Editing Moine geology of the Great Glen - an excursion

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Localities 14.1 and 14.2 are a related pair of exposures that occur in the River Lochy within the valley bottom of the Great Glen north of Fort William. Due to ease of access, it is suggested to visit Locality 14.1 first and then from here proceed to Locality 14.2. Allocate 2-3 hours for these sites.
 
Localities 14.1 and 14.2 are a related pair of exposures that occur in the River Lochy within the valley bottom of the Great Glen north of Fort William. Due to ease of access, it is suggested to visit Locality 14.1 first and then from here proceed to Locality 14.2. Allocate 2-3 hours for these sites.
  
=== Locality 14.1 Torcastle, River Lochy: northern outcrop. [NN 135 791] ===
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== Locality 14.1 Torcastle, River Lochy: northern outcrop. [NN 135 791] ==
  
 
Torcastle, River Lochy: northern outcrop. ([[Media:EGS_MOI_Fig_14_01.jpg|(Fig. 14.1)]], inset 1). A highly deformed fault-bounded sliver of probable Glenfinnan Group protolith within the core of the Great Glen Fault Zone, with structural evidence for sinistral shear.  
 
Torcastle, River Lochy: northern outcrop. ([[Media:EGS_MOI_Fig_14_01.jpg|(Fig. 14.1)]], inset 1). A highly deformed fault-bounded sliver of probable Glenfinnan Group protolith within the core of the Great Glen Fault Zone, with structural evidence for sinistral shear.  
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A good place to study the original protolith is at 1B [[Media:EGS_MOI_Fig_14_02.jpg|(Fig. 14.2)]]. Here, the rocks are polished clean by the river water and reveal bands of quartzite and arkosic psammite along with micaceous partitions. Within the latter, the fine mm-scale through-going shears can be seen. Also note the presence of a retrogressed metabasite. These Moine rocks are lithologically most similar to the Glenfinnan Group (Stewart ''et al''., 2000). In the northern third of the exposure, altered lamprophyre dykes are assigned to a regional Permo-Carboniferous swarm (Baxter & Mitchell, 1984). These are locally truncated by minor faults, indicating that at least some of the deformation here relates to later, post-Caledonian movement. In the area by the narrow rapids in the northern corner of the exposure (1C on [[Media:EGS_MOI_Fig_14_02.jpg|(Fig. 14.2)]]), note how the fractures that trend WNW swing round from the main flat low-lying belt that occurs in the shallow stream bed north of this outcrop. These faults are thought to represent antithetic shears to the main NE-trending high strain belts and is a pattern repeated at the next set of exposures.
 
A good place to study the original protolith is at 1B [[Media:EGS_MOI_Fig_14_02.jpg|(Fig. 14.2)]]. Here, the rocks are polished clean by the river water and reveal bands of quartzite and arkosic psammite along with micaceous partitions. Within the latter, the fine mm-scale through-going shears can be seen. Also note the presence of a retrogressed metabasite. These Moine rocks are lithologically most similar to the Glenfinnan Group (Stewart ''et al''., 2000). In the northern third of the exposure, altered lamprophyre dykes are assigned to a regional Permo-Carboniferous swarm (Baxter & Mitchell, 1984). These are locally truncated by minor faults, indicating that at least some of the deformation here relates to later, post-Caledonian movement. In the area by the narrow rapids in the northern corner of the exposure (1C on [[Media:EGS_MOI_Fig_14_02.jpg|(Fig. 14.2)]]), note how the fractures that trend WNW swing round from the main flat low-lying belt that occurs in the shallow stream bed north of this outcrop. These faults are thought to represent antithetic shears to the main NE-trending high strain belts and is a pattern repeated at the next set of exposures.
  
=== Locality 14.2 Torcastle, River Lochy: southern outcrop. [NN 132 786] ===
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== Locality 14.2 Torcastle, River Lochy: southern outcrop. [NN 132 786] ==
  
 
Torcastle, River Lochy: southern outcrop. ([[Media:EGS_MOI_Fig_14_01.jpg|(Fig. 14.1)]], inset 1, and [[Media:EGS_MOI_Fig_14_03.jpg|(Fig. 14.3)]]). A highly deformed fault-bounded sliver of probable Glenfinnan Group protolith within the core of the Great Glen Fault Zone, with structural evidence for sinistral shear.  
 
Torcastle, River Lochy: southern outcrop. ([[Media:EGS_MOI_Fig_14_01.jpg|(Fig. 14.1)]], inset 1, and [[Media:EGS_MOI_Fig_14_03.jpg|(Fig. 14.3)]]). A highly deformed fault-bounded sliver of probable Glenfinnan Group protolith within the core of the Great Glen Fault Zone, with structural evidence for sinistral shear.  
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A study of this Clunes Tonalite pluton has concluded that intrusion occurred synchronous with sinistral displacements along the Great Glen Fault (Stewart ''et al., ''2001). This is based on observations of a sinistral swing of the magmatic fabric at its northeastern margin, together with intrusion into a ductile sinistral shear zone within the marginal Moine host rock. U-Pb zircon dating of the tonalite has yielded an age of 428 ± 2 Ma which is thus interpreted to date early sinistral displacements (Stewart ''et al., ''2001). Near the shoreline of Loch Lochy the northeastern end of the pluton is displaced by a post-Caledonian brittle fault demonstrating dextral offset. As you proceed to drive north, note the deeply incised gorges running down the eastern side of the valley. The streams flowing down-slope here are actually very small, and the deep-cutting incision is an indicator as to how intensely fractured and shattered the bedrock is adjacent to the fault.
 
A study of this Clunes Tonalite pluton has concluded that intrusion occurred synchronous with sinistral displacements along the Great Glen Fault (Stewart ''et al., ''2001). This is based on observations of a sinistral swing of the magmatic fabric at its northeastern margin, together with intrusion into a ductile sinistral shear zone within the marginal Moine host rock. U-Pb zircon dating of the tonalite has yielded an age of 428 ± 2 Ma which is thus interpreted to date early sinistral displacements (Stewart ''et al., ''2001). Near the shoreline of Loch Lochy the northeastern end of the pluton is displaced by a post-Caledonian brittle fault demonstrating dextral offset. As you proceed to drive north, note the deeply incised gorges running down the eastern side of the valley. The streams flowing down-slope here are actually very small, and the deep-cutting incision is an indicator as to how intensely fractured and shattered the bedrock is adjacent to the fault.
  
=== Locality 14.4 Loch Lochy shoreline [NN 255 918] ===
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== Locality 14.4 Loch Lochy shoreline [NN 255 918] ==
  
 
Loch Lochy shoreline [[Media:EGS_MOI_Fig_14_01.jpg|(Fig. 14.1)]]. Fault-bounded block of gneiss of probable Moinian affinity on the SE side of the Great Glen Fault.  
 
Loch Lochy shoreline [[Media:EGS_MOI_Fig_14_01.jpg|(Fig. 14.1)]]. Fault-bounded block of gneiss of probable Moinian affinity on the SE side of the Great Glen Fault.  
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Continue along the A82 for a few km until the road drops down to run straight alongside the shoreline of Loch Lochy and pull into the parking layby on the left-hand side. Cross the road to exposures of steep-dipping psammitic gneisses by the waterfall; this is best seen by following the track up just above the fall. These rocks are part of a narrow fault-bounded block that makes up most of the lower valley side south of the Great Glen Fault in this area and northwards until pinching out into the centre of the fault zone near Fort Augustus. They are unlike rocks of the Grampian and Dalradian groups seen locally, which are more flaggy, finer grained and less gneissic in appearance. Also in thin-section, metamorphic textures of Dalradian and Grampian rocks are almost always polygonal in appearance, suggesting a single major phase of metamorphic growth. In contrast, thin-sections of the rocks exposed here reveal complex textures with sutured grain boundaries and very coarse crystals enclosing subgrains, indicating a secondary grain growth over an earlier metamorphic texture, typical of microtextures seen throughout the Moine. The status of these rocks is uncertain, but they appear Moine-like and it is possible that they represent a slice of Moinian material derived from the NW side of the fault and transplanted SE of the axis during strike-slip displacements. Alternatively they may correlate with gneissic basement material underlying the Grampian terrane (Badenoch Group) that has been uplifted adjacent to the fault in response to transpressional displacements.
 
Continue along the A82 for a few km until the road drops down to run straight alongside the shoreline of Loch Lochy and pull into the parking layby on the left-hand side. Cross the road to exposures of steep-dipping psammitic gneisses by the waterfall; this is best seen by following the track up just above the fall. These rocks are part of a narrow fault-bounded block that makes up most of the lower valley side south of the Great Glen Fault in this area and northwards until pinching out into the centre of the fault zone near Fort Augustus. They are unlike rocks of the Grampian and Dalradian groups seen locally, which are more flaggy, finer grained and less gneissic in appearance. Also in thin-section, metamorphic textures of Dalradian and Grampian rocks are almost always polygonal in appearance, suggesting a single major phase of metamorphic growth. In contrast, thin-sections of the rocks exposed here reveal complex textures with sutured grain boundaries and very coarse crystals enclosing subgrains, indicating a secondary grain growth over an earlier metamorphic texture, typical of microtextures seen throughout the Moine. The status of these rocks is uncertain, but they appear Moine-like and it is possible that they represent a slice of Moinian material derived from the NW side of the fault and transplanted SE of the axis during strike-slip displacements. Alternatively they may correlate with gneissic basement material underlying the Grampian terrane (Badenoch Group) that has been uplifted adjacent to the fault in response to transpressional displacements.
  
=== Locality 14.5 Loch Oich shoreline [NN 304 985] ===
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== Locality 14.5 Loch Oich shoreline [NN 304 985] ==
  
 
Loch Oich shoreline [[Media:EGS_MOI_Fig_14_01.jpg|(Fig. 14.1)]]. Moinian rocks SE of the Great Glen Fault, severely altered by fault-zone fluids.  
 
Loch Oich shoreline [[Media:EGS_MOI_Fig_14_01.jpg|(Fig. 14.1)]]. Moinian rocks SE of the Great Glen Fault, severely altered by fault-zone fluids.  
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These highly retrogressed, altered and cataclastically deformed rocks, perhaps best termed ‘hydrated cataclasites’, dominate most of the outcrop within this sliver of Moine-like rock and suggest deformation occurred within a seismogenic upper-crustal environment characterised by brittle fragmentation and dilatancy leading to high fluid influx. This deformation is interpreted as pre-Devonian in age on the basis that a fault-bounded sliver of Old Red Sandstone is seen to lie unconformably upon these cataclasites and is comparatively free of fracturing and evidence of fluid flow. Similar highly altered and shattered cataclasites may well underlie the whole core of the fault zone, explaining why the fault zone was so prone to excavation during glaciation. The Moinian protolith seen here (and at the previous locality) is similar to that seen at Torcastle, but the type of fault rock produced is quite different, a product of shearing in the upper crust, in contrast to inferred mid-crustal shearing seen at Torcastle. Such comparisons suggest the present configuration of fault-bound units is a consequence of late-Caledonian or later differential uplift along the length of the structure.
 
These highly retrogressed, altered and cataclastically deformed rocks, perhaps best termed ‘hydrated cataclasites’, dominate most of the outcrop within this sliver of Moine-like rock and suggest deformation occurred within a seismogenic upper-crustal environment characterised by brittle fragmentation and dilatancy leading to high fluid influx. This deformation is interpreted as pre-Devonian in age on the basis that a fault-bounded sliver of Old Red Sandstone is seen to lie unconformably upon these cataclasites and is comparatively free of fracturing and evidence of fluid flow. Similar highly altered and shattered cataclasites may well underlie the whole core of the fault zone, explaining why the fault zone was so prone to excavation during glaciation. The Moinian protolith seen here (and at the previous locality) is similar to that seen at Torcastle, but the type of fault rock produced is quite different, a product of shearing in the upper crust, in contrast to inferred mid-crustal shearing seen at Torcastle. Such comparisons suggest the present configuration of fault-bound units is a consequence of late-Caledonian or later differential uplift along the length of the structure.
  
=== Locality 14.6 Kilfinnan Burn [NN 277 957] ===
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== Locality 14.6 Kilfinnan Burn [NN 277 957] ==
  
 
Kilfinnan Burn [[Media:EGS_MOI_Fig_14_01.jpg|(Fig. 14.1)]]. Granite veining within Loch Eil Group in the Great Glen Fault Zone.  
 
Kilfinnan Burn [[Media:EGS_MOI_Fig_14_01.jpg|(Fig. 14.1)]]. Granite veining within Loch Eil Group in the Great Glen Fault Zone.  
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Before leaving, look across Loch Lochy to the huge gulleys incised into the valleysides on the SE of the Great Glen, indicating the weak nature of the highly shattered bedrock. Note also how the gulleys are very wide upslope, but then narrow abruptly. Thus sudden change marks the boundary between, upslope, shattered Moine (e.g. Locality 14.4) and Grampian Group bedrock and, downslope, less fractured Old Red Sandstone which occurs as a local fault-bounded sliver. Such evidence suggests that the majority of fracturing observed along the Great Glen Fault Zone relates to pre-Old Red Sandstone sinistral displacements. From here continue on to Inverness and then to Locality 14.7.
 
Before leaving, look across Loch Lochy to the huge gulleys incised into the valleysides on the SE of the Great Glen, indicating the weak nature of the highly shattered bedrock. Note also how the gulleys are very wide upslope, but then narrow abruptly. Thus sudden change marks the boundary between, upslope, shattered Moine (e.g. Locality 14.4) and Grampian Group bedrock and, downslope, less fractured Old Red Sandstone which occurs as a local fault-bounded sliver. Such evidence suggests that the majority of fracturing observed along the Great Glen Fault Zone relates to pre-Old Red Sandstone sinistral displacements. From here continue on to Inverness and then to Locality 14.7.
  
=== Locality 14.7 Rosemarkie [NH 773 627 to NH 765 615] ===
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== Locality 14.7 Rosemarkie [NH 773 627 to NH 765 615] ==
  
 
Rosemarkie ([[Media:EGS_MOI_Fig_14_01.jpg|(Fig. 14.1)]], inset 2, and [[Media:EGS_MOI_Fig_14_05.jpg|(Fig. 14.5)]]). Interleaved Lewisian and Moine rocks with intrusive granites, showing evidence for ductile deformation that may be related to displacement along the Great Glen Fault.  
 
Rosemarkie ([[Media:EGS_MOI_Fig_14_01.jpg|(Fig. 14.1)]], inset 2, and [[Media:EGS_MOI_Fig_14_05.jpg|(Fig. 14.5)]]). Interleaved Lewisian and Moine rocks with intrusive granites, showing evidence for ductile deformation that may be related to displacement along the Great Glen Fault.  

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