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== Jurassic ==
 
== Jurassic ==
  
The Lias Group is a sequence of richly fossiliferous mudstones, calcareous in part, with subsidiary shallow marine sandstones and ironstones of about 420 m maximum thickness in the Cleveland Basin ([[Jurassic, Tertiary and Quaternary around Great Ayton and Roseberry Topping, Cleveland Hills - an excursion|Excursion 11]], [[Lower Jurassic rocks between Staithes and Port Mulgrave - an excursion|Excursion 16]], [[Lower–Middle Jurassic sequences between Whitby and Saltwick - an excursion|Excursion 17]], [[Lower and Middle Jurassic rocks between Robin Hood's Bay and Hawsker Bottoms - an excursion|Excursion 18]]). '''Concretions''' are abundant. A cyclic repetition of thin limestones and shales is very evident in the lower part of the succession. In contrast to the two preceding systems, abundant '''ammonites''' allow a very precise dating and correlation of the sequence. Other fossils include a wide range of bivalves, brachiopods, '''belemnites''', less common '''echinoderms''', marine reptiles and wood. The ironstones, alum shales and jet were of economic importance historically.
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The Lias Group is a sequence of richly fossiliferous mudstones, calcareous in part, with subsidiary shallow marine sandstones and ironstones of about 420 m maximum thickness in the Cleveland Basin ([[Jurassic, Tertiary and Quaternary around Great Ayton and Roseberry Topping, Cleveland Hills - an excursion|Excursion 11]][[Lower Jurassic rocks between Staithes and Port Mulgrave - an excursion|Excursion 16]], [[Lower–Middle Jurassic sequences between Whitby and Saltwick - an excursion|Excursion 17]], [[Lower and Middle Jurassic rocks between Robin Hood's Bay and Hawsker Bottoms - an excursion|Excursion 18]]). Concretions are abundant. A cyclic repetition of thin limestones and shales is very evident in the lower part of the succession. In contrast to the two preceding systems, abundant ammonites allow a very precise dating and correlation of the sequence. Other fossils include a wide range of bivalves, brachiopods, belemnites, less common echinoderms, marine reptiles and wood. The ironstones, alum shales and jet were of economic importance historically.
  
Uplift and gentle folding terminated this marine sequence and the Middle Jurassic consists of about 250 m of fluviatile and deltaic sandstones, siltstones, shales and minor coals with marine intercalations, resting unconformably on various levels of the Lower Jurassic ([[Jurassic, Tertiary and Quaternary around Great Ayton and Roseberry Topping, Cleveland Hills - an excursion|Excursion 11]], [[Lower–Middle Jurassic sequences between Whitby and Saltwick - an excursion|Excursion 17]], [[Lower and Middle Jurassic rocks between Robin Hood's Bay and Hawsker Bottoms - an excursion|Excursion 18]], [[Middle–Upper Jurassic sequence between Layton Bay and Yons Nab - an excursion|Excursion 19]]). These deposits were laid down by south-flowing river systems originating immediately to the north. South of Yorkshire, they pass transitionally into the fully marine sequence of central and southern England ([[:File:YGS_YORKROCK_FIG_03_00.jpg|Figure 3]]d). Fossils are often common in the marine interbeds and the marshy delta-top environment supported a rich flora which is locally well preserved. A transgression returned fully marine conditions to the area late in the Middle Jurassic. The first sediments deposited were a variety of sandstones, shales and limestones up to 50 m thick, many highly fossiliferous, principally with bivalves and ammonites.
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Uplift and gentle folding terminated this marine sequence and the Middle Jurassic consists of about 250 m of fluviatile and deltaic sandstones, siltstones, shales and minor coals with marine intercalations, resting unconformably on various levels of the Lower Jurassic ([[Jurassic, Tertiary and Quaternary around Great Ayton and Roseberry Topping, Cleveland Hills - an excursion|Excursion 11]], [[Lower–Middle Jurassic sequences between Whitby and Saltwick - an excursion|Excursion 17]], [[Lower and Middle Jurassic rocks between Robin Hood's Bay and Hawsker Bottoms - an excursion|Excursion 18]], [[Middle–Upper Jurassic sequence between Layton Bay and Yons Nab - an excursion|Excursion 19]]). These deposits were laid down by south-flowing river systems originating immediately to the north. South of Yorkshire, they pass transitionally into the fully marine sequence of central and southern England ([[:File:YGS_YORKROCK_FIG_03_00.jpg|Figure 3]]d). Fossils are often common in the marine interbeds and the marshy delta-top environment supported a rich flora which is locally well preserved. A transgression returned fully marine conditions to the area late in the Middle Jurassic. The first sediments deposited were a variety of sandstones, shales and limestones up to 50 m thick, many highly fossiliferous, principally with bivalves and ammonites.
  
Minor episodes of gentle warping and erosion broke up the Upper Jurassic succession on the Yorkshire coast ([[Middle–Upper Jurassic sequence between Layton Bay and Yons Nab - an excursion|Excursion 19]], [[Jurassic, Cretaceous and Quaternary rocks of Filey Bay and Speeton - an excursion|Excursion 20]]), whilst inland and to the south of the county the widely uniform muds of the Oxford Clay became established. This facies only extended to the coast in upper Oxford Clay times where it is 45 m thick. There, it is succeeded by a highly variable 100 m thick complex of limestones, including coral patch-reefs, and fine-grained calcareous sandstones. These too show lateral facies changes north of the Market Weighton area to marine muds typical of much of the eastern Midlands.
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Minor episodes of gentle warping and erosion broke up the Upper Jurassic succession on the Yorkshire coast ([[Middle–Upper Jurassic sequence between Layton Bay and Yons Nab - an excursion|Excursion 19]], [[Jurassic, Cretaceous and Quaternary rocks of Filey Bay and Speeton - an excursion|Excursion 20]]), whilst inland and to the south of the county the widely uniform muds of the Oxford Clay became established. This facies only extended to the coast in upper Oxford Clay times where it is 45 m thick. There, it is succeeded by a highly variable too m thick complex of limestones, including coral patch-reefs, and fine-grained calcareous sandstones. These too show lateral facies changes north of the Market Weighton area to marine muds typical of much of the eastern Midlands.
  
Renewed transgression extended marine clay facies back across the Yorkshire area with the deposition of the Kimmeridge Clay, the most laterally persistent and uniform of all late Jurassic sediments and the principal source rock for North Sea oil. Unfortunately, the unit is poorly exposed in Yorkshire although in boreholes it may reach about 270 m in thickness. The latest Jurassic and very earliest Cretaceous strata are not represented in Yorkshire because renewed earth movements resulted in the uplift of an extensive landmass extending from central England into western Europe.
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Renewed transgression extended marine clay facies back across the Yorkshire area with the deposition of the Kimmeridge Clay, the most laterally persistent and uniform of all late Jurassic sediments and the principal source rock for North Sea oil. Unfortunately, the unit is poorly exposed in Yorkshire although in boreholes it may reach about 270 m in thickness. The latest Jurassic and very earliest Cretaceous strata are not represented in Yorkshire because renewed earth movements resulted in the uplift of an extensive landmass extending from central England into western Europe.
  
 
== Cretaceous ==
 
== Cretaceous ==
Transgression early in the Cretaceous returned marine conditions to Yorkshire with the deposition of a richly fossiliferous sequence of soft shales and clay some 100 m thick (Excursion 20). The principal fossils are ammonites and belemnites and almost the whole of the lower Cretaceous is represented by this uniform sequence, making it the best and most complete example of this period in the U.K. Towards the end of early Cretaceous time, the supply of mud ceased and the sea deepened and cleared. This began the period of maximum transgression in the Mesozoic ([[:File:YGS_YORKROCK_FIG_03_00.jpg|Figure 3]]e), with uniform marine conditions across the whole of western Europe ([[Jurassic and Cretaceous rocks of the Market Weighton area - an excursion|Excursion 15]], [[Jurassic, Cretaceous and Quaternary rocks of Filey Bay and Speeton - an excursion|Excursion 20]], [[Chalk of Flamborough Head - an excursion|Excursion 21]]). Calcareous oozes formed, composed almost wholly of the minute platelets of '''coccoliths''' (unicellular algae) which abounded in surface waters. Initially, iron oxides stained the oozes and up to 30 m of red '''chalk''' is recorded in southeast Yorkshire, with varied faunas of '''sponges''', bivalves, brachiopods, echinoids, and crinoids. In the late Cretaceous, the red chalk is succeeded by the familiar white chalk which reaches a maximum thickness locally of 500 m. The rock is more marly and harder, due to calcite cementation, than in southern England and '''flints''' are present between about 40 m and 210 m above the base. The fauna is similar to that of the red chalk, although ammonites are also sporadically found. Distinctive laterally continuous marl and flint layers have proved excellent marker beds for local correlation between sections. The highest parts of the chalk do not crop out in Yorkshire, although they are present in part under the glacial deposits of Holderness.
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Transgression early in the Cretaceous returned marine conditions to Yorkshire with the deposition of a richly fossiliferous sequence of soft shales and clay some too m thick (Excursion 20). The principal fossils are ammonites and belemnites and almost the whole of the lower Cretaceous is represented by this uniform sequence, making it the best and most complete example of this period in the U.K.
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Towards the end of early Cretaceous time, the supply of mud ceased and the sea deepened and cleared. This began the period of maximum transgression in the Mesozoic ([[:File:YGS_YORKROCK_FIG_03_00.jpg|Figure 3]]e), with uniform marine conditions across the whole of western Europe ([[Jurassic and Cretaceous rocks of the Market Weighton area - an excursion|Excursion 15]], [[Jurassic, Cretaceous and Quaternary rocks of Filey Bay and Speeton - an excursion|Excursion 20]], [[Chalk of Flamborough Head - an excursion|Excursion 21]]). Calcareous oozes formed, composed almost wholly of the minute platelets of coccoliths (unicellular algae) which abounded in surface waters. Initially, iron oxides stained the oozes and up to 30 m of red chalk is recorded in southeast Yorkshire, with varied faunas of sponges, bivalves, brachiopods, echinoids, and crinoids. In the late Cretaceous, the red chalk is succeeded by the familiar white chalk which reaches a maximum thickness locally of 500 m. The rock is more marly and harder, due to calcite cementation, than in southern England and flints are present between about 4.0 m and 210 m above the base. The fauna is similar to that of the red chalk, although ammonites are also sporadically found. Distinctive laterally continuous marl and flint layers have proved excellent marker beds for local correlation between sections. The highest parts of the chalk do not crop out in Yorkshire, although they are present in part under the glacial deposits of Holderness.
  
 
== 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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