Editing Cheviot — early Devonian volcanic rocks, granite and basement - an excursion

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== Geological background ==
 
== Geological background ==
  
The Cheviot volcano was erupted during the early Devonian period just to the north of the Iapetus '''Suture''', the boundary between the Laurentian and Avalonian continents that marks the site of the Iapetus Ocean. The first outpourings of this subaerial volcano were of '''agglomerate''' and '''breccia''' which rest '''unconformably''' on a basement of '''folded''' sedimentary rocks of Silurian age, but most of the volcano consists of '''andesite''' lavas. '''Rhyolite''' lavas and thin interlayers of '''tuff''' are uncommon. Towards the end of its life granite was intruded into the central vent of the volcano. This is a zoned intrusion and, unusually for a granite, it contains the mineral '''pyroxene'''. There are many '''dykes, sills''' and '''laccoliths,''' which intruded the lava pile either while it was being built or soon after. The country rock adjacent to the granite was thermally metamorphosed during its emplacement, but also associated with it was a phase of intense hydrothermal alteration, locally involving the formation of the mineral '''tourmaline'''.
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The Cheviot volcano was erupted during the early Devonian period just to the north of the Iapetus Suture, the boundary between the Laurentian and Avalonian continents that marks the site of the Iapetus Ocean. The first outpourings of this subaerial volcano were of agglomerate and breccia which rest unconformably on a basement of folded sedimentary rocks of Silurian age, but most of the volcano consists of andesite lavas. Rhyolite lavas and thin interlayers of tuff are uncommon. Towards the end of its life granite was intruded into the central vent of the volcano. This is a zoned intrusion and, unusually for a granite, it contain the mineral pyroxene. There are many dykes, sills and laccoliths, which intruded the lava pile either while it was being built or soon after. The country rock adjacent to the granite was thermally metamorphosed during its emplacement, but also associated with it was a phase of intense hydrothermal alteration, locally involving the formation of the mineral tourmaline.
  
On the Scottish side, the volcanic rocks are overlain by Upper Devonian sandstones, but in Northumberland they are overlain by the Lower Carboniferous Cementstone Group, a cyclical sequence of sandy mudstone, shale, '''cementstone''', '''dolomitic''' limestone and sandstone. '''Basaltic''' lavas occur at the base of the Carboniferous succession and within the Cementstone Group. In places, there is a basal Carboniferous '''conglomerate''' containing cobbles and boulders of andesite and granite. The latter indicates that much of the volcano, perhaps three quarters of it, had been eroded to unroof the granite intrusion before the conglomerate was laid down.
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On the Scottish side, the volcanic rocks are overlain by Upper Devonian sandstones, but in Northumberland they are overlain by the Lower Carboniferous Cementstone Group, a cyclical sequence of sandy mudstone, shale, cementstone, dolomitic limestone and sandstone. Basaltic lavas occur at the base of the Carboniferous succession and within the Cementstone Group. In places, there is a basal Carboniferous conglomerate containing cobbles and boulders of andesite and granite. The latter indicates that much of the volcano, perhaps three quarters of it, had been eroded to unroof the granite intrusion before the conglomerate was laid down.
  
The Silurian sedimentary rocks forming the basement are tightly folded, but the lavas, though tilted gently to the east, are not folded. Many '''faults''' and shatter zones cross the volcanic terrain, including a major one along Harthope Burn, traced for 20 km.
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The Silurian sedimentary rocks forming the basement are tightly folded, but the lavas, though tilted gently to the east, are not folded. Many faults and shatter zones cross the volcanic terrain, including a major one along Harthope Burn, traced for 20 km.
  
The next major event in the area was the Quaternary glaciation. The hills were covered in ice during the late Devensian and remnant spreads of ''till'' are common. There are vast expanses of peat bog and head, immediately post glacial in origin. Alluvium and alluvial terraces occur in the valleys.
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The next major event in the area was the Quaternary glaciation. The hills were covered in ice during the late Devensian and remnant spreads of till are common. There are vast expanses of peat bog and head, immediately post glacial in origin. Alluvium and alluvial terraces occur in the valleys.
  
 
Rock exposure in the Cheviots is sparse, presenting severe problems for reconstructing the geological history. Thus it is important to accept that there may be other interpretations to those given here.
 
Rock exposure in the Cheviots is sparse, presenting severe problems for reconstructing the geological history. Thus it is important to accept that there may be other interpretations to those given here.

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