Editing Lower Carboniferous of Bewcastle and Gilsland - an excursion

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== Geological background ==
 
== Geological background ==
  
The area covered comprises a mixture of rolling agricultural land and peat covered moorland rising to 500 m above sea level. It is dominated geologically by Carboniferous sandstones, marine limestones and shales of the Lower, Middle and Upper Border groups, and the Liddesdale Group ([[:File:YGS_NORTROCK_FIG_05_1.jpg|Figure 5.1]]). These are '''unconformably''' overlain by Permian basal '''breccias''' and shales, and a thick blanket of glacial deposits.
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The area covered comprises a mixture of rolling agricultural land and peat covered moorland rising to 500 m above sea level. It is dominated geologically by Carboniferous sandstones, marine limestones and shales of the Lower, Middle and Upper Border groups, and the Liddesdale Group ([[:File:YGS_NORTROCK_FIG_05_1.jpg|Figure 5.1]]). These are unconformably overlain by Permian basal breccias and shales, and a thick blanket of glacial deposits.
  
The Carboniferous rocks of the area were deposited in a shallow trough bordered to the north by the subdued '''Caledonian''' Mountains of the Southern Uplands and, to the south, by the Alston Block. These upstanding margins of the trough were sources of '''clastic''' sediment during Lower Border Group times, but large drainage systems from the north and east dominated deposition through much of the later Carboniferous. At times of high clastic sediment input or lower sea level, extensive deltas, sometimes topped by coal-forming swamps, '''prograded''' into the shallow gulf-like sea which occupied the trough in this area. Periodically, reduced sediment supply or higher sea level led to northeastward '''transgression''' of marine conditions. Thus the sandstones, shales and limestones of the Lower and Upper Border groups encountered at the localities detailed below reflect the complex interplay of these fluviodeltaic and marine depositional systems.
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The Carboniferous rocks of the area were deposited in a shallow trough bordered to the north by the subdued Caledonian Mountains of the Southern Uplands and, to the south, by the Alston Block. These upstanding margins of the trough were sources of elastic sediment during Lower Border Group times, but large drainage systems from the north and east dominated deposition through much of the later Carboniferous. At times of high clastic sediment input or lower sea level, extensive deltas, sometimes topped by coal-forming swamps, prograded into the shallow gulf-like sea which occupied the trough in this area. Periodically, reduced sediment supply or higher sea level led to northeastward transgression of marine conditions. Thus the sandstones, shales and limestones of the Lower and Upper Border groups encountered at the localities detailed below reflect the complex interplay of these fluviodeltaic and marine depositional systems.
  
During deposition of the Liddesdale Group fully marine conditions dominated the area. The Southern Uplands and Alston Block, until this time the margins to the basin, were finally breached by the sea and limestone was laid down in laterally persistent beds, some of which extended north into the Midland Valley of Scotland and south into Yorkshire. In mid-Carboniferous time thick sands prograded from the northeast (Millstone Grit). The late Carboniferous saw the development of swamp conditions and deposition of the Coal Measures. Regional '''tectonic''' compression during the late Carboniferous and early Permian resulted in uplift and prolonged subaerial erosion. The late Permian transgression, from the south, deposited the St. Bees and Kirklington sandstones, widely used in local buildings.
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During deposition of the Liddesdale Group fully marine conditions dominated the area. The Southern Uplands and Alson Block, until this time the margins to the basin, were finally breached by the sea and limestone was laid down in laterally persistent beds, some of which extended north into the Midland Valley of Scotland and south into Yorkshire. In mid-Carboniferous time thick sands prograded from the northeast (Millstone Grit). The late Carboniferous saw the development of swamp conditions and deposition of the Coal Measures. Regional tectonic compression during the late Carboniferous and early Permian resulted in uplift and prolonged subaerial erosion. The late Permian transgression, from the south, deposited the St. Bees and Kirklington sandstones, widely used in local buildings.
  
The present drainage systems were established during Tertiary uplift. Pleistocene glacial advance and retreat covered the area in thick '''till''', overlain by sands and gravels, but in Holocene times the major rivers have re-located in their pre-glacial courses. Post-glacial climates favoured the formation of an extensive upland peat blanket.
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The present drainage systems were established during Tertiary uplift. Pleistocene glacial advance and retreat covered the area in thick till, overlain by sands and gravels, but in Holocene times the major rivers have re-located in their pre-glacial courses. Post-glacial climates favoured the formation of an extensive upland peat blanket.
  
More recent history is also of considerable interest. There are scattered signs of ancient British settlements throughout the area, and from about 120 A.D. Bewcastle was the site of the Roman fort of ''Banna. ''After Roman occupation the fort became an Anglo-Saxon settlement and a well preserved, carved sandstone cross, now located in the churchyard, dates from this period. Subsequent Norse occupation, for a time under the chieftain Beuth, gave Bewcastle its name. The church and castle which now stand on the site of the fort were built during the reign of Edward I in about 1291. The castle was destroyed by Cromwell's troops in 1641.
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More recent history is also of considerable interest. There are scattered signs of ancient British settlements throughout the area, and from about 120 A.D. Bewcastle was the site of the Roman fort ''ofBanna. ''After Roman occupation the fort became an Anglo-Saxon settlement and a well preserved, carved sandstone cross, now located in the churchyard, dates from this period. Subsequent Norse occupation, for a time under the chieftain Beuth, gave Bewcastle its name. The church and castle which now stand on the site of the fort were built during the reign of Edward I in about 1291. The castle was destroyed by Cromwell's troops in 1641.
  
 
Further details of the geology can be found in Day (1970).
 
Further details of the geology can be found in Day (1970).

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