South west Scotland - introduction - Revision history

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	The thrust deformation was a diachronous process, becoming progressively younger southwards. The NW part of the Lower Palaeozoic outcrop was affected in the late Ordovician and the SE part in the mid-Silurian. The first phase of deformation at the thrust front was accompanied by refolding within the thrust hinterland to accommodate the progressive steepening of the thrust sheets. However, a different factor came into play in the mid-Silurian with the increased importance of sinistral shear. Prior to that time deformation seems to have been more or less orthogonal and early-formed fold hinges generally have only a gentle plunge to NE or SW and an axial planar cleavage. However, from the mid-Silurian onwards, steeply plunging folds with an S-shaped down-plunge profile were superimposed on the thrust hinterland to refold earlier structures and cleavage. Simultaneously the deformation at the thrust front, by then coincident with the SE part of the Southern Uplands, changed in character so that the first folds formed there have variable, locally steeply plunging hinges; cleavage formed at this stage may transect the folds markedly. This diachronous tectonic history, spanning much of the Silurian or more, is the local manifestation of the Caledonian Orogeny, the mountain-building episode which accompanied the destruction of the Iapetus Ocean.		The thrust deformation was a diachronous process, becoming progressively younger southwards. The NW part of the Lower Palaeozoic outcrop was affected in the late Ordovician and the SE part in the mid-Silurian. The first phase of deformation at the thrust front was accompanied by refolding within the thrust hinterland to accommodate the progressive steepening of the thrust sheets. However, a different factor came into play in the mid-Silurian with the increased importance of sinistral shear. Prior to that time deformation seems to have been more or less orthogonal and early-formed fold hinges generally have only a gentle plunge to NE or SW and an axial planar cleavage. However, from the mid-Silurian onwards, steeply plunging folds with an S-shaped down-plunge profile were superimposed on the thrust hinterland to refold earlier structures and cleavage. Simultaneously the deformation at the thrust front, by then coincident with the SE part of the Southern Uplands, changed in character so that the first folds formed there have variable, locally steeply plunging hinges; cleavage formed at this stage may transect the folds markedly. This diachronous tectonic history, spanning much of the Silurian or more, is the local manifestation of the Caledonian Orogeny, the mountain-building episode which accompanied the destruction of the Iapetus Ocean.
	[[File:P521148.jpg\|500px\|thumbnail\|left\|A fossil specimen of Monograptus convolutus Hisinger, 1837. A fossil graptolite. (Graptolithina.) 236 metres due north of bridge over Barntimpen Burn, Dumfriesshire, Scotland.This specimen of Monograptus convolutus is from the Llandovery, convolutus Biozone of the Silurian and was found 236 metres due north of the bridge over Barntimpen Burn. British Geological Survey Biostratigraphy Collection number GSE 15160. Monograptus is characterized by a rhabdosome with a single stipe which could be straight or curved. It is uniserial (stipes consisting of a single series of thecae) and scandent (stipes grow upwards from the sicula with the thecae growing outwards). Monograptus is entirely a Silurian form. Graptolites have been used to identify beds of the same age in different parts of the world. This has been made easier by the fact that the animals floated abundantly in the open sea and hence spread far and wide.]]		[[File:P521148.jpg\|500px\|thumbnail\|left\|A fossil specimen of Monograptus convolutus Hisinger, 1837. A fossil graptolite. (Graptolithina.) 236 metres due north of bridge over Barntimpen Burn, Dumfriesshire, Scotland.This specimen of ''Monograptus convolutus'' is from the Llandovery, convolutus Biozone of the Silurian and was found 236 metres due north of the bridge over Barntimpen Burn. British Geological Survey Biostratigraphy Collection number GSE 15160. Monograptus is characterized by a rhabdosome with a single stipe which could be straight or curved. It is uniserial (stipes consisting of a single series of thecae) and scandent (stipes grow upwards from the sicula with the thecae growing outwards). Monograptus is entirely a Silurian form. Graptolites have been used to identify beds of the same age in different parts of the world. This has been made easier by the fact that the animals floated abundantly in the open sea and hence spread far and wide.]]
	A numbering system has been widely applied to the Southern Uplands fold sequence. The early, thrust-related deformation is deemed Dl, accommodation structures in the thrust hinterland are grouped together as D2, and the late sinistral shear is identified as D3. When specifically discussing fold hinges or cleavage planes D may be replaced by F or S respectively. Thus the D1 deformation produced F1 fold hinges and an S1 slaty cleavage. This shorthand will be used in many of the excursion accounts.		A numbering system has been widely applied to the Southern Uplands fold sequence. The early, thrust-related deformation is deemed Dl, accommodation structures in the thrust hinterland are grouped together as D2, and the late sinistral shear is identified as D3. When specifically discussing fold hinges or cleavage planes D may be replaced by F or S respectively. Thus the D1 deformation produced F1 fold hinges and an S1 slaty cleavage. This shorthand will be used in many of the excursion accounts.

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	The thrust deformation was a diachronous process, becoming progressively younger southwards. The NW part of the Lower Palaeozoic outcrop was affected in the late Ordovician and the SE part in the mid-Silurian. The first phase of deformation at the thrust front was accompanied by refolding within the thrust hinterland to accommodate the progressive steepening of the thrust sheets. However, a different factor came into play in the mid-Silurian with the increased importance of sinistral shear. Prior to that time deformation seems to have been more or less orthogonal and early-formed fold hinges generally have only a gentle plunge to NE or SW and an axial planar cleavage. However, from the mid-Silurian onwards, steeply plunging folds with an S-shaped down-plunge profile were superimposed on the thrust hinterland to refold earlier structures and cleavage. Simultaneously the deformation at the thrust front, by then coincident with the SE part of the Southern Uplands, changed in character so that the first folds formed there have variable, locally steeply plunging hinges; cleavage formed at this stage may transect the folds markedly. This diachronous tectonic history, spanning much of the Silurian or more, is the local manifestation of the Caledonian Orogeny, the mountain-building episode which accompanied the destruction of the Iapetus Ocean.		The thrust deformation was a diachronous process, becoming progressively younger southwards. The NW part of the Lower Palaeozoic outcrop was affected in the late Ordovician and the SE part in the mid-Silurian. The first phase of deformation at the thrust front was accompanied by refolding within the thrust hinterland to accommodate the progressive steepening of the thrust sheets. However, a different factor came into play in the mid-Silurian with the increased importance of sinistral shear. Prior to that time deformation seems to have been more or less orthogonal and early-formed fold hinges generally have only a gentle plunge to NE or SW and an axial planar cleavage. However, from the mid-Silurian onwards, steeply plunging folds with an S-shaped down-plunge profile were superimposed on the thrust hinterland to refold earlier structures and cleavage. Simultaneously the deformation at the thrust front, by then coincident with the SE part of the Southern Uplands, changed in character so that the first folds formed there have variable, locally steeply plunging hinges; cleavage formed at this stage may transect the folds markedly. This diachronous tectonic history, spanning much of the Silurian or more, is the local manifestation of the Caledonian Orogeny, the mountain-building episode which accompanied the destruction of the Iapetus Ocean.
			[[File:P521148.jpg\|500px\|thumbnail\|left\|A fossil specimen of Monograptus convolutus Hisinger, 1837. A fossil graptolite. (Graptolithina.) 236 metres due north of bridge over Barntimpen Burn, Dumfriesshire, Scotland.This specimen of Monograptus convolutus is from the Llandovery, convolutus Biozone of the Silurian and was found 236 metres due north of the bridge over Barntimpen Burn. British Geological Survey Biostratigraphy Collection number GSE 15160. Monograptus is characterized by a rhabdosome with a single stipe which could be straight or curved. It is uniserial (stipes consisting of a single series of thecae) and scandent (stipes grow upwards from the sicula with the thecae growing outwards). Monograptus is entirely a Silurian form. Graptolites have been used to identify beds of the same age in different parts of the world. This has been made easier by the fact that the animals floated abundantly in the open sea and hence spread far and wide.]]
	A numbering system has been widely applied to the Southern Uplands fold sequence. The early, thrust-related deformation is deemed Dl, accommodation structures in the thrust hinterland are grouped together as D2, and the late sinistral shear is identified as D3. When specifically discussing fold hinges or cleavage planes D may be replaced by F or S respectively. Thus the D1 deformation produced F1 fold hinges and an S1 slaty cleavage. This shorthand will be used in many of the excursion accounts.		A numbering system has been widely applied to the Southern Uplands fold sequence. The early, thrust-related deformation is deemed Dl, accommodation structures in the thrust hinterland are grouped together as D2, and the late sinistral shear is identified as D3. When specifically discussing fold hinges or cleavage planes D may be replaced by F or S respectively. Thus the D1 deformation produced F1 fold hinges and an S1 slaty cleavage. This shorthand will be used in many of the excursion accounts.

	During the final stages of deformation a widespread dyke swarm was intruded into the region. Felsites, porphyritic microdiorites and lamprophyres are all present and some have been foliated by the later tectonic episodes. Other dykes are entirely post-tectonic and have been radiometrically dated at about 395-418 Ma (Rock et al., 1986). The climax of igneous activity was reached as deformation ended with the intrusion of the major granitic plutons. These have all been dated radiometrically to within a few million years of 400 Ma (Halliday et al., 1980). Evidence pertaining to the timing of deformation and intrusion is summarised in Figure 4.		During the final stages of deformation a widespread dyke swarm was intruded into the region. Felsites, porphyritic microdiorites and lamprophyres are all present and some have been foliated by the later tectonic episodes. Other dykes are entirely post-tectonic and have been radiometrically dated at about 395-418 Ma (Rock et al., 1986). The climax of igneous activity was reached as deformation ended with the intrusion of the major granitic plutons. These have all been dated radiometrically to within a few million years of 400 Ma (Halliday et al., 1980). Evidence pertaining to the timing of deformation and intrusion is summarised in Figure 4.
	[[File:P521148.jpg\|400px\|thumbnail\|left\|A fossil specimen of Monograptus convolutus Hisinger, 1837. A fossil graptolite. (Graptolithina.) 236 metres due north of bridge over Barntimpen Burn, Dumfriesshire, Scotland.This specimen of Monograptus convolutus is from the Llandovery, convolutus Biozone of the Silurian and was found 236 metres due north of the bridge over Barntimpen Burn. British Geological Survey Biostratigraphy Collection number GSE 15160. Monograptus is characterized by a rhabdosome with a single stipe which could be straight or curved. It is uniserial (stipes consisting of a single series of thecae) and scandent (stipes grow upwards from the sicula with the thecae growing outwards). Monograptus is entirely a Silurian form. Graptolites have been used to identify beds of the same age in different parts of the world. This has been made easier by the fact that the animals floated abundantly in the open sea and hence spread far and wide.]]

	=== Graptolite biostratigraphy of south-west Scotland: R A Hughes ===		=== Graptolite biostratigraphy of south-west Scotland: R A Hughes ===

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	During the final stages of deformation a widespread dyke swarm was intruded into the region. Felsites, porphyritic microdiorites and lamprophyres are all present and some have been foliated by the later tectonic episodes. Other dykes are entirely post-tectonic and have been radiometrically dated at about 395-418 Ma (Rock et al., 1986). The climax of igneous activity was reached as deformation ended with the intrusion of the major granitic plutons. These have all been dated radiometrically to within a few million years of 400 Ma (Halliday et al., 1980). Evidence pertaining to the timing of deformation and intrusion is summarised in Figure 4.		During the final stages of deformation a widespread dyke swarm was intruded into the region. Felsites, porphyritic microdiorites and lamprophyres are all present and some have been foliated by the later tectonic episodes. Other dykes are entirely post-tectonic and have been radiometrically dated at about 395-418 Ma (Rock et al., 1986). The climax of igneous activity was reached as deformation ended with the intrusion of the major granitic plutons. These have all been dated radiometrically to within a few million years of 400 Ma (Halliday et al., 1980). Evidence pertaining to the timing of deformation and intrusion is summarised in Figure 4.
			[[File:P521148.jpg\|400px\|thumbnail\|left\|A fossil specimen of Monograptus convolutus Hisinger, 1837. A fossil graptolite. (Graptolithina.) 236 metres due north of bridge over Barntimpen Burn, Dumfriesshire, Scotland.This specimen of Monograptus convolutus is from the Llandovery, convolutus Biozone of the Silurian and was found 236 metres due north of the bridge over Barntimpen Burn. British Geological Survey Biostratigraphy Collection number GSE 15160. Monograptus is characterized by a rhabdosome with a single stipe which could be straight or curved. It is uniserial (stipes consisting of a single series of thecae) and scandent (stipes grow upwards from the sicula with the thecae growing outwards). Monograptus is entirely a Silurian form. Graptolites have been used to identify beds of the same age in different parts of the world. This has been made easier by the fact that the animals floated abundantly in the open sea and hence spread far and wide.]]

	=== Graptolite biostratigraphy of south-west Scotland: R A Hughes ===		=== Graptolite biostratigraphy of south-west Scotland: R A Hughes ===

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	== South-west Scotland introduction to geology ==		== South-west Scotland introduction to geology ==
			[[File:P005979.jpg\|500px\|thumbnail\|left\|Whitehouse Shore, 3 km. S. of Girvan (Ardmillan Lodge in background). Turbidite sequence. General view (looking east-north-east) of thin-bedded turbidite sequence, Lower Whitehouse Group. Way-up is to the left-hand side. P005979. ]]

	The rolling hills of south-west Scotland are underlain principally by Lower Palaeozoic elastic sedimentary rocks and Caledonian granitic plutons. Drainage in the region is controlled in the main by Permo-Carboniferous faults trending SSE, which define the margins of half-graben sedimentary basins. The Upper Palaeozoic basin infills have been preferentially eroded so that they now coincide with the main river valleys. Rivers such as the Cree, Ken and Nith flow generally southwards to the Solway Firth, the site of another major Upper Palaeozoic basin. The Lower Palaeozoic rocks are exposed on the west coast of the Rhins of Galloway and along much of the Solway coast westward from about Dalbeattie. The lower-lying part of the region, mainly in the south along the Solway coast, carries an extensive cover of glacial deposits, with drumlins particularly well developed between Wigtown and Glenluce.		The rolling hills of south-west Scotland are underlain principally by Lower Palaeozoic elastic sedimentary rocks and Caledonian granitic plutons. Drainage in the region is controlled in the main by Permo-Carboniferous faults trending SSE, which define the margins of half-graben sedimentary basins. The Upper Palaeozoic basin infills have been preferentially eroded so that they now coincide with the main river valleys. Rivers such as the Cree, Ken and Nith flow generally southwards to the Solway Firth, the site of another major Upper Palaeozoic basin. The Lower Palaeozoic rocks are exposed on the west coast of the Rhins of Galloway and along much of the Solway coast westward from about Dalbeattie. The lower-lying part of the region, mainly in the south along the Solway coast, carries an extensive cover of glacial deposits, with drumlins particularly well developed between Wigtown and Glenluce.

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 In the latter half of the last century Charles Lapworth, working in the Moffatdale area of the Southern Uplands (now the type area for the Moffat Shale Group) first recognised that graptolites occur in distinct assemblages, and established the sequence of assemblages and zones which record the evolution of the graptolites through geological time (Figure 5). Lapworth was the first person <sub>t</sub>o demonstrate the use of graptolites as a biostratigraphical tool, and his work has survived the test of time to the extent that his sequence of zones (Lapworth, 1878) remains almost entirely unchanged, and is today used throughout the world. His achievement was extraordinary, and without the tool of graptolite biostratigraphy the recognition of the gross structure of the Southern Uplands as an imbricate thrust stack would have been impossible. It is no exaggeration to state that graptolite biostratigraphy has been and will remain entirely fundamental to solving the great geological complexity of the Southern Uplands. Excursion 18 gives a selection of localities where they can be found. Further information on this fascinating group of animals is given by Palmer and Rickards (1991).
 === Turbidite sedimentology: M C Akhurst ===
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 The pattern of regional metamorphism found in south-west Scotland cannot easily be modelled in terms of a stratigraphical burial pattern whereby grade increases into older strata with increasing thickness of overburden (Roberts et al., 1991). Had the succession initially acquired such a pattern of burial metamorphism and subsequently been imbricated and rotated, older strata would still show higher grades than younger strata, whatever the final structure. The regional pattern in the Ordovician and parts of the Llandovery outcrop is the reverse of that generated by stratigraphical burial, in that grade may increase into younger tracts of strata. Such a pattern indicates that, from time to time, younger strata were buried beneath older strata, and in turn this suggests that the metamorphic pattern was generated by thrust-related tectonism. In the Ordovician outcrop the metamorphic pattern suggests that tracts comprising NW-younging Kirkcolm, Portpatrick and Shinnel formations were sequentially underthrust and buried beneath the older Corsewall Formation, which formed the upper unit of a thrust stack. It appears that a depth-related pattern of metamorphism was acquired after the strata were steepened, but imbrication continued to modify the pattern after burial. On the Rhins of Galloway, the widespread occurrence of diagenetic grade mudstones and shales in the SE-younging strata of the southernmost Gala Group outcrop is consistent with overthrusting to the upper part of the thrust stack (McCurry and Anderson, 1989). The outcrop of diagenetic zone strata has an abrupt southern termination at a significant metamorphic discontinuity across which grade and probable depth of burial increase sharply into the Hawick Group.
 [[File:SWScotlandUpperPalaeozoicGeology.jpg|thumbnail|Principal features of Upper Palaeozoic geology in south-west Scotland.]]
 === Upper Palaeozoic to Quaternary regional geology: A A McMillan and A D McAdam ===
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 [[File:SWScotlandCarboniferousFossils.jpg|thumbnail|A selection of characteristic Carboniferous fossils. a. ''Latiproductus latissimus ''(Sowerby) x 1. b, c. ''Syringothyris cuspidata ''(Sowerby) x 1/2 , dorsal valve and posterior view. d, e. ''Lithostrotion ''sp. x 1/2 with cross-section x l. f. ''Siphonodendron junceum ''(Fleming) X 1 g. ''Schellwienella ''x l , dorsal view. h. ''Prothyris ''sp. x 3 ''1. Gigantoproductus giganteus ''(Sowerby) x 1/2 Illustrations b, d, e, g, hand i are reproduced by permission of the Natural History Museum, London.]]
 === Carboniferous palaeontology: P J Brand ===

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	=== Graptolite biostratigraphy of south-west Scotland: R A Hughes ===		=== Graptolite biostratigraphy of south-west Scotland: R A Hughes ===
	[[File:SWScotlandOrdovicianSilurianBiozones.jpg\|400px\|thumbnail\|The sequence of Ordovician and Silurian graprolite biozones present in south-west Scotland, with line-drawings of selected graptolites, arranged in stratigraphical order. The graprolites are approximately x 1, and the species are: a. Nemagmptus gracilis (Hall) b. Climacograptus bicornis (Hall) c. Dicranograptus clingani Carruthers d. Orthograptus abbreviatus Elles & Wood e. Dicellograptus anceps (Nicholson) f. Parakiclograptus acuminatus (Nicholson) g. Atavograptus atavus (Jones) h. Monograptus triangulatus (Harkness) 1. Coronograptus gregarius (Lapworth) J. Monograptus convolutus (Hisinger) k. Monograptus sedgwickii (Portlock) l. Monograptus turriculatus (Barrande) m. Monograptus crispus Lapworth n. Monoclimacis griestoniensis (Nicol) o. Monoclimacis crenulata (Elles & Wood) p. Cyrtograptus rigidus Tullberg q. Monograptus flexilis Elles r. Cyrtograptus lundgreni Tullberg]]		[[File:SWScotlandOrdovicianSilurianBiozones.jpg\|400px\|thumbnail\|The sequence of Ordovician and Silurian graprolite biozones present in south-west Scotland, with line-drawings of selected graptolites, arranged in stratigraphical order. The graprolites are approximately x 1, and the species are: a. ''Nemagmptus gracilis ''(Hall) b. ''Climacograptus bicornis ''(Hall) c. ''Dicranograptus clingani ''Carruthers d. ''Orthograptus abbreviatus ''Elles & Wood e. ''Dicellograptus anceps ''(Nicholson) f. ''Parakiclograptus acuminatus ''(Nicholson) g. ''Atavograptus atavus ''(Jones) h. ''Monograptus triangulatus ''(Harkness) 1. ''Coronograptus gregarius ''(Lapworth) J. ''Monograptus convolutus ''(Hisinger) k. ''Monograptus sedgwickii ''(Portlock) l. ''Monograptus turriculatus ''(Barrande) m. ''Monograptus crispus ''Lapworth n. ''Monoclimacis griestoniensis ''(Nicol) o. ''Monoclimacis crenulata ''(Elles & Wood) p. ''Cyrtograptus rigidus ''Tullberg q. ''Monograptus flexilis ''Elles r. ''Cyrtograptus lundgreni ''Tullberg ]]
	Graptolites are a long extinct group of tiny colonial animals which formed the greater part of the Ordovician and Silurian oceanic plankton. They are the most useful fossil group in Southern Uplands biostratigraphy. Each graptolite colony was composed of many (over 1000 in some cases) polyp-like animals (zooids) which lived in linked cuplike structures (thecae). These collectively comprised a single skeletal structure, called a rhabdosome, made of a protein-like material. Graptolite rhabdosomes are normally a few centimetres in length, but range from almost microscopic to a metre or so in extreme cases; all are slender and delicate. During Ordovician and Silurian times the graptolites evolved an extraordinary variety of shapes (Figure 5), which may have been adaptations to differing feeding strategies or to life in various parts of the water column. When the graptolites are well preserved, this variety enables today's palaeontologists to distinguish different groups and species, essential to the practice of biostratigraphy.		Graptolites are a long extinct group of tiny colonial animals which formed the greater part of the Ordovician and Silurian oceanic plankton. They are the most useful fossil group in Southern Uplands biostratigraphy. Each graptolite colony was composed of many (over 1000 in some cases) polyp-like animals (zooids) which lived in linked cuplike structures (thecae). These collectively comprised a single skeletal structure, called a rhabdosome, made of a protein-like material. Graptolite rhabdosomes are normally a few centimetres in length, but range from almost microscopic to a metre or so in extreme cases; all are slender and delicate. During Ordovician and Silurian times the graptolites evolved an extraordinary variety of shapes (Figure 5), which may have been adaptations to differing feeding strategies or to life in various parts of the water column. When the graptolites are well preserved, this variety enables today's palaeontologists to distinguish different groups and species, essential to the practice of biostratigraphy.

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	Some 15 000 years ago the ice sheets began to melt. The higher hills became free of ice while glaciers still filled valleys. Meltwater from the ice cut channels, many now left as dry valleys, and deposited sand and gravel as eskers and kames and as raised beaches along the coast. Rivers have continued this process, depositing alluvial clay, silt, sand and gravel in their floodplains. However, the drainage pattern has been fundamentally influenced by the underlying geology. The main river valleys follow the trend of the Upper Palaeozoic basins, the softer rocks of which were preferentially eroded by the ice.		Some 15 000 years ago the ice sheets began to melt. The higher hills became free of ice while glaciers still filled valleys. Meltwater from the ice cut channels, many now left as dry valleys, and deposited sand and gravel as eskers and kames and as raised beaches along the coast. Rivers have continued this process, depositing alluvial clay, silt, sand and gravel in their floodplains. However, the drainage pattern has been fundamentally influenced by the underlying geology. The main river valleys follow the trend of the Upper Palaeozoic basins, the softer rocks of which were preferentially eroded by the ice.

			[[File:SWScotlandCarboniferousFossils.jpg\|thumbnail\|A selection of characteristic Carboniferous fossils. a. ''Latiproductus latissimus ''(Sowerby) x 1. b, c. ''Syringothyris cuspidata ''(Sowerby) x 1/2 , dorsal valve and posterior view. d, e. ''Lithostrotion ''sp. x 1/2 with cross-section x l. f. ''Siphonodendron junceum ''(Fleming) X 1 g. ''Schellwienella ''x l , dorsal view. h. ''Prothyris ''sp. x 3 ''1. Gigantoproductus giganteus ''(Sowerby) x 1/2 Illustrations b, d, e, g, hand i are reproduced by permission of the Natural History Museum, London.]]

	Figure 9 A selection of characteristic Carboniferous fossils.		Figure 9 A selection of characteristic Carboniferous fossils.

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	Large volumes of sediment can be carried in suspension within the turbid flow. As current velocity wanes the coarsest sand and pebbles are deposited, followed by progressively finer-grained sand to form a graded bed (Figure 6). Deposition of fine sand and silt follows and is associated with the development firstly of planar lamination and then of ripple cross-lamination, reflecting changes in the flow regime of the current. Bedding in the fine sandstone and siltstone may also be contorted or convoluted where the rapidly deposited sediment was very wet and unstable. The ripple cross-laminated sediment (another indication of way up) grades into finely laminated or homogeneous silt-stone and mudstone. These finest-grained sediments, which are darker in colour, accumulated much more slowly. The resulting mudstone or shale beds include both the finest sediment from the turbidity current and hemipelagic deep-sea sediment. The turbidites are predominantly unfossiliferous, although graptolites may be preserved in the finer-grained bed tops, perhaps only as broken pieces. However, if time is available between flows for a hemipelagic interval to develop, the resulting shale bed may carry a rich graptolite fauna.		Large volumes of sediment can be carried in suspension within the turbid flow. As current velocity wanes the coarsest sand and pebbles are deposited, followed by progressively finer-grained sand to form a graded bed (Figure 6). Deposition of fine sand and silt follows and is associated with the development firstly of planar lamination and then of ripple cross-lamination, reflecting changes in the flow regime of the current. Bedding in the fine sandstone and siltstone may also be contorted or convoluted where the rapidly deposited sediment was very wet and unstable. The ripple cross-laminated sediment (another indication of way up) grades into finely laminated or homogeneous silt-stone and mudstone. These finest-grained sediments, which are darker in colour, accumulated much more slowly. The resulting mudstone or shale beds include both the finest sediment from the turbidity current and hemipelagic deep-sea sediment. The turbidites are predominantly unfossiliferous, although graptolites may be preserved in the finer-grained bed tops, perhaps only as broken pieces. However, if time is available between flows for a hemipelagic interval to develop, the resulting shale bed may carry a rich graptolite fauna.

	~~Figure 6 Divisions within an idealised turbidite bed after Bouma (1962) and Pickering et al. (1989).~~

	Generation and flow of turbidity currents and subsequent deposition of turbidite beds, are sudden, catastrophic events. Turbidity currents can be initiated, for example, by collapse of unstable sediments on a submarine slope or by an earthquake shock. The resuspended sediment may be transferred for hundreds of kilometres on to the deep ocean floor. Currents are first funnelled down the continental slope via submarine canyons. Most sediment is then deposited as the current slows at the foot of the slope and spreads out from the confinement of the canyon. Accumulated deposits from many turbidity currents form a fan that has its apex at the canyon mouth (Figure 7). Submarine fans can be very large structures; modern examples are tens to hundreds of kilometres across.		Generation and flow of turbidity currents and subsequent deposition of turbidite beds, are sudden, catastrophic events. Turbidity currents can be initiated, for example, by collapse of unstable sediments on a submarine slope or by an earthquake shock. The resuspended sediment may be transferred for hundreds of kilometres on to the deep ocean floor. Currents are first funnelled down the continental slope via submarine canyons. Most sediment is then deposited as the current slows at the foot of the slope and spreads out from the confinement of the canyon. Accumulated deposits from many turbidity currents form a fan that has its apex at the canyon mouth (Figure 7). Submarine fans can be very large structures; modern examples are tens to hundreds of kilometres across.

	The sediments deposited from a single turbidity current are not of uniform character along the length of the flow. Close to the source the turbidite bed will be dominated by coarse-grained graded sandstone, the lowest of the Bouma divisions (Figure 6), but far away from the source only fine-grained silt-stone and mudstone of the higher divisions will occur. The complete Bouma sequence will only be deposited at intermediate positions. Studies of many ancient and modern submarine fan sequences have recognised inner, middle and outer fan divisions (Figure 7). The inner fan, nearest the sediment source, is crossed by a single channel within which turbidite deposition is mostly confined. Fine-grained sediment in suspension is deposited laterally to the channel. Inner fan deposits are, therefore, made up of coarse-grained sandstones and conglomerates forming lenses within a background of laminated mudstone. Down-current, in the middle fan setting, the channel divides and becomes much shallower. Turbidites dominated by the graded and planar-bedded divisions are deposited as more extensive, sheet-like beds. In the outer fan setting turbidity currents spread out over the fan surface to give laterally extensive beds with complete Bouma sequences. Finally, at locations farthest from the submarine canyon mouth, turbidite units may consist of only laminated siltstone and mudstone. Thus, by noting the style of bedding and sedimentary structures in a sequence of turbidites, it is possible to recognise where on a submarine fan they were deposited.		The sediments deposited from a single turbidity current are not of uniform character along the length of the flow. Close to the source the turbidite bed will be dominated by coarse-grained graded sandstone, the lowest of the Bouma divisions (Figure 6), but far away from the source only fine-grained silt-stone and mudstone of the higher divisions will occur. The complete Bouma sequence will only be deposited at intermediate positions. Studies of many ancient and modern submarine fan sequences have recognised inner, middle and outer fan divisions (Figure 7). The inner fan, nearest the sediment source, is crossed by a single channel within which turbidite deposition is mostly confined. Fine-grained sediment in suspension is deposited laterally to the channel. Inner fan deposits are, therefore, made up of coarse-grained sandstones and conglomerates forming lenses within a background of laminated mudstone. Down-current, in the middle fan setting, the channel divides and becomes much shallower. Turbidites dominated by the graded and planar-bedded divisions are deposited as more extensive, sheet-like beds. In the outer fan setting turbidity currents spread out over the fan surface to give laterally extensive beds with complete Bouma sequences. Finally, at locations farthest from the submarine canyon mouth, turbidite units may consist of only laminated siltstone and mudstone. Thus, by noting the style of bedding and sedimentary structures in a sequence of turbidites, it is possible to recognise where on a submarine fan they were deposited.

	~~Figure 7 Depositional environments on a submarine fan.~~

	An extensive review of this remarkable group of sedimentary rocks is given by Pickering et al. (1989). Detailed sedimentological analyses are provided by Nilsen (1978) and Walker and Mutti (1973).		An extensive review of this remarkable group of sedimentary rocks is given by Pickering et al. (1989). Detailed sedimentological analyses are provided by Nilsen (1978) and Walker and Mutti (1973).
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	The pattern of regional metamorphism found in south-west Scotland cannot easily be modelled in terms of a stratigraphical burial pattern whereby grade increases into older strata with increasing thickness of overburden (Roberts et al., 1991). Had the succession initially acquired such a pattern of burial metamorphism and subsequently been imbricated and rotated, older strata would still show higher grades than younger strata, whatever the final structure. The regional pattern in the Ordovician and parts of the Llandovery outcrop is the reverse of that generated by stratigraphical burial, in that grade may increase into younger tracts of strata. Such a pattern indicates that, from time to time, younger strata were buried beneath older strata, and in turn this suggests that the metamorphic pattern was generated by thrust-related tectonism. In the Ordovician outcrop the metamorphic pattern suggests that tracts comprising NW-younging Kirkcolm, Portpatrick and Shinnel formations were sequentially underthrust and buried beneath the older Corsewall Formation, which formed the upper unit of a thrust stack. It appears that a depth-related pattern of metamorphism was acquired after the strata were steepened, but imbrication continued to modify the pattern after burial. On the Rhins of Galloway, the widespread occurrence of diagenetic grade mudstones and shales in the SE-younging strata of the southernmost Gala Group outcrop is consistent with overthrusting to the upper part of the thrust stack (McCurry and Anderson, 1989). The outcrop of diagenetic zone strata has an abrupt southern termination at a significant metamorphic discontinuity across which grade and probable depth of burial increase sharply into the Hawick Group.		The pattern of regional metamorphism found in south-west Scotland cannot easily be modelled in terms of a stratigraphical burial pattern whereby grade increases into older strata with increasing thickness of overburden (Roberts et al., 1991). Had the succession initially acquired such a pattern of burial metamorphism and subsequently been imbricated and rotated, older strata would still show higher grades than younger strata, whatever the final structure. The regional pattern in the Ordovician and parts of the Llandovery outcrop is the reverse of that generated by stratigraphical burial, in that grade may increase into younger tracts of strata. Such a pattern indicates that, from time to time, younger strata were buried beneath older strata, and in turn this suggests that the metamorphic pattern was generated by thrust-related tectonism. In the Ordovician outcrop the metamorphic pattern suggests that tracts comprising NW-younging Kirkcolm, Portpatrick and Shinnel formations were sequentially underthrust and buried beneath the older Corsewall Formation, which formed the upper unit of a thrust stack. It appears that a depth-related pattern of metamorphism was acquired after the strata were steepened, but imbrication continued to modify the pattern after burial. On the Rhins of Galloway, the widespread occurrence of diagenetic grade mudstones and shales in the SE-younging strata of the southernmost Gala Group outcrop is consistent with overthrusting to the upper part of the thrust stack (McCurry and Anderson, 1989). The outcrop of diagenetic zone strata has an abrupt southern termination at a significant metamorphic discontinuity across which grade and probable depth of burial increase sharply into the Hawick Group.

	Figure 8 Principal features of Upper Palaeozoic geology in south-west Scotland.		Figure 8
			[[File:SWScotlandUpperPalaeozoicGeology.jpg\|thumbnail\|Principal features of Upper Palaeozoic geology in south-west Scotland.]]
	=== Upper Palaeozoic to Quaternary regional geology: A A McMillan and A D McAdam ===		=== Upper Palaeozoic to Quaternary regional geology: A A McMillan and A D McAdam ===
	Following the Caledonian Orogeny and intrusion of the Galloway granitic plutons, the Southern Upland region was subjected to uplift and erosion. Extension of the crust from late Devonian times led to the development of major sedimentary basins in the Midland Valley to the north and the Northumberland—Solway Trough to the south. Associated north- and NW-trending normal faults, which can be traced through the Southern Uplands, may have been active during early Carboniferous times, perhaps initiated by minor dextral movement on existing faults of Caledonoid (NE) trend (McMillan and Brand, 1995). Small half-graben basins defined by these faults as at Thornhill, Sanquhar and Loch Ryan (Figure 8), preserve a fragmentary record of fluviatile, estuarine and marginal marine sedimentation from Devonian to late Carboniferous times. A subsequent extensional event in early Permian times, generated a short-lived volcanic episode as arid climatic conditions developed and extensive desert dune sandstones and associated breccia fans covered much of the region.		Following the Caledonian Orogeny and intrusion of the Galloway granitic plutons, the Southern Upland region was subjected to uplift and erosion. Extension of the crust from late Devonian times led to the development of major sedimentary basins in the Midland Valley to the north and the Northumberland—Solway Trough to the south. Associated north- and NW-trending normal faults, which can be traced through the Southern Uplands, may have been active during early Carboniferous times, perhaps initiated by minor dextral movement on existing faults of Caledonoid (NE) trend (McMillan and Brand, 1995). Small half-graben basins defined by these faults as at Thornhill, Sanquhar and Loch Ryan (Figure 8), preserve a fragmentary record of fluviatile, estuarine and marginal marine sedimentation from Devonian to late Carboniferous times. A subsequent extensional event in early Permian times, generated a short-lived volcanic episode as arid climatic conditions developed and extensive desert dune sandstones and associated breccia fans covered much of the region.

	~~Table 2~~ Upper Carboniferous and Permian stratigraphy in south-west Scotland. This table does not show relative thickness of different groups, formations and members.		[[File:SWScotlandLowerCarboniferousStratigraphy.jpg\|thumbnail\|Upper Carboniferous and Permian stratigraphy in south-west Scotland. This table does not show relative thickness of different groups, formations and members.]]

	The north Solway coast, effectively the northern margin of the Solway Basin, provides the best exposed record of early Carboniferous sedimentary rocks, together with remnants of late Devonian red beds (Table 1). The latter comprise conglomerates and arenites deposited in fans and braided channels under hot, arid conditions. Calcareous palaeosols developed locally (Leeder, 1976). Following deposition of the Devonian strata, short-lived volcanic activity produced the lavas of the Birrenswark Volcanic Formation. The volcanism may be related to the extensional event which formed the Solway Basin (Leeder, 1982, 1988).		The north Solway coast, effectively the northern margin of the Solway Basin, provides the best exposed record of early Carboniferous sedimentary rocks, together with remnants of late Devonian red beds (Table 1). The latter comprise conglomerates and arenites deposited in fans and braided channels under hot, arid conditions. Calcareous palaeosols developed locally (Leeder, 1976). Following deposition of the Devonian strata, short-lived volcanic activity produced the lavas of the Birrenswark Volcanic Formation. The volcanism may be related to the extensional event which formed the Solway Basin (Leeder, 1982, 1988).

	The NE-trending North Solway Fault which forms the northern boundary to the Solway Basin is probably a reactivated Caledonoid structure. Coarse clastic rocks of Dinantian age in the Rerrick (Wall Hill to Rascarrel Bay) and Colvend (Castlehill Point to Portling Bay) outliers provide evidence for periodic syn-sedimentary dip-slip movement on the fault (Deegan, 1973; Ord et al., 1988). Syn-sedimentary deformation of hanging-wall strata increases towards the fault. Cyclicity within alluvial fan deposits of the Rerrick Outlier may be attributed to the interplay between tectonic subsidence and changing sea level. Further away from the margin, Lower to Upper Border Group strata (Courceyan to Asbian, Table 1) of the Kirkbean-Southerness area (Craig, 1956) and at Langholm (Lumsden et al., 1967) also show evidence of changing sea level. Cyclically interbedded mudstones, siltstones and sandstones with thin cementstones and thin coals with seatearths reflect changing depositional environments in low-lying coastal areas. Locally, as at Powillimount on the Solway coast, there is convincing evidence in the Thirlstane Sandstone Member of synsedimentary seismic activity (Ord et al., 1988).		The NE-trending North Solway Fault which forms the northern boundary to the Solway Basin is probably a reactivated Caledonoid structure. Coarse clastic rocks of Dinantian age in the Rerrick (Wall Hill to Rascarrel Bay) and Colvend (Castlehill Point to Portling Bay) outliers provide evidence for periodic syn-sedimentary dip-slip movement on the fault (Deegan, 1973; Ord et al., 1988). Syn-sedimentary deformation of hanging-wall strata increases towards the fault. Cyclicity within alluvial fan deposits of the Rerrick Outlier may be attributed to the interplay between tectonic subsidence and changing sea level. Further away from the margin, Lower to Upper Border Group strata (Courceyan to Asbian, Table 1) of the Kirkbean-Southerness area (Craig, 1956) and at Langholm (Lumsden et al., 1967) also show evidence of changing sea level. Cyclically interbedded mudstones, siltstones and sandstones with thin cementstones and thin coals with seatearths reflect changing depositional environments in low-lying coastal areas. Locally, as at Powillimount on the Solway coast, there is convincing evidence in the Thirlstane Sandstone Member of synsedimentary seismic activity (Ord et al., 1988).
			[[File:SWScotlandUpperCarboniferousStratigraphy.jpg\|thumbnail\|Upper Carboniferous and Permian statigraphy in south-west Scotland.]]

	Marine inundations are also recorded at higher stratigraphical levels in the Lower and Upper Liddesdale groups (Asbian to Brigantian) at Kelhead near Annan, and at Langholm and Thornhill. At Thornhill, the Closeburn Limestone Formation records a marginal and probably quite short-lived marine incursion. These strata represent the earliest record of a breach through the Southern Uplands linking the Solway Basin to Sanquhar and the Midland Valley. Later Carboniferous sedimentation in the Southern Uplands was probably also restricted, particularly during Namurian times, but an attenuated Coal Measures sequence at Thornhill (Table 2) can be correlated with those at Canonbie and Sanquhar. At Thornhill, late-Carboniferous to Permo-Triassic oxidation has resulted in reddening of strata and replacement of coal. A fragmentary Upper Carboniferous sequence is also present at Loch Ryan. There has long been speculation on the extent of late Carboniferous to Permian sedimentation over the Southern Uplands. Pringle and Richey (1931) opined that it was once much more extensive and considered that the present distribution of strata could be attributed to post-Carboniferous downfaulting.		Marine inundations are also recorded at higher stratigraphical levels in the Lower and Upper Liddesdale groups (Asbian to Brigantian) at Kelhead near Annan, and at Langholm and Thornhill. At Thornhill, the Closeburn Limestone Formation records a marginal and probably quite short-lived marine incursion. These strata represent the earliest record of a breach through the Southern Uplands linking the Solway Basin to Sanquhar and the Midland Valley. Later Carboniferous sedimentation in the Southern Uplands was probably also restricted, particularly during Namurian times, but an attenuated Coal Measures sequence at Thornhill (Table 2) can be correlated with those at Canonbie and Sanquhar. At Thornhill, late-Carboniferous to Permo-Triassic oxidation has resulted in reddening of strata and replacement of coal. A fragmentary Upper Carboniferous sequence is also present at Loch Ryan. There has long been speculation on the extent of late Carboniferous to Permian sedimentation over the Southern Uplands. Pringle and Richey (1931) opined that it was once much more extensive and considered that the present distribution of strata could be attributed to post-Carboniferous downfaulting.

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Turbidite sedimentology: M C Akhurst

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	=== Turbidite sedimentology: M C Akhurst ===		=== Turbidite sedimentology: M C Akhurst ===
			[[File:SWScotlandDepositionalEnvironments.jpg\|thumbnail\|Depositional environments on a submarine fan. ]]
	Exposures of Lower Palaeozoic rocks in south-west Scotland are predominantly of interbedded greywacke, siltstone, mudstone, shale and, more rarely, conglomerate. The term greywacke refers to hard, poorly sorted sandstones containing grains of quartz, feldspar, dark ferromagnesian minerals and rock fragments set in a clay matrix which constitutes more than about 15 per cent of the rock. The sandstone beds may range from a few centimetres to more than 2 m thick. Bedding relationships are consistent: the sandstone has a sharp base and grades up into siltstone and shale, reflecting their origin in a single depositional event. Sediment is transported downslope as a turbulent current. As the current slows, first coarse-grained then fine-grained sediment is deposited. The resulting graded bed is called a turbidite and shows a characteristic set of sedimentary structures forming a `Bouma sequence' (Figure 6) (Bouma, 1962). Turbidity currents can flow very fast; one generated by an earthquake off eastern Canada in 1929 flowed at up to 20 metres/second (about 45 mph). Turbidity currents can erode the sea bed prior to deposition of the coarse-grained sediment. The erosive patterns, preserved as positive casts or 'sole structures' on the base of a turbidite bed, will then give a clear indication of way-up in the sedimentary sequence. Flute casts are distinctive sole structures, deepest and narrowest at the up-current end, which can be used to deduce the flow direction of the turbidity current. Linear groove casts are not quite so useful in this respect; they lie parallel to the current trend but do not show which way it flowed. Loading of sand into the underlying turbidite may also produce sole structures; sand at the base of the bed sinks into the underlying mud to form distinctive bulbous structures called load casts. Tapered 'flames' of mobile mud may penetrate up into the sand between the load casts.		Exposures of Lower Palaeozoic rocks in south-west Scotland are predominantly of interbedded greywacke, siltstone, mudstone, shale and, more rarely, conglomerate. The term greywacke refers to hard, poorly sorted sandstones containing grains of quartz, feldspar, dark ferromagnesian minerals and rock fragments set in a clay matrix which constitutes more than about 15 per cent of the rock. The sandstone beds may range from a few centimetres to more than 2 m thick. Bedding relationships are consistent: the sandstone has a sharp base and grades up into siltstone and shale, reflecting their origin in a single depositional event. Sediment is transported downslope as a turbulent current. As the current slows, first coarse-grained then fine-grained sediment is deposited. The resulting graded bed is called a turbidite and shows a characteristic set of sedimentary structures forming a `Bouma sequence' (Figure 6) (Bouma, 1962). Turbidity currents can flow very fast; one generated by an earthquake off eastern Canada in 1929 flowed at up to 20 metres/second (about 45 mph). Turbidity currents can erode the sea bed prior to deposition of the coarse-grained sediment. The erosive patterns, preserved as positive casts or 'sole structures' on the base of a turbidite bed, will then give a clear indication of way-up in the sedimentary sequence. Flute casts are distinctive sole structures, deepest and narrowest at the up-current end, which can be used to deduce the flow direction of the turbidity current. Linear groove casts are not quite so useful in this respect; they lie parallel to the current trend but do not show which way it flowed. Loading of sand into the underlying turbidite may also produce sole structures; sand at the base of the bed sinks into the underlying mud to form distinctive bulbous structures called load casts. Tapered 'flames' of mobile mud may penetrate up into the sand between the load casts.

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	=== Graptolite biostratigraphy of south-west Scotland: R A Hughes ===		=== Graptolite biostratigraphy of south-west Scotland: R A Hughes ===
			[[File:SWScotlandOrdovicianSilurianBiozones.jpg\|400px\|thumbnail\|The sequence of Ordovician and Silurian graprolite biozones present in south-west Scotland, with line-drawings of selected graptolites, arranged in stratigraphical order. The graprolites are approximately x 1, and the species are: a. Nemagmptus gracilis (Hall) b. Climacograptus bicornis (Hall) c. Dicranograptus clingani Carruthers d. Orthograptus abbreviatus Elles & Wood e. Dicellograptus anceps (Nicholson) f. Parakiclograptus acuminatus (Nicholson) g. Atavograptus atavus (Jones) h. Monograptus triangulatus (Harkness) 1. Coronograptus gregarius (Lapworth) J. Monograptus convolutus (Hisinger) k. Monograptus sedgwickii (Portlock) l. Monograptus turriculatus (Barrande) m. Monograptus crispus Lapworth n. Monoclimacis griestoniensis (Nicol) o. Monoclimacis crenulata (Elles & Wood) p. Cyrtograptus rigidus Tullberg q. Monograptus flexilis Elles r. Cyrtograptus lundgreni Tullberg]]
	Graptolites are a long extinct group of tiny colonial animals which formed the greater part of the Ordovician and Silurian oceanic plankton. They are the most useful fossil group in Southern Uplands biostratigraphy. Each graptolite colony was composed of many (over 1000 in some cases) polyp-like animals (zooids) which lived in linked cuplike structures (thecae). These collectively comprised a single skeletal structure, called a rhabdosome, made of a protein-like material. Graptolite rhabdosomes are normally a few centimetres in length, but range from almost microscopic to a metre or so in extreme cases; all are slender and delicate. During Ordovician and Silurian times the graptolites evolved an extraordinary variety of shapes (Figure 5), which may have been adaptations to differing feeding strategies or to life in various parts of the water column. When the graptolites are well preserved, this variety enables today's palaeontologists to distinguish different groups and species, essential to the practice of biostratigraphy.		Graptolites are a long extinct group of tiny colonial animals which formed the greater part of the Ordovician and Silurian oceanic plankton. They are the most useful fossil group in Southern Uplands biostratigraphy. Each graptolite colony was composed of many (over 1000 in some cases) polyp-like animals (zooids) which lived in linked cuplike structures (thecae). These collectively comprised a single skeletal structure, called a rhabdosome, made of a protein-like material. Graptolite rhabdosomes are normally a few centimetres in length, but range from almost microscopic to a metre or so in extreme cases; all are slender and delicate. During Ordovician and Silurian times the graptolites evolved an extraordinary variety of shapes (Figure 5), which may have been adaptations to differing feeding strategies or to life in various parts of the water column. When the graptolites are well preserved, this variety enables today's palaeontologists to distinguish different groups and species, essential to the practice of biostratigraphy.

	Graptolites inhabited the waters of the ancient oceans, and many different species co-existed at any one time. After death the graptolites sank to the sea bed. Graptolites today can be extremely abundant in `hemipelagite' rock sequences, such as the Moffat Shale, formed mainly of very fine-grained sediment. Some Moffat Shale bedding planes are covered with graptolite remains. In marked contrast, graptolites are generally rare within the thick greywacke sequences, where hemipelagic sedimentation was mostly overwhelmed by coarse-grained sediment from continental sources, transported and deposited by frequent turbidity currents.		Graptolites inhabited the waters of the ancient oceans, and many different species co-existed at any one time. After death the graptolites sank to the sea bed. Graptolites today can be extremely abundant in `hemipelagite' rock sequences, such as the Moffat Shale, formed mainly of very fine-grained sediment. Some Moffat Shale bedding planes are covered with graptolite remains. In marked contrast, graptolites are generally rare within the thick greywacke sequences, where hemipelagic sedimentation was mostly overwhelmed by coarse-grained sediment from continental sources, transported and deposited by frequent turbidity currents.
			[[File:Fig 06.jpg\|thumbnail\|Divisions within an idealised turbidite bed after Bouma (1962) and Pickering et al. (1989).]]
	~~Figure 4 Summary of information used to establish controls on the timing of Caledonian deformation in the Southern Uplands~~ (~~cf. Barnes~~ et al., 1989~~, fig~~. 1)

	The different graptolite species on a Moffat Shale bedding plane constitute a sample of the graprolite population which existed at the time of deposition, and collectively are called an assemblage. Distinct assemblages of graptolite species are unique to individual graptolite zones, each of which represents a small slice of geological time. By calibrating the sequence of Silurian graptolite zones against the radiometric time-scale for the Silurian, we know for example that the duration of some of the Silurian graptolite zones was considerably less than one million years. The graptolites were a rapidly evolving group, especially in the Silurian, and different assemblages of species representing different zones are therefore indicative of different geological time periods.		The different graptolite species on a Moffat Shale bedding plane constitute a sample of the graprolite population which existed at the time of deposition, and collectively are called an assemblage. Distinct assemblages of graptolite species are unique to individual graptolite zones, each of which represents a small slice of geological time. By calibrating the sequence of Silurian graptolite zones against the radiometric time-scale for the Silurian, we know for example that the duration of some of the Silurian graptolite zones was considerably less than one million years. The graptolites were a rapidly evolving group, especially in the Silurian, and different assemblages of species representing different zones are therefore indicative of different geological time periods.