Editing Geology of Eyemouth and Burnmouth - an excursion

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== Geological background ==
 
== Geological background ==
  
By the early Silurian, the microcontinent of Eastern Avalonia, including the area of England, Wales and southeast Ireland, had moved northwards to lie in close proximity to the margin of Laurentia ([[:File:YGS_NORTROCK_FIG_00_3.jpg|Figure 3]]a). The intervening Iapetus Ocean had been reduced to the width of a narrow seaway. On the Laurentian side; thick deposits of poorly sorted sand-grade sediments, derived from rising land to the north, began to cover the thin sequences of black, '''graptolitic''' shales that had accumulated on deep shelf and, in some places, oceanic crust. These coarser sediments were introduced by '''turbidity flows''', each resulting in a few cms to 1 m+ thick greywacke, usually separated by thin bands of background mud-grade sediment, sometimes graptolitic. In southern Scotland, these beds extend from the Llandovery up into the Wenlock when sedimentation ceased as the seaway narrowed, filled up and was eventually uplifted as a result of the final oblique compression between Avalonia, Baltica and Laurentia at the end of the '''Caledonian Orogenic Cycle''' ([[:File:YGS_NORTROCK_FIG_00_3.jpg|Figure 3]]a). In the Southern Uplands, the Lower Palaeozoic rocks are arranged in a series of northeast–southwest trending '''fault'''-defined slices, variously interpreted as reflecting a syn-depositional '''accretionary prism''', or the post-depositional '''thrust''' slicing of the thick sediment wedge on the Laurentian margin by the late Silurian compressional event. In the Eyemouth area, individual greywacke beds are seldom more than 0.5 m thick and are strongly folded; the sequence is unfossiliferous but is compared to the Hawick Group elsewhere and is thus most likely to be of Llandovery age.
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By the early Silurian, the microcontinent of Eastern Avalonia, including the area of England, Wales and southeast Ireland, had moved northwards to lie in close proximity to the margin of Laurentia ([[:File:YGS_NORTROCK_FIG_00_3.jpg|Figure 3]]a). The intervening Iapetus Ocean had been reduced to the width of a narrow seaway. On the Laurentian side; thick deposits of poorly sorted sand grade sediments, derived from rising land to the north, began to cover the thin sequences of black, graptolitic shales that had accumulated on deep shelf and, in some places, oceanic crust. These coarser sediments were introduced by turbidity flows, each resulting in a few cms to 1 m+ thick greywacke, usually separated by thin bands of background mud-grade sediment, sometimes graptolitic. In southern Scotland, these beds extend from the Llandovery up into the Wenlock when sedimentation ceased as the seaway narrowed, filled up and was eventually uplifted as a result of the final oblique compression between Avalonia, Baltica and Laurentia at the end of the Caledonian Orogenic Cycle ([[:File:YGS_NORTROCK_FIG_00_3.jpg|Figure 3]]a). In the Southern Uplands, the Lower Palaeozoic rocks are arranged in a series of northeast–southwest trending fault-defined slices, variously interpreted as reflecting a syn-depositional accretionary prism, or the post-depositional thrust slicing of the thick sediment wedge on the Laurentian margin by the late Silurian compressional event. In the Eyemouth area, individual greywacke beds are seldom more than 0.5 m thick and are strongly folded; the sequence is unfossiliferous but is compared to the Hawick Group elsewhere and is thus most likely to be of Llandovery age.
  
 
'''Magmas''' generated at depth by the Caledonian continental collision resulted in extensive late Silurian/early Devonian intrusions and volcanic activity, represented by '''dykes''' and extensive lavas, '''agglomerates''' and '''tuffs''' in the Eyemouth area and, to the southwest, by the Cheviot Volcano (Excursion 4). Locally, contemporary sediments were largely derived from the newly formed volcanic centres. More earth movements in the mid Devonian produced gentle folds in the early Devonian rocks. In the late Devonian, this hilly landscape of Lower Palaeozoic greywackes and younger volcanic rocks was deeply weathered in a semi-arid climate and the debris stripped off by flash floods to form fans of breccias and channel sands on the lower ground. By the early Carboniferous, the relief was somewhat reduced and the Cementstone Group was deposited on a broad floodplain crossed by meandering to low sinuosity channels with the formation of early diagenetic '''cementstones''' during periods of non-deposition. Subsequently, rivers flowing from the east and northeast brought thick sequences of fluviodeltaic '''cross-bedded''' medium- to coarse-grained sandstones into the area, building out to the southwest. The Fell Sandstone Group succeeds the Cementstones on the coast south of Burnmouth.
 
'''Magmas''' generated at depth by the Caledonian continental collision resulted in extensive late Silurian/early Devonian intrusions and volcanic activity, represented by '''dykes''' and extensive lavas, '''agglomerates''' and '''tuffs''' in the Eyemouth area and, to the southwest, by the Cheviot Volcano (Excursion 4). Locally, contemporary sediments were largely derived from the newly formed volcanic centres. More earth movements in the mid Devonian produced gentle folds in the early Devonian rocks. In the late Devonian, this hilly landscape of Lower Palaeozoic greywackes and younger volcanic rocks was deeply weathered in a semi-arid climate and the debris stripped off by flash floods to form fans of breccias and channel sands on the lower ground. By the early Carboniferous, the relief was somewhat reduced and the Cementstone Group was deposited on a broad floodplain crossed by meandering to low sinuosity channels with the formation of early diagenetic '''cementstones''' during periods of non-deposition. Subsequently, rivers flowing from the east and northeast brought thick sequences of fluviodeltaic '''cross-bedded''' medium- to coarse-grained sandstones into the area, building out to the southwest. The Fell Sandstone Group succeeds the Cementstones on the coast south of Burnmouth.

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