Editing Glaciations and stadials, Palaeogene volcanic districts of Scotland

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== Dimlington Stadial ==
 
== Dimlington Stadial ==
 
===  Main Late Devensian Glaciation ===
 
===  Main Late Devensian Glaciation ===
The extent of former glaciers has been deciphered by establishing the source of glacial erratics in the tills and moraines, and by recognising glacial trimlines. Trimlines define the highest levels to which glacier ice has actively eroded, or trimmed, frost-shattered bedrock and debris on protruding peaks (nunataks). Above the trimline, frost-shattered bedrock will persist but on the lower slopes, below the trimline, the bedrock will be glacially moulded and striated ([[Media:P580489.jpg|P580489]]). Glacially transported boulders will be present below the trimline but otherwise most of the loose material that characterises nunataks will have been removed. The mineralogy of weathered materials may differ; gibbsite is common in the clay fraction of debris on former nunataks but it is rare, or absent, below the trimlines. Subsequent events may obscure or eliminate the evidence of trimlines, for example the downslope movement of scree debris (Ballantyne, 1997, 1999).
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The extent of former glaciers has been deciphered by establishing the source of glacial erratics in the tills and moraines, and by recognising glacial trimlines. Trimlines define the highest levels to which glacier ice has actively eroded, or trimmed, frost-shattered bedrock and debris on protruding peaks (nunataks). Above the trimline, frost-shattered bedrock will persist but on the lower slopes, below the trimline, the bedrock will be glacially moulded and striated (P580489). Glacially transported boulders will be present below the trimline but otherwise most of the loose material that characterises nunataks will have been removed. The mineralogy of weathered materials may differ; gibbsite is common in the clay fraction of debris on former nunataks but it is rare, or absent, below the trimlines. Subsequent events may obscure or eliminate the evidence of trimlines, for example the downslope movement of scree debris (Ballantyne, 1997, 1999).
  
The Inner Hebrides and Arran were almost entirely covered by a thick regional ice sheet during the early part of the Late Devensian ([[Media:P914157.png|P914157]]). The ice sheet extended west of the Inner Hebrides, but probably not as far as St Kilda, which supported local glaciers. Later, as the ice sheet diminished, independent ice domes formed over the Outer Hebrides, and the islands of Skye, Mull, Arran and probably Rum.
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The Inner Hebrides and Arran were almost entirely covered by a thick regional ice sheet during the early part of the Late Devensian (P914157). The ice sheet extended west of the Inner Hebrides, but probably not as far as St Kilda, which supported local glaciers. Later, as the ice sheet diminished, independent ice domes formed over the Outer Hebrides, and the islands of Skye, Mull, Arran and probably Rum.
  
 
Skye was an important centre for ice sheet glaciation and the Cuillin is an outstanding area for the study of glacial geomorphology (e.g. Gordon and Sutherland, 1993). The distribution of glacial striae, ice-moulded rocks and erratics all indicate that the Cuillin and the Red Hills were not over-ridden by ice from the Scottish mainland during the Main Late Devensian glaciation. Instead, independent ice caps developed and these deflected the ice that flowed westward from the mainland. The distribution of glacial erratics from the mainland over a range of altitudes on Raasay and Scalpay to the north, and on Soay to the south, and their absence from upland areas of central Skye, defines the area unaffected by mainland ice (Harker, 1904, but see also Ballantyne, 1990).
 
Skye was an important centre for ice sheet glaciation and the Cuillin is an outstanding area for the study of glacial geomorphology (e.g. Gordon and Sutherland, 1993). The distribution of glacial striae, ice-moulded rocks and erratics all indicate that the Cuillin and the Red Hills were not over-ridden by ice from the Scottish mainland during the Main Late Devensian glaciation. Instead, independent ice caps developed and these deflected the ice that flowed westward from the mainland. The distribution of glacial erratics from the mainland over a range of altitudes on Raasay and Scalpay to the north, and on Soay to the south, and their absence from upland areas of central Skye, defines the area unaffected by mainland ice (Harker, 1904, but see also Ballantyne, 1990).
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Mainland erratics have not, however, been found on the high ground in northern Eigg. Till deposits attributable to the Main Late Devensian Glaciation are common on Rum and small patches of till occur on the other islands, but glaciofluvial deposits are everywhere rare. There are two prominent north-east-trending morainic ridges in north-east Eigg. The eastern ridge is most likely part of a large crag-and-tail structure produced by ice flowing around the north end of Eigg, whereas the western ridge is a lateral moraine formed as the same ice mass decayed. Good examples of kettleholes occur in Cleadale, on Eigg.
 
Mainland erratics have not, however, been found on the high ground in northern Eigg. Till deposits attributable to the Main Late Devensian Glaciation are common on Rum and small patches of till occur on the other islands, but glaciofluvial deposits are everywhere rare. There are two prominent north-east-trending morainic ridges in north-east Eigg. The eastern ridge is most likely part of a large crag-and-tail structure produced by ice flowing around the north end of Eigg, whereas the western ridge is a lateral moraine formed as the same ice mass decayed. Good examples of kettleholes occur in Cleadale, on Eigg.
  
The glaciation of Mull during the Loch Lomond Stadial (see below) removed much of the evidence for earlier glaciations from the central parts of the island. A local ice dome was established over the hills of central Mull during the Main Late Devensian Glaciation, diverting the mainland ice down Glen Forsa and south-west along Glen More and Loch na Keal (Bailey et al., 1924; [[Media:P914158.png|P914158]]). Ben More and other high peaks probably formed nunataks (Ballantyne, 1999).
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The glaciation of Mull during the Loch Lomond Stadial (see below) removed much of the evidence for earlier glaciations from the central parts of the island. A local ice dome was established over the hills of central Mull during the Main Late Devensian Glaciation, diverting the mainland ice down Glen Forsa and south-west along Glen More and Loch na Keal (Bailey et al., 1924; P914158). Ben More and other high peaks probably formed nunataks (Ballantyne, 1999).
  
On the south side of Loch na Keal, for some distance either side of the outlet of the Scarisdale River, bare basalt slabs on the shoreface have been moulded into a striking assemblage of smoothed, fluted, channel-like forms a metre of so in depth and a few metres in length ([[Media:P580490.jpg|P580490]]). These are termed 'p-forms', and are excellent examples of structures that probably originated by a combination of erosion by glacial meltwaters and overlying ice (Gray, 1981; Walker et al., 1992). At the northern end of Loch Don, deltaic sands and gravels were deposited where south- and south-east-flowing meltwater streams entered the sea, to form the lower levels of the 'Loch Don Sand-Moraine' (Bailey et al., 1924; Benn and Evans, 1993; see below).
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On the south side of Loch na Keal, for some distance either side of the outlet of the Scarisdale River, bare basalt slabs on the shoreface have been moulded into a striking assemblage of smoothed, fluted, channel-like forms a metre of so in depth and a few metres in length (P580490). These are termed 'p-forms', and are excellent examples of structures that probably originated by a combination of erosion by glacial meltwaters and overlying ice (Gray, 1981; Walker et al., 1992). At the northern end of Loch Don, deltaic sands and gravels were deposited where south- and south-east-flowing meltwater streams entered the sea, to form the lower levels of the 'Loch Don Sand-Moraine' (Bailey et al., 1924; Benn and Evans, 1993; see below).
  
 
The Ardnamurchan peninsula was completely over-ridden by mainland ice during the Main Late Devensian Glaciation but apparently escaped renewed glaciation during the Loch Lomond Stadial. Evidence for ice movement from the south-east is provided by numerous striae and by the distribution and character of glacial erratics, including many derived from the Strontian Granite Pluton which occur, for example, on Ben Hiant. Much ice sculpting of the bare rocks has occurred and the hills formed by the gabbroic ring-intrusions now provide numerous examples of roches moutonnées.
 
The Ardnamurchan peninsula was completely over-ridden by mainland ice during the Main Late Devensian Glaciation but apparently escaped renewed glaciation during the Loch Lomond Stadial. Evidence for ice movement from the south-east is provided by numerous striae and by the distribution and character of glacial erratics, including many derived from the Strontian Granite Pluton which occur, for example, on Ben Hiant. Much ice sculpting of the bare rocks has occurred and the hills formed by the gabbroic ring-intrusions now provide numerous examples of roches moutonnées.
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During the Loch Lomond Stadial (11 000 to 10 000 years BP; Table 17), small ice fields and glaciers became re-established in the mountainous areas of the district, leading to renewed valley and corrie glaciation, and to the modification or erosion of older glacial deposits; a distinctive suite of deposits is associated with this glaciation.
 
During the Loch Lomond Stadial (11 000 to 10 000 years BP; Table 17), small ice fields and glaciers became re-established in the mountainous areas of the district, leading to renewed valley and corrie glaciation, and to the modification or erosion of older glacial deposits; a distinctive suite of deposits is associated with this glaciation.
  
In Skye, glacial activity was centred over the Cuillin and the western Red Hills ([[Media:P914159.png|P914159]]a). In addition, there were two small glaciers in north-east-facing valleys on the Trotternish escarpment and glaciers in Kylerhea Glen and Glen Arroch in eastern Skye. The extent of the Cuillin ice field has been defined by the distribution and orientation of glacial striae, roches moutonnées and moraines, and by the lithologies and distribution patterns of glacial erratics. Outlet glaciers flowed northwards down Glen Sligachan into glens Drynoch and Varragill, and into Loch Sligachan;others were located along the north sides of the western and eastern Red Hills. To the south, a large glacier flowed into Loch Slapin and outlet glaciers formed in Srath na Creitheach and from Loch Coruisk into Loch Scavaig. Nunataks protruding through the icefield included the main ridge of the Cuillin together with Sgùrr na Stri and Blà Bheinn, and Marsco and Glamaig in the western Red Hills.
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In Skye, glacial activity was centred over the Cuillin and the western Red Hills (P914159a). In addition, there were two small glaciers in north-east-facing valleys on the Trotternish escarpment and glaciers in Kylerhea Glen and Glen Arroch in eastern Skye. The extent of the Cuillin ice field has been defined by the distribution and orientation of glacial striae, roches moutonnées and moraines, and by the lithologies and distribution patterns of glacial erratics. Outlet glaciers flowed northwards down Glen Sligachan into glens Drynoch and Varragill, and into Loch Sligachan;others were located along the north sides of the western and eastern Red Hills. To the south, a large glacier flowed into Loch Slapin and outlet glaciers formed in Srath na Creitheach and from Loch Coruisk into Loch Scavaig. Nunataks protruding through the icefield included the main ridge of the Cuillin together with Sgùrr na Stri and Blà Bheinn, and Marsco and Glamaig in the western Red Hills.
  
Corries such as Coir' a' Ghrunnda, Coire Lagan, An Garbh-choire and Coir'-uisg, with their precipitous sides, rock falls and striated, ice-scoured rock in the corrie floors, provide some of the most spectacular glacial landforms in the district ([[Media:P521672.jpg|P521672]], [[Media:P580489.jpg|P580489]]). Moraines and glacial trimlines generally help to define the extent and thickness of the corrie glaciers, although in the Red Hills this evidence is generally obscured by scree. Tills are common, and typically have a sheet-like geometry within topographical depressions, although more-linear deposits are also present in the form of moraine ridges, as in the area around Sligachan. Excellent examples of moraines formed at glacier margins occur at the mouth of Coir' a' Ghrunnda in the Cuillin and at Coire Fearchair in the eastern Red Hills. Hummocky moraine is common, containing material supplied from both subglacial and supraglacial environments. In Gleann Torra-mhichaig, east of Glamaig, chains of hummocks oblique to the valley floor are attributed to intermittent bulldozing by advancing ice during the 'overall decay' of the valley glacier. In Coire Choinnich at the head of Loch Ainort, a chaotic assemblage of hummocks, non-aligned ridges and fluvial terrace accumulations resulted from the in-situ decay of less-active glacier ice.
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Corries such as Coir' a' Ghrunnda, Coire Lagan, An Garbh-choire and Coir'-uisg, with their precipitous sides, rock falls and striated, ice-scoured rock in the corrie floors, provide some of the most spectacular glacial landforms in the district (P521672, P580489). Moraines and glacial trimlines generally help to define the extent and thickness of the corrie glaciers, although in the Red Hills this evidence is generally obscured by scree. Tills are common, and typically have a sheet-like geometry within topographical depressions, although more-linear deposits are also present in the form of moraine ridges, as in the area around Sligachan. Excellent examples of moraines formed at glacier margins occur at the mouth of Coir' a' Ghrunnda in the Cuillin and at Coire Fearchair in the eastern Red Hills. Hummocky moraine is common, containing material supplied from both subglacial and supraglacial environments. In Gleann Torra-mhichaig, east of Glamaig, chains of hummocks oblique to the valley floor are attributed to intermittent bulldozing by advancing ice during the 'overall decay' of the valley glacier. In Coire Choinnich at the head of Loch Ainort, a chaotic assemblage of hummocks, non-aligned ridges and fluvial terrace accumulations resulted from the in-situ decay of less-active glacier ice.
  
 
Depositional features from the Loch Lomond Stadial are abundant in the mountains in the southern half of Rum, where there is evidence that twelve local glaciers formed. One occupied Coire nan Gruund and, at the lower end of the corrie, the Kinloch—Dibidil Path crosses a well-defined moraine made of ultrabasic blocks derived from the east side of Hallival and Askival. Rock slabs north of the moraine show north-north-east-directed glacial striae from the Loch Lomond Stadial glaciation cutting across north-west-directed striae from the earlier, Main Late Devensian Glaciation. In upper Glen Harris, linear moraine ridges parallel to the direction of ice movement are strikingly developed in Atlantic Corrie.
 
Depositional features from the Loch Lomond Stadial are abundant in the mountains in the southern half of Rum, where there is evidence that twelve local glaciers formed. One occupied Coire nan Gruund and, at the lower end of the corrie, the Kinloch—Dibidil Path crosses a well-defined moraine made of ultrabasic blocks derived from the east side of Hallival and Askival. Rock slabs north of the moraine show north-north-east-directed glacial striae from the Loch Lomond Stadial glaciation cutting across north-west-directed striae from the earlier, Main Late Devensian Glaciation. In upper Glen Harris, linear moraine ridges parallel to the direction of ice movement are strikingly developed in Atlantic Corrie.
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A wealth of glacial erosional and depositional features is preserved on the Western Granite hills of Rum, including a fine arcuate terminal moraine composed of microgranite blocks north of Sron an t-Saighdeir. There is also a superb blockfield, of supposed Loch Lomond Stadial age on the southern slopes of Orval and Sròn an t-Saighdeir (Ballantyne and Wain-Hobson, 1980). This has been modified by later solifluction processes, with the development of stone stripes on the lower, steeper slopes.
 
A wealth of glacial erosional and depositional features is preserved on the Western Granite hills of Rum, including a fine arcuate terminal moraine composed of microgranite blocks north of Sron an t-Saighdeir. There is also a superb blockfield, of supposed Loch Lomond Stadial age on the southern slopes of Orval and Sròn an t-Saighdeir (Ballantyne and Wain-Hobson, 1980). This has been modified by later solifluction processes, with the development of stone stripes on the lower, steeper slopes.
  
During the Loch Lomond Stadial, Mull supported an ice sheet that extended from Loch Bà to Loch Spelve, with Beinn Talaidh and other high hills in central Mull forming nunataks ([[Media:P914159.png|P914159]]b). The most striking scenic features of Mull attributable to glacial processes date from this episode. Glaciers from the main area of ice reached the sea at several points, as did local glaciers flowing north-north-east off the hills into the Sound of Mull in the vicinity of Craignure. Outwash deposits formed at the foot of Loch Bà and Glen Forsa. Terminal moraines are present at Kinlochspelve and at the northern end of Loch Don, where Late-glacial deltaic deposits of the Loch Don Sand Moraine (see above) are partly covered by till from the Loch Lomond Stadial glaciation (Benn and Evans, 1993). Inland, there are widespread deposits of hummocky morainic drift, for example east of Craig in Glen More ([[Media:P580488.jpg|P580488]]). Linear drift deposits, or fluted moraines, occur at several localities; well-defined examples on the north-west shoulder of Sgurr Dearg were formed by glaciers converging to the north-west.
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During the Loch Lomond Stadial, Mull supported an ice sheet that extended from Loch Bà to Loch Spelve, with Beinn Talaidh and other high hills in central Mull forming nunataks (P914159b). The most striking scenic features of Mull attributable to glacial processes date from this episode. Glaciers from the main area of ice reached the sea at several points, as did local glaciers flowing north-north-east off the hills into the Sound of Mull in the vicinity of Craignure. Outwash deposits formed at the foot of Loch Bà and Glen Forsa. Terminal moraines are present at Kinlochspelve and at the northern end of Loch Don, where Late-glacial deltaic deposits of the Loch Don Sand Moraine (see above) are partly covered by till from the Loch Lomond Stadial glaciation (Benn and Evans, 1993). Inland, there are widespread deposits of hummocky morainic drift, for example east of Craig in Glen More (P580488). Linear drift deposits, or fluted moraines, occur at several localities; well-defined examples on the north-west shoulder of Sgurr Dearg were formed by glaciers converging to the north-west.
  
 
The northern hills of Arran supported valley glaciers during the Loch Lomond Stadial and the till formed at this stage augmented earlier till (see above). Valley glaciers from a small ice field centred on the head of Glen Iorsa extended down glens Catacol, Easan Biorach and Iorsa. Glaciers also occupied North Glen Sannox and the upper parts of glens Rosa and Sannox. Small corrie glaciers formed on the eastern side of the Goat Fell ridge and north-west and north of Beinn Bharrain. There are two generations of moraine in the northern hills. One group of fairly well defined moraines occurs at low levels throughout the area and is covered with grass and heather, for example in lower Glen Rosa. The second group is formed of fresh moraine ridges, studded with boulders and relatively free of vegetation. The latter moraines generally occur above 450 m OD, and are almost entirely restricted to corries in the east of the northern hills, for example at the head of Glen Rosa, on the north side of Casteal Abhail, and to the south-east of Beinn Tarsuinn. Substantial corrie glaciers evidently persisted in the eastern part of the northern hills after most of the ground to the west had become free of ice.
 
The northern hills of Arran supported valley glaciers during the Loch Lomond Stadial and the till formed at this stage augmented earlier till (see above). Valley glaciers from a small ice field centred on the head of Glen Iorsa extended down glens Catacol, Easan Biorach and Iorsa. Glaciers also occupied North Glen Sannox and the upper parts of glens Rosa and Sannox. Small corrie glaciers formed on the eastern side of the Goat Fell ridge and north-west and north of Beinn Bharrain. There are two generations of moraine in the northern hills. One group of fairly well defined moraines occurs at low levels throughout the area and is covered with grass and heather, for example in lower Glen Rosa. The second group is formed of fresh moraine ridges, studded with boulders and relatively free of vegetation. The latter moraines generally occur above 450 m OD, and are almost entirely restricted to corries in the east of the northern hills, for example at the head of Glen Rosa, on the north side of Casteal Abhail, and to the south-east of Beinn Tarsuinn. Substantial corrie glaciers evidently persisted in the eastern part of the northern hills after most of the ground to the west had become free of ice.

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