenic in origin. 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. Authigenic flint clays consist of sediments altered in place after deposition as beds within acid, such as peat, accumulating within swamps and mires.

Flint clay can also be associated with other non-coal related kaolin deposits, so not all flint clay is a seat-earth.