Editing Geological history of Yorkshire

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== Tertiary ==
 
== Tertiary ==
A major retreat of the sea, together with uplift at the end of the Cretaceous, means that Tertiary sediments may never have been deposited in Yorkshire, although thick sequences are known offshore beneath the North Sea. These earth movements lifted the northwest part of the British Isles and were largely responsible for the prevailing shallow southeasterly tilt of the Mesozoic rocks in eastern and southern Britain. The only undoubted Tertiary rock in the county is the Cleveland '''Dyke''' ([[:File:YGS_YORKROCK_FIG_01_00.jpg|Figure 1]]), a '''thoeliitic basalt''' intrusion up to 25 m wide and a distant representative of the Tertiary igneous complex of Mull. It has been worked almost to exhaustion, mainly for roadstone in a series of quarries from Great Ayton across the Cleveland Hills ([[Jurassic, Tertiary and Quaternary around Great Ayton and Roseberry Topping, Cleveland Hills - an excursion|Excursion 11]]).
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A major retreat of the sea, together with uplift at the end of the Cretaceous, means that Tertiary sediments may never have been deposited in Yorkshire, although thick sequences are known offshore beneath the North Sea. These earth movements lifted the northwest part of the British Isles and were largely responsible for the prevailing shallow southeasterly tilt of the Mesozoic rocks in eastern and southern Britain. The only undoubted Tertiary rock in the county is the Cleveland Dyke ([[:File:YGS_YORKROCK_FIG_01_00.jpg|Figure 1]]), a thoeliitic basalt intrusion up to 25 m wide and a distant representative of the Tertiary igneous complex of Mull. It has been worked almost to exhaustion, mainly for roadstone in a series of quarries from Great Ayton across the Cleveland Hills ([[Jurassic, Tertiary and Quaternary around Great Ayton and Roseberry Topping, Cleveland Hills - an excursion|Excursion 11]]).
  
 
==Quaternary ==
 
==Quaternary ==
  
The most recent geological activity, that of the Quaternary, has had a profound influence on the physiography, superficial deposits and soils of the county ([[Quaternary geology and geomorphology of the area around Kisdon, upper Swaledale - an excursion|Excursion 3]], [[Jurassic, Tertiary and Quaternary around Great Ayton and Roseberry Topping, Cleveland Hills - an excursion|Excursion 11]], [[Quaternary features of Scugdale, northwest Cleveland Hills - an excursion|Excursion 12]], [[Jurassic, Cretaceous and Quaternary rocks of Filey Bay and Speeton - an excursion|Excursion 20]], and briefly elsewhere). The whole area was probably covered by ice sheets on many occasions, but the bulk of the evidence relates to the last glacial stage, the Devensian. Glacial and '''glaciofluvial''' sediments of this age cover northwest Yorkshire and the lowlands of the Vale of York, Teeside and Holderness ([[:File:YGS_YORKROCK_FIG_03_00.jpg|Figure 3]]f). Elsewhere, periglacial processes redistributed existing superficial sediments as sheets of '''gelifluctate''' and thin sheets of loess across hillside slopes and plateaux, often now mixed in the present topsoil. In the Pennine dales and Cleveland Hills, the effects of glacial meltwater are preserved in the form of channels and outwash deposits. The lowlands of the Vales of Mowbray, York and Pickering, and in Holderness, contain complex associations of till, sand and gravel, and laminated lake clays. '''Erratics''' in these deposits are derived from the Lake District, the Cheviot Hills, Scotland and, in the eastern part of the county, Scandinavia. Locally, interglacial deposits with mammal remains survive in caves, and interglacial shorelines indicate the positions of former high sea levels, similar to that of the present day.
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The most recent geological activity, that of the Quaternary, has had a profound influence on the physiography, superficial deposits and soils of the county ([[Quaternary geology and geomorphology of the area around Kisdon, upper Swaledale - an excursion|Excursion 3]], [[Jurassic, Tertiary and Quaternary around Great Ayton and Roseberry Topping, Cleveland Hills - an excursion|Excursion 11]], [[Quaternary features of Scugdale, northwest Cleveland Hills - an excursion|Excursion 12]], [[Jurassic, Cretaceous and Quaternary rocks of Filey Bay and Speeton - an excursion|Excursion 20]], and briefly elsewhere). The whole area was probably covered by ice sheets on many occasions, but the bulk of the evidence relates to the last glacial stage, the Devensian. Glacial and glaciofluvial sediments of this age cover northwest Yorkshire and the lowlands of the Vale of York, Teeside and Holderness ([[:File:YGS_YORKROCK_FIG_03_00.jpg|Figure 3]]f). Elsewhere, periglacial processes redistributed existing superficial sediments as sheets of gelifluctate and thin sheets of loess across hillside slopes and plateaux, often now mixed in the present topsoil. In the Pennine dales and Cleveland Hills, the effects of glacial meltwater are preserved in the form of channels and outwash deposits. The lowlands of the Vales of Mowbray, York and Pickering, and in Holderness, contain complex associations of till, sand and gravel, and laminated lake clays. Erratics in these deposits are derived from the Lake District, the Cheviot Hills, Scotland and, in the eastern part of the county, Scandinavia. Locally, interglacial deposits with mammal remains survive in caves, and interglacial shorelines indicate the positions of former high sea levels, similar to that of the present day.
  
It is only some 17 000 <sup>14</sup>C yrs (see '''dating''') since the Devensian ice sheet began its retreat from Yorkshire, and the last ice in the northwest Pennine fells disappeared only about 10 500 <sup>14</sup>C yrs ago. With post-glacial sea level rise, the county has gradually assumed the familiar form of the present day. This rise is still continuing, albeit very slowly, and coastal erosion in areas such as Holderness, for example, is causing the shoreline to retreat on average by about 1.5&nbsp;m per year. However we are long past the climatic optimum of the present interglacial and in the distant future (in human terms) the glaciers could return to Yorkshire.
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It is only some 17 000 <sup>14</sup>C yrs (see dating) since the Devensian ice sheet began its retreat from Yorkshire, and the last ice in the northwest Pennine fells disappeared only about 10 500 <sup>14</sup>C yrs ago. With post-glacial sea level rise, the county has gradually assumed the familiar form of the present day. This rise is still continuing, albeit very slowly, and coastal erosion in areas such as Holderness, for example, is causing the shoreline to retreat on average by about 1.5 m per year. However we are long past the climatic optimum of the present interglacial and in the distant future (in human terms) the glaciers could return to Yorkshire.
  
  

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