Editing Geological history of Yorkshire

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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.
 
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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