[A local term of unknown origin; cf. TILL sb., boulder-clay.] The thin stratum of fire-clay, etc., usually underlying a coal-seam; under-clay; the floor or bottom of a seam of coal.
1329–30. Durham Acc. Rolls (Surtees), 515. Quatuor bayardours portantibus Thill et focale in abbathiam per x septimanas, xxiij s. vj d. Ibid. (1454–5), 634. Operanti circa le ryddyng ac adquisicione de le Thill pro eodem furno. Ibid. (1500–1), 657. Pro iiijor plaustr. de lez thillstone, xvj d.
1708. J. C., Compl. Collier (1845), 39. Sometimes a Pit may happen to have a Hitch or Dipping of the Thill or Bottom orihe Way.
1851. Greenwell, Coal-trade Terms Northumb. & Durh., 54. Thill, the floor of a seam of coal.
1867. W. W. Smyth, Coal, 25. The floor, thill, or seat…, of the coal is an underclay.
1878. Lebour, Geol. Northumberland & Durh. (1886), iii. 12. There is a strict analogy between these peat-marls and clays and the ‘thills’ or ‘underclays’ of many coals.
1881. Borings & Sinkings, II. 4 (E.D.D.). Grey thill with water.
1887. Woodward, Geol. Eng. & Wales (ed. 2), 179. The Underclay is known as ‘Spavin’ in Yorkshire; as ‘Thill’ in Durham; as ‘Warrant’ or ‘Seat-earth’ in Lancashire; and as ‘Bottomstone’ or ‘Pouncin’ in South Wales.
1894. Heslop, Northumb. Gloss., s.v., The underlayer of a coal seam frequently consists of a thin bed of fireclay; hence thin strata of that material are called thill, irrespective of their position with regard to a seam of coal.
