Editing Carboniferous rocks of the Roman Wall and Haltwhistle Burn - an excursion

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== Geological background ==
 
== Geological background ==
  
The striking landforms of the area arise from a combination of three factors. Firstly, the succession exposed, which ranges from the Jew (=Oxford) Limestone (Dinantian) to the Oakwood Limestone (Namurian) and consists of sediments deposited in typical '''Yoredale''' cycles. This repeated sequence of limestone, shale, sandstone, shale, coal produces an alternation of hard and soft beds with very different weathering characteristics. Into this succession was intruded the Whin Sill, of variable thickness and frequently '''transgressing''' from one horizon to another, but very durable and resistant to weathering in comparison to the enclosing sediments. Secondly, the post-Carboniferous uplift of the Cheviot '''pluton''' resulted in doming of the sediments laid down in the Northumberland Trough, imparting a significant southerly '''dip''' to the strata in the southwest of the county. Dips of around 15°, coupled with the alternation of hard and soft beds, produced a strong dip and scarp topography. Thirdly, although these conditions occur over most of Northumberland, the spectacular scenery displayed in the Roman Wall area is not evident elsewhere. This is because ice movement in south Northumberland was from west to east, parallel to the '''strike''' of the beds, as demonstrated by an '''erratic''' of Cumbrian Shap '''Granite''' found in the area by Johnson (1952). The movement of ice along the strike direction will have preserved, and possibly enhanced the dip-scarp topography, whereas movement of ice across the strike fills in the shale depressions with glacial debris, flattening out the topography. Drilling in east Northumberland has also revealed strong dip and scarp topography in rockhead underlying the flat ground surface, with thickness of glacial '''till''' from 0 to 80 m.
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The striking landforms of the area arise from a combination of three factors. Firstly, the succession exposed, which ranges from the Jew (=Oxford) Limestone (Dinantian) to the Oakwood Limestone (Namurian) and consists of sediments deposited in typical '''Yoredale''' cycles. This repeated sequence of limestone, shale, sandstone, shale, coal produces an alternation of hard and soft beds with very different weathering characteristics. Into this succession was intruded the Whin Sill, of variable thickness and frequently '''transgressing''' from one horizon to another, but very durable and resistant to weathering in comparison to the enclosing sediments. Secondly, the post-Carboniferous uplift of the Cheviot '''pluton''' resulted in doming of the sediments laid down in the Northumberland Trough, imparting a significant southerly '''dip''' to the strata in the southwest of the county. Dips of around 15°, coupled with the alternation of hard and soft beds, produced a strong dip and scarp topography. Thirdly, although these conditions occur over most of Northumberland, the spectacular scenery displayed in the Roman Wall area is not evident elsewhere. This is because ice movement in south Northumberland was from west to east, parallel to the '''strike''' of the beds, as demonstrated by an '''erratic''' of Cumbrian Shap '''Granite''' found in the area by Johnson (1952). The movement of ice along the strike direction will have preserved, and possibly enhanced the dip-scarp topography, whereas movement of ice across the strike fills in the shale depressions with glacial debris, flattening out the topography. Drilling in east Northumberland has also revealed strong dip and scarp topography in rockhead underlying the flat ground surface, with thickness of glacial '''till''' from o-80 m.
  
The Roman Wall is an outstanding structure, literally and metaphorically, accentuated in the excursion area by its location on the scarp edge of the Whin Sill outcrop. It is 118 km long, 4.6–6.1 m high and 2.1–3.1 m thick, with accompanying forts and turrets, crossing from one side of the country to the other. Archaeological investigations have revealed that the Wall is a complex frontier zone with a great wealth of Roman remains. In AD 121 Hadrian set out on a tour to examine the military dispositions in his empire and during his visit to Britain he initiated the construction of the wall which had been proposed by his predecessor Trajan to define the limits of the Roman Empire and to defend the Stanegate frontier road. The building of the Wall seems to have been completed by AD 140, and it continued to operate as a fortification and customs control until the final withdrawal of the Romans from Britain in AD 400.
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The Roman Wall is an outstanding structure, literally and metaphorically, accentuated in the excursion area by its location on the scarp edge of the Whin Sill outcrop. It is 118 km long, 4.6–6.1 m high and 2.1–3.1 m thick, with accompanying forts and turrets, crossing from one side of the country to the other. Archaeological investigations have revealed that the Wall is a complex frontier zone with a great wealth of Roman remains. In AD 121 Hadrian set out on a tour to examine the military dispositions in his empire and during his visit to Britain he initiated the construction of the wall which had been proposed by his predecessor Trajan to define the limits of the Roman Empire and to defend the Stanegate frontier road. The building of the Wall seems to have been completed by AD 140, and it continued to operate as a fortification and customs control until the final withdrawal of the Romans from Britain in AD 400.
  
Where the wall was stone built, as far west as Gilsland, the dressed faces were constructed almost exclusively of Carboniferous sandstones, quarried locally. The core was filled with any available rock material. The Romans found the Whin Sill too hard to work and where the wall crosses the dolerite outcrop on Limestone Bank [NY 875 715], the excavation of the vallum is incomplete.
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Where the wall was stone built, as far west as Gilsland, the dressed faces were constructed almost exclusively of Carboniferous sandstones, quarried locally. The core was filled with any available rock material. The Romans found the Whin Sill too hard to work and where the wall crosses the dolerite outcrop on Limestone Bank [NY 875 715], the excavation of the vallum is incomplete.
  
Over the centuries the wall has been gradually degraded, mainly because the dressed stone was an easy source of building material for local farms, houses and castles. The greatest degradation took place following the 1745 Jacobite rebellion when General Wade constructed an east–west road, still known in the north as the Military Road (B6318), to speed troop travel across the country. The first 48 km west from Newcastle were built almost wholly on top of the foundations of the Wall, and only the Wall's construction on the rugged scarp edge of the Whin Sill in the area west of Carrawburgh saved it from total destruction.
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Over the centuries the wall has been gradually degraded, mainly because the dressed stone was an easy source of building material for local farms, houses and castles. The greatest degradation took place following the 1745 Jacobite rebellion when General Wade constructed an east–west road, still known in the north as the Military Road (B6318), to speed troop travel across the country. The first 48 km west from Newcastle were built almost wholly on top of the foundations of the Wall, and only the Wall's construction on the rugged scarp edge of the Whin Sill in the area west of Carrawburgh saved it from total destruction.
  
 
== Excursion details ==
 
== Excursion details ==

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