Editing Geological history of Yorkshire

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Renewed subsidence in Permian times placed Yorkshire near the western margin of a broad basin extending across northern Europe into Poland, situated in the hot, dry belt about 12–13° north of the equator. Basal and marginal piedmont '''breccia''' wedges and sheets are associated with patchy continental dune sands of presumed late Cisuralian age ([[:File:YGS_YORKROCK_FIG_03_00.jpg|Figure 3]]c) ([[Permian rocks of south-central Yorkshire - an excursion|Excursion 14]]), resting with gentle unconformity on the Carboniferous ([[:File:YGS_YORKROCK_FIG_01_00.jpg|Figure 1]], [[:File:YGS_YORKROCK_FIG_02_00.jpg|Figure 2]]). At the beginning of the Guaclalupian, the basin, then well below sea level, was flooded, extensively reworking the sands and introducing a period of marine and hypersaline conditions in a fluctuating '''epicontinental''' sea. Four major cycles of limestones, later altered to dolomites, and succeeding '''evaporites''', including '''gypsum''' (now '''anhydrite'''), '''halite''' and potash salts, were developed. These cycles resulted from periodic recharge of the basin by normal marine waters, from which limestones, 'many oolitic and locally fossiliferous, were formed around the margins. Only the limestones and dolomites are prominent at outcrop in a narrow north–south belt bisecting the county, the evaporites being reduced to thin, silty dissolution residues ([[Permian and Carboniferous rocks of Knaresborough - an excursion|Excursion 13]], [[Permian rocks of south-central Yorkshire - an excursion|Excursion 14]]). In the first cycle, fossiliferous '''bryozoan''' and '''stromatolitic''' patch reefs formed, but in the third cycle the limestones contain only a few plant and invertebrate species. Limestones of the second and fourth cycles do not crop out in Yorkshire but are present in thicker sequences in the subsurface to the east, where potash is mined at Boulby near the coast west-northwest of Whitby. Towards the end of the Permian, continental, water-lain red sandstones, siltstones and mudstones gradually filled the basin from the west.
 
Renewed subsidence in Permian times placed Yorkshire near the western margin of a broad basin extending across northern Europe into Poland, situated in the hot, dry belt about 12–13° north of the equator. Basal and marginal piedmont '''breccia''' wedges and sheets are associated with patchy continental dune sands of presumed late Cisuralian age ([[:File:YGS_YORKROCK_FIG_03_00.jpg|Figure 3]]c) ([[Permian rocks of south-central Yorkshire - an excursion|Excursion 14]]), resting with gentle unconformity on the Carboniferous ([[:File:YGS_YORKROCK_FIG_01_00.jpg|Figure 1]], [[:File:YGS_YORKROCK_FIG_02_00.jpg|Figure 2]]). At the beginning of the Guaclalupian, the basin, then well below sea level, was flooded, extensively reworking the sands and introducing a period of marine and hypersaline conditions in a fluctuating '''epicontinental''' sea. Four major cycles of limestones, later altered to dolomites, and succeeding '''evaporites''', including '''gypsum''' (now '''anhydrite'''), '''halite''' and potash salts, were developed. These cycles resulted from periodic recharge of the basin by normal marine waters, from which limestones, 'many oolitic and locally fossiliferous, were formed around the margins. Only the limestones and dolomites are prominent at outcrop in a narrow north–south belt bisecting the county, the evaporites being reduced to thin, silty dissolution residues ([[Permian and Carboniferous rocks of Knaresborough - an excursion|Excursion 13]], [[Permian rocks of south-central Yorkshire - an excursion|Excursion 14]]). In the first cycle, fossiliferous '''bryozoan''' and '''stromatolitic''' patch reefs formed, but in the third cycle the limestones contain only a few plant and invertebrate species. Limestones of the second and fourth cycles do not crop out in Yorkshire but are present in thicker sequences in the subsurface to the east, where potash is mined at Boulby near the coast west-northwest of Whitby. Towards the end of the Permian, continental, water-lain red sandstones, siltstones and mudstones gradually filled the basin from the west.
  
These continental clastic deposits are unfossiliferous and span the Permo-Triassic boundary, which is consequently difficult to define. In addition, the soft sandstones, '''marls''' and evaporites of the Triassic, deposited along the western margin of an epicontinental North Sea Basin are rarely seen at outcrop, forming low ground largely mantled by glacial deposits in the Vales of Mowbray and York. The early Triassic (Sherwood Sandstone) consists of fluvial sandstones, with some flood-plain mudstones and siltstones and, in the south of the county, pebble beds deposited by north-flowing river systems. These are overlain with gentle unconformity by a thin concentrate deposit and then by dolomitic and silty mudstones (often red) and evaporites (principally halite) of the Mercia Mudstone, the result of waters from the Tethys Ocean flooding into the North Sea Basin, forming a shallow hypersaline marine environment. Macrofossils are rare but plant spores allow dating and correlation of the lower part of the sequence. Renewed transgression at the end of the Triassic produced thin, shaly mudstones with a '''bivalve''' fauna, bone beds and argillaceous limestones overlain by soft lagoonal mudstones. The total thickness of Triassic deposits increases from about 400 m at outcrop to some 700 m in the subsurface beneath east Yorkshire.
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These continental clastic deposits are unfossiliferous and span the Permo-Triassic boundary, which is consequently difficult to define. In addition, the soft sandstones, marls and evaporites of the Triassic, deposited along the western margin of an epicontinental North Sea Basin are rarely seen at outcrop, forming low ground largely mantled by glacial deposits in the Vales of Mowbray and York. The early Triassic (Sherwood Sandstone) consists of fluvial sandstones, with some flood-plain mudstones and siltstones and, in the south of the county, pebble beds deposited by north-flowing river systems. These are overlain with gentle unconformity by a thin concentrate deposit and then by dolomitic and silty mudstones (often red) and evaporites (principally halite) of the Mercia Mudstone, the result of waters from the Tethys Ocean flooding into the North Sea Basin, forming a shallow hypersaline marine environment. Macrofossils are rare but plant spores allow dating and correlation of the lower part of the sequence. Renewed transgression at the end of the Triassic produced thin, shaly mudstones with a bivalve fauna, bone beds and argillaceous limestones overlain by soft lagoonal mudstones. The total thickness of Triassic deposits increases from about 400 m at outcrop to some 700 m in the subsurface beneath east Yorkshire.
  
 
== Triassic ==
 
== Triassic ==

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