Carboniferous rocks totalling 3442 m in thickness were drilled in the Seal Sands No. 1 Borehole at Teesmouth, Cleveland, in 1974–75. Cores taken in the middle and towards the bottom of the Carboniferous section show that the strata are broadly horizontal so the thicknesses drilled are close to true stratigraphical thicknesses. The borehole passed through 727 m of Triassic and Permian strata, which lie unconformably on the Carboniferous rocks. The latter comprise a Mississippian succession of Viséan–Serpukhovian (Holkerian–?Arnsbergian) age that is dominated by near-shore, shallow-water deposits with numerous but mainly thin coal seams. The Serpukhovian (Pendleian–?Arnsbergian) succession of shales and siltstones with thin sandstones, several thin limestones and rare coals is assigned to the Stainmore Formation (135 m). Below this, a thick calcareous interval of limestone and interbedded mudstone is believed to be the Great Limestone (110 m), which forms the top member of the underlying upper Viséan (Brigantian)–lower Serpukhovian (Pendleian) Alston Formation. The remaining strata of the Alston Formation (782 m) are equivalent to the 10 principal cyclothems (Four Fathom–Peghorn limestones) of the Alston Block, together with the underlying Birkdale Limestone at the base of the Alston Formation from which the lowest Brigantian microfossils have been recorded. The underlying Great Scar Limestone Group of late Asbian age (148 m) includes pale limestones of the Melmerby Scar Limestone Formation, similar to the limestones of the same formation on the Alston Block, interbedded with sandstones and mudstones. Below the Great Scar Limestone Group, the new informally defined Cleveland Group comprises three new informally defined formations. At the top of the group, the Greatham Formation (388 m) comprises limestones interbedded with mudstone and sandstone in cyclic sequences of late Asbian age. Below this, the informally defined Teesmouth Formation (539 m), also of late Asbian age, is a more terrigenous succession with numerous coal seams. The oldest unit in the borehole, the informally defined Seal Sands Formation (1338 m), comprises finely interbedded, black fissile mudstone, siltstone, fine-grained sandstone, thin black peloidal limestone beds, black chert and many coal seams. This distinctly black succession is mostly Asbian in age, but Holkerian microfossils occur in the lower part, from 3621 m down to the bottom of the hole at 4169 m. Correlations are made with the Alston Block succession and that of the Stainmore Basin. The unusually thick Mississippian succession, especially in the Asbian (>1800 m thick), is in accordance with the borehole's location within the eastern part of the Stainmore Basin, where a thick Carboniferous sequence is inferred from geophysical data. It is also comparable with a thick Asbian succession in the Northumberland Basin.