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Underclays typically show considerable evidence of having been altered by plant activity and Pedogenesis and are either in whole or in part buried soils, called paleosols. As documented in various detailed studies,Driese, S.G., and E.G. Ober, 2005, Paleopedologic and paleohydrologic records of precipitation seasonality from Early Pennsylvanian "underclay" paleosols, U.S.A., Journal of Sedimentary Research. v. 75, no. 6, pp. 997-1010.Huddle, J.W., and S.H. Patterson, 1961, Origin of Pennsylvanian underclay and related seat rocks, Geological Society of America Bulletin. vo. 72, pp. 1643-1660.Joeckel, R.N., 1995a, Paleosols below the Ames marine unit (Upper Pennsylvanian, Conemaugh Group) in the Appalachian Basin, U.S.A.: variability on an ancient depositional landscape, Journal of Sedimentary Research. v. A65, no. 2, pp. 393-407.Joeckel, R.M., 1995b, Tectonic and paleoclimatic significance of a prominent upper Pennsylvanian (Virgilian/Stephanian) weathering profile, Iowa and Nebraska, USA, Palaeogeography, Palaeoeclimatology, Palaeoecology. v. 118, pp. 159-179. underclays and seatearths typically exhibit features characteristic of soil profile development. Depending on the specific underclay, these soil features can include some combination of Soil formation slickensides, pedogenic ped structures, Cutans, different types of pedogenic microfabrics, rhizocretions, caliche nodules, root moulds, and Soil horizon. 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 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 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 Pedogenesis 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.Gardner, T.W., E.G. Williams, and P.W. Holbrook, 1988, Pedogenesis of some Pennsylvanian underclays; ground-water, topography, and tectonic controls in J. Reinhardt and W.R. Sigleo, eds., Paleosols and Weathering Through Geologic Time: principles and Applications. Geological Society of America Special Paper. no. 216, pp. 81-102. Ober, E.G.., and S.G. Driese, 2003, The paleohydrologic history of coal underclays based upon Pennsylvanian paleosols in eastern Tennessee. Geological Society of America Abstracts with Programs v. 35, no. 6, p. 601
Flint clay can be either detrital or Authigenesis 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 stratum accumulated. Authigenic flint clays consist of sediments altered in place after deposition as beds within acid, such as peat, accumulating within swamps and .
Flint clays associated with coal typically occur as thin, laterally continuous layers (bands), called "tonsteins", found within coal beds. 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.Burger, K., and H.H. Damberger, 1985, Tonsteins in the Coalfields of Western Europe and North America. in Compte Rendu 4:433-448, IXICC International Congress on Carboniferous Stratigraphy and Geology, Southern Illinois University Press.Outerbridge, W.F., 2003, Isopach map and regional correlations of the Fire Clay tonstein, central Appalachian Basin. Open-File Report 03-351. United States Geological Survey.
As defined, ganisters can be created by either the cementation of quartzose by surficial soil-forming processes to form silcrete, or by Diagenesis cementation within the subsurface. 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 .Gibling, M.R., and B.P. Rust, 1992, Silica-cemented paleosols (ganisters) in the Pennsylvanian Waddens Cove Formation, Nova Scotia, Canada in K.H. Wolf and G.V. Chilingarian, George, eds., Diagenesis, III. Developments in Sedimentology. v. 47, pp. 621-655 Perciveil, C.J., 1982, Paleosols containing an albic horizon: examples from the upper Carboniferous of northern Britain in V.P. Wright, ed., pp. 87-111, Paleosols: Their Recognition and Interpretation. Princeton, Princeton University Press Percival, C.J., 1983, The Firestone Sill Ganister, Namurian, northern England—the A2 horizon of a podzol or podzolic palaeosol, Sedimentary Geology. v. 36, no. 1, pp. 41-49. Modern formation of ganisters has been observed in the Okavango Delta of Botswana.McCarthy, T.S. and W.N. Ellery, 1995, Sedimentation on the distal reaches of the Okavango Fan, Botswana, and its bearing on calcrete and silcrete (ganister) formation, Journal of Sedimentary Research. vol. A65, no. 1, pp. 77-90.
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