Figures, plates and tables

Figures

(Figure 1) Topography of the Glencoe area.

(Figure 2) Simplified map, generalised succession and cross-section showing the geology of the Glencoe area. The cross-section is drawn as if viewed looking towards the south, which is the view seen southwards from the main road (A82T) travelling west from the vicinity of the Kingshouse Hotel [NN 26 54] to the lower end of Glen Coe [NN 12 56]. See p.5 for key.

(Figure 3) Distribution of Siluro-Devonian volcanic and plutonic rocks showing faults that were active during the magmatic activity.

(Figure 4) (right) Main features of calderas, caldera-forming eruptions and the associated phenomena. a. Valles caldera, Jemez Mountains, New Mexico, USA (image generated by H P Foote; geology from USGS map I-571, 1970). The main topographical depression in the summit of the volcano is the caldera. This formed via two large-scale explosive eruptions between 1 and 1.5 million years ago. The entire volcano records some 13 million years of activity. The caldera wall shows degradation by collapse, with a typically scalloped form and with wedges of collapse breccia forming part of the caldera fill. Ignimbrites emplaced during the caldera-forming eruptions form fans on the outer flanks of the volcano and a large part of the fill in the caldera. Post-caldera resurgence of magma into the volcano has caused the intracaldera ignimbrites to be forced upwards, forming a central resurgent dome, with an extensional graben across its apex, and a discontinuous ring of vents with lava flows. b. Sketch illustrating eruption within a multi-subsidence, piecemeal caldera. Hypothetical volcano illustrating a large-scale eruption with associated progressive deposition of ignimbrite from the base of the pyroclastic current and collapse of developing volcanotectonic fault scarps. Downsag with related extensional opening of crevasses is depicted for the ongoing eruption. The diagram illustrates how the complexity of an early stage of caldera collapse, represented by the lower of the two intracaldera ignimbrites, can by obscured by burial. The Glencoe Caldera-volcano Complex records seven caldera-forming eruptions with deposition of intracaldera ignimbrites; most involved both downsag and piecemeal volcanotectonic faulting.

(Figure 5) Models of cauldron subsidence. a. The original models of cauldron subsidence derived from studies at Glen Coe (modified after Clough et al., 1909). b. Model of asymmetrical subsidence of a coherent caldera-floor block (after Roberts, 1974). Note the depiction of pronounced inward dip (downward convergence) of the bounding faults. This geometry is implausible for straightforward central-block subsidence and does not occur in reality.

(Figure 6) Structural cross-sections through Dalradian rocks along the classic Loch Leven section, just to the north of the Glencoe Caldera–volcano Complex. a. After Treagus (1974). b.After Hickman (1978). BSL Ballachulish Slide; BS Ballachulish Syncline; BWA Blackwater Antiform; BWS Blackwater Synform; KA Kinlochleven Anticline; MA Mamore Antiform; SBS Stob Ban Synform; TS Treig Syncline

(Figure 7) Major structural features of the Glencoe Caldera-volcano Complex, highlighting intrusions and faults active during volcanism (Etive Dyke Swarm not shown).

(Figure 8) Outcrop of the Basal Andesite Sill-complex relative to the structural framework.

(Figure 9) Vertical section of the peperitic upper contact of one of the basal andesite sills, below Aonach Dubh [1408 5594]. The vesicles were formed in the sandstone by gen- eration of steam in the original wet sediment, and, together with the in-situ swarms of andesite grains formed by magma-water interaction within the sedimentary host, are evidence that the magma was intrusive.

(Figure 10) (right) Stob Dearg [22 54] (original drawings by K Lancaster) (P611773). a and b) Viewed towards the south-west, showing lower elements of the intracaldera stratigraphy and the cross-cutting rhyolite interpreted as infilling the vent from which the Upper Etive Rhyolite was erupted. c. Viewed towards the north-west, showing the rhyolite-filled vent of the Upper Etive Rhyolite and thickening of the Lower Three Sisters Ignimbrite across the Chasm step-fault system. Views b and c both show the restriction of the Kingshouse Breccias at palaeofault scarps that trend north- eastwards, at right angles to the Chasm step-fault system and Glencoe Graben. In view b, three broad V-shaped outcrops of breccia and conglomerate (labelled p) are remnants of a single infilled palaeocanyon that was cut into the Lower Etive Rhyolite parallel to the Glencoe Graben. The outcrops are remnants of the south- west side of the palaeocanyon, which must have trended north-westwards, parallel to the cliff faces (despite their appearance, they are not complete cross-sections of three separate features). 1 Central Buttress; 2 North Buttress; 3 Great Gully Buttress; 4 Broad Buttress; 5 Tulaich Buttress; 6 Waterslide; CRT Crowberry Ridge Tuffs; DA Dalradian metamorphic basement; KHB Kingshouse Breccias; KHT Kingshouse Tuffs; LER Lower Etive Rhyolite; LTS Lower Three Sisters Ignimbrite; MER Middle Etive Rhyolite; RGT Raven’s Gully Tuffs; UER Upper Etive Rhyolite; USA Upper Streaky Andesites; UTS Upper Three Sisters Ignimbrite

(Figure 11) Kingshouse Tuffs (KHT). a Distribution. b Schematic diagram showing a north-west section parallel to the axis of the Glencoe Graben, and the setting of the Kingshouse Tuffs eruption and related deposits. This site was located at the intersection of the Glencoe Graben and a cross-graben between the Glen Etive Fault and the Devil’s Staircase Fault; thus it was tectonically controlled, as were the alluvial fans of the Kingshouse Breccias (KHB). BAS Basal Andesite Sill-complex; DA Dalradian metamorphic basement; DSF Devil’s Staircase Fault; GEF Glen Etive Fault; NEGF Northeastern Graben Fault; OF Ossian Fault; QCF Queen’s Cairn Fault; WCF White Corries Fault

(Figure 12) (right) Kingshouse Tuffs (KHT) at North Buttress on Stob Dearg [NN 2270 5476]. a. Schematic log of the sequence that is part of the outer flank of a tuff cone. It shows an upward change from predominantly massive breccia with abundant lithic fragments to finer grained tuffs characterised by planar and low-angle cross-stratification with layers rich in accretionary lapilli. This section records phreatomagmatic activity transitional from early vent-clearing explosions to later development of a tall eruption column that produced energetic pyroclastic density currents and substantial ash fall. b Cross-stratified silicic tuffs showing low-amplitude bedforms and low-angle scour surfaces recording the passage of highly unsteady, energetic pyroclastic density currents (P611782). c Massive to weakly bedded tuffaceous lithic breccias and lithic-rich tuff formed from initial, vent-clearing, wet explosions; the lithic fragments are quartzite fragments from the underlying Kingshouse Breccias (P611783).

(Figure 13) Lower Etive Rhyolite distribution and key features.

(Figure 14) Schematic cross-section illustrating ignimbrites and breccias restricted at and near the Southwestern Graben Fault (zone) in Coire nam Beitheach [NN 139 547], north-west of the Queen’s Cairn Fault. Original near-vertical fault scarps that ponded the Lower Etive Rhyolite (LER) have been rotated by downsag towards the Glencoe Graben. Blocks and megablocks of Lower Etive Rhyolite have been incorporated at several horizons within the ponded Upper Three Sisters Ignimbrite. Overlying Church Door Buttress Breccias include mesobreccias that were shed from the Southwestern Graben Fault and show evidence of loading into hot ignimbrite; andesite-dominated breccias higher in the section were shed from scarps cutting the Basal Andesite Sill-complex farther to the south-west.

(Figure 15) Vertical section through the Kingshouse Tuffs and Lower Etive Rhyolite, north-west of the Ossian Fault, on G-Buttress, Aonach Dubh [NN 143 554].

(Figure 16) (left) Glencoe Graben: schematic diagrams showing a north-west to south-east section parallel to the axis of the graben and the contrasting eruptions of the Upper Etive Rhyolite and Lower Three Sisters Ignimbrite. a. The three Etive rhyolite eruptions formed mainly lava-like ignimbrites from pyroclastic fountains at the sites of tuff-cones that were active during the initial phreatomagmatic explosivity. The vents were located centrally, at the intersection of the Glencoe Graben and the cross-graben bounded by the Devil’s Staircase and Glen Etive faults. They probably formed in or close to the Chasm Fault system, on the north-east side of the main graben. b. The Lower Three Sisters Ignimbrite appears to have been erupted from the Northeastern Graben Fault (zone) at a location towards the south-east of the volcano complex; its emplacement involved both volcanotectonic faulting and downsag. Buoyant-convective ash plumes that would have risen above both the vent and the pyroclastic currents are omitted for clarity. BAS Basal Andesite Sill-complex; CRT Crowberry Ridge Tuffs; DA Dalradian metamorphic basement; DSF Devil’s Staircase Fault; GEF Glen Etive Fault; KHB Kingshouse Breccias; KHT Kingshouse Tuffs; LER Lower Etive Rhyolite; LTS Lower Three Sisters Ignimbrite; NEGF Northeastern Graben Fault; OF Ossian Fault; QCF Queen’s Cairn Fault; RGT Raven’s Gully Tuffs; UER Upper Etive Rhyolite; WCF White Corries Fault

(Figure 17) Distribution of the Three Sisters ignimbrites, associated sedimentary rocks and contemporary volcanotectonic features. LQB Lower Queen’s Cairn Breccias; LTS Lower Three Sisters Ignimbrite; QCC Queen’s Cairn Conglomerates; UTS Upper Three Sisters Ignimbrite

(Figure 18) Schematic cross-section of the Lower Three Sisters Ignimbrite and its contact relationships transverse to the Glencoe Graben, south-east of the Glen Etive Fault. 1 Mesobreccias and megabreccias shed from fault scarps of the Etive rhyolites during early phases of caldera subsidence in the course of the Lower Three Sisters eruption. 2 Eutaxitic tuff with mesobreccia layers showing onlap and fanning dips that respectively record progressive aggradation of the ignimbrite and progressive downsag during the course of the eruption. 3 Extensional crevasse formed along the Northeastern Graben Fault to accommodate the downsag towards the south-west.

(Figure 19) Texture of coarse-grained lithofacies of the Lower Queen’s Cairn Breccias showing near jigsaw-fitting of clasts and sparse matrix of similar material, which together indicate fragmentation at a late stage of transport. Such textures in sedimentary deposits are characteristically formed in debris avalanches.

(Figure 20) (right) Schematic sections north-west to south-east parallel to the axis of the Glencoe Graben. a. The tectonic development that led to emplacement of the Lower Queen’s Cairn Breccias (LQB). The breccias are banked against a retrogressively degraded scarp of the Queen’s Cairn Fault and spread to the south and south-east across the alluvial surface formed of Queen’s Cairn conglomerates, sandstones, and siltstones (QCC).b. The ensuing eruption that formed the Upper Three Sisters Ignimbrite (UTS) and led to volcano-tectonic subsidence involving both faulting and downsag. Buoyant-convective ash plumes that would have risen above both the vent and the pyroclastic currents are omitted for clarity.BAS Basal Andesite Sill-complex; CRT Crowberry Ridge Tuffs; DA Dalradian metamorphic basement; DSF Devil’s Staircase Fault; GEF Glen Etive Fault; KHB Kingshouse Breccias; KHT Kingshouse Tuffs; LER Lower Etive Rhyolite; LTS Lower Three Sisters Ignimbrite; NEGF Northeastern Graben Fault; OF Ossian Fault; QCF Queen’s Cairn Fault; RGT Raven’s Gully Tuffs; UER Upper Etive Rhyolite; UTS Upper Three Sisters Ignimbrite; WCF White Corries Fault

(Figure 21) Schematic log of the Upper Three Sisters Ignimbrite in Coire nam Beitheach [NN 1414 5473] showing stratified mesobreccias towards the base and, at around 65 m, the characteristic increase from moderate to dense welding picked out by intensification of the eutaxitic fabric (flattening of fiamme). The stratigraphical levels of the ignimbrite shown in (Plate 19a), (Plate 19b) are also located.

(Figure 22) Locations of outcrops of the Upper Queen’s Cairn Breccias (UQB), the Church Door Buttress Breccias (CDB), and the diverse sedimentary rocks that infill the Glas Choire palaeocanyon and overlie a fluvially eroded surface to the north-west (Glas Choire Sandstone Member; GCS). The inset box shows the location of the detail provided in (Figure 23).

(Figure 23) Locations and key features of individual elements of the Church Door Buttress Breccias and of the distal Glas Choire alluvial deposits, in the north-west of the caldera-volcano complex (see (Figure 14) and (Figure 22)).

(Figure 24) Alluvial architecture of the fill within the Glas Choire palaeocanyon.a. Schematic section of the Glas Choire palaeocanyon fill, showing four groups of stacked and infilled channels, with overbank siltstone that lies on the shoulder of the palaeocanyon. The uppermost siltstones are lacustrine in origin. b. Detail of the alluvial architecture: vertical scale is approximately 2 the horizontal scale.

(Figure 25) Ring-fault system, associated fault-intrusions and the passively emplaced Clach Leathad Pluton.

(Figure 26) (left) Simplified conceptual model proposed to explain occurrences at Glen Coe of fault-intrusions with planar inner (caldera-side) contacts and extremely irregular outer contacts (see text). a. This shows the setting of a hypothetical hydrothermal system at considerable depths (at least several hundreds of metres) where it will be intersected by a dilating caldera-fault plane. The figure shows a potential releasing bend in the incipient fault plane, although in reality dilatation may be more general where the fault dips outwards or where it is more irregularly curved and juxtaposes parts with different curvatures. The depicted isolation of the hydrothermal system is a diagrammatic simplification. b. The hydrothermal system is shown here as a localised network of fractures containing superheated water under high confining pressure. In reality such a hydrothermal reservoir would probably have greater vertical extent and be connected to both meteoric and magmatic water sources at depth, and to fumaroles at the surface. X0 is an arbitrary reference point in the block that subsides. c. The proposed immediate effect of rapid subsidence on the caldera fault (X⁰ to X¹): rapid dilatation causes the hydrothermal system to explode, via transformation of superheated liquid water to vapour and vigorous expansion of vapour. Such processes would most probably be followed rapidly by ascent of fragmented melts from depth, these too having been disrupted by volatile exsolution and expansion due to pressure relief. d. The final form of the opposed contacts of the fault-intrusion, as seen at outcrop at Glen Coe: the overall shape and relative dimensions of the intrusion are hypothetical. The vertical distance moved by the reference point (X⁰ to X²) would be of the order of several hundreds of metres and the duration of that subsidence a matter of only hours or a few days.

(Figure 27) Relative timing of events affecting the Glencoe area over the last 40 000 years (or since the last Glacial Maximum). The graph on the left shows the oxygen isotope record from the Greenland (GISP) ice-core, a proxy for Northern Hemisphere temperature.

(Figure 28) A reconstruction of the Glencoe glacier approximately 12 000 years ago, during the Loch Lomond Stadial. View towards south-west.

Plates

(Plate 1) Satellite view showing the location of the Glencoe area in Scotland. BGS enhanced image © NERC, 2005. Grid lines in white show latitude and longitude; National Grid is indicated along the margin of the image.

(Plate 2a) Welded ignimbrite. a. At outcrop: the black lenticular fiamme represent pumice fragments that collapsed within the hot- state compacted ash matrix. Locally the flattening fabric is wrapped around a rock fragment (just below centre) that was rigid (P611765).

(Plate 2b) Welded ignimbrite. b. Under the microscope: a eutaxitic texture can be seen. Brown to colourless glass shards are strongly flattened and wrapped in a ductile fashion around a rigid crystal of quartz (centre). Field of view is 4 mm wide: plane-polarised light (P611766).

(Plate 3) Basal Andesite Sill-complex with overlying Kingshouse Tuffs and stratified Lower Etive Rhyolite, viewed looking east-south-east towards Aonach Dubh [NN 15 56]; the valley to the left is Glen Coe and the summit to the right is Stob Coire nan Lochan (P611767). BAS Basal Andesite Sill-complex; DA Dalradian metamorphic basement; KHT Kingshouse Tuffs; LER Lower Etive Rhyolite

(Plate 4) Basal Andesite Sill-complex, Kingshouse Tuffs and Lower Etive Rhyolite cut by strands of the Ossian Fault in the north face of Aonach Dubh [NN 15 56]. A palaeocanyon filled with conglomerate overlain by ignimbrite is located on the trace of the right-hand fault strand and is part of the extensive unconformity surface that cuts the sill-complex. The Lower Etive Rhyolite thickens considerably to the right (north-west) across both fault strands; the right-hand strand shows reactivation in the opposite sense (down to the south-east) (P611768). BAS Basal Andesite Sill-complex; KHB Kingshouse Breccias; KHT Kingshouse Tuffs; LER Lower Etive Rhyolite; UER Upper Etive Rhyolite

(Plate 5) Basal Andesite Sill-complex (BAS) and unconformity on Dalradian metamorphic basement (DA) including palaeocanyons on north side of Glen Coe [NN 15 57]. The sills and unconformity dip south- wards and are cut by a strand of the ring-fault system across the crest of the Aonach Eagach (serrated ridge at middle left) and beyond it on the north side. This dip away from the fault towards the inside of the caldera-volcano complex, and similar geometry elsewhere, was originally interpreted as due to subsidence between inward-dipping faults, but is now interpreted as early caldera downsag (see p.87) (P611769).

(Plate 6a) Basal Andesite Sill-complex in Coire nam Beitheach [NN 139 551]. a. Peperitic and autobrecciated top of a sill near the top of the sill stack (P611770).

(Plate 6b) Basal Andesite Sill-complex in Coire nam Beitheach [NN 139 551]. b. Detail showing jigsaw fit of andesite fragments, due to in situ brecciation, within a purple, homogeneous fine-grained sandstone host. Drink can is 6 cm in diameter (P611771).

(Plate 7) Peperitic top of the uppermost sill of the Basal Andesite Sill-complex (BAS), unconformably overlain by thinly bedded sandstone (KHB) that locally forms the base of the Etive Rhyolite Member. This exposure, high on the very steep buttressed slopes north-east of Coire nam Beitheach [142 554], estab- lishes the intrusive nature of the entire succession of andesitic sheets in this vicinity. The absence of coeval andesitic lavas here, together with several other lines of evidence (see text), is taken to indicate that the unconformity could represent a time break of the order of hundreds of thousands of years, or more (P611772).

(Plate 8a) Psammite breccias. a Psammite talus-breccia at the foot of a small fault scarp (out of view to left), overlain by poorly sorted psammite breccias of the Kingshouse Breccias, near the Waterslide on Central Buttress, Stob Dearg [NN 2282 5440] (P611774).

(Plate 8b) Psammite breccias. b Massive to weakly stratified psammite breccias with cross- stratified quartzose sandstone infilling a scour in the Kingshouse Breccias, at the Waterslide [NN 2284 5454] (P611775).

(Plate 9) Poorly sorted breccia and conglomerate (Kingshouse Breccias) dominated by clasts of andesite (purple) and of Rannoch Moor granite (pink), with interca- lated sandstones. These deposits, which show imbrication of the large clasts, form part of a small wedge mainly of unstratified breccia that extends south- westwards from the Northeastern Graben Fault in Cam Ghleann [NN 2484 5181] (P611776).

(Plate 10) Parallel-bedded distal Kingshouse Tuffs, 7 km from the vent, showing layers rich in small accretionary lapilli (centre); F Buttress on Aonach Dubh [NN 1432 5550] (P611777).

(Plate 11a) Indicators of the position of the vent of the Kingshouse Tuffs (KHT). a. Asymmetrical ballistic-bomb impact sag in bedded and cross-stratified silicic tuffs, indicating that the vent was located to the west or south-west (away from the viewer). The impacted beds lie on a surface scoured by the passage of one or more pyroclastic density currents; Great Gully Buttress, Stob Dearg [NN 226 548] (P611778).

(Plate 11b) Indicators of the position of the vent of the Kingshouse Tuffs (KHT). b. Cross-stratified silicic tuffs showing five scour (truncation) surfaces (indicated by arrows); the strata record deposition and migration of low-amplitude ash dunes down-current from the south-west (right to left), alternating with erosion. The uppermost scour surface is locally steep and here was plastered by damp ash, also indicating that the current direction was from the south-west. The dip of the beds, which is towards the south-west, resulted from caldera downsag; Broad Buttress, Stob Dearg [NN 225 549] (P611779).

(Plate 12a) Aqueous litho- facies of the Kingshouse Tuffs. a. View towards the south-west showing the flanks of Sròn na Creise (buttress on right hand side) and Stob a’ Ghlais Choire (middle summit) with an extensive outcrop of the Kingshouse Tuffs, including the thick lacustrine succession formed in a small fault-bounded basin (P611780).

(Plate 12b) Aqueous litho- facies of the Kingshouse Tuffs. b. Flame structures and convolute laminations in silicic tuffs deposited from aqueous suspension (parallel-stratified tuffs) and by turbidity currents (mainly massive division showing near-vertical water-escape structures near its middle); Sròn na Creise [NN 241 525] (P611781).

(Plate 13a) Lower Etive Rhyolite with sparse feldspar crystals; Stob Dearg [NN 220 552]. Although texturally similar to rhyolite lava at outcrop, the overall field relationships indicate an explosive, pyroclastic origin, so that the rock is referred to as lava-like ignimbrite. a Fine parallel flow-lamination (P611784).

(Plate 13b) Lower Etive Rhyolite with sparse feldspar crystals; Stob Dearg [NN 220 552]. Although texturally similar to rhyolite lava at outcrop, the overall field relationships indicate an explosive, pyroclastic origin, so that the rock is referred to as lava-like ignimbrite. a Fine parallel flow-lamination (P611784). b Convolute flow-lamination and a rare lithic fragment (arrow) (P611785).

(Plate 14a) North face of Aonach Dubh [NN 15 56] and northern side of Glen Coe [NN 16 57]. a. The Upper Etive Rhyolite (UER) becomes thicker across one strand of the Ossian Fault (OF) due to syn-eruptive down-to-the-south-east (to the left) movement; this is opposite to the offset that formed during eruption of the Lower Etive Rhyolite (LER). Ponding of the Lower Three Sisters Ignimbrite (LTS) and thickness change of the Upper Three Sisters Ignimbrite (UTS) are also evident across this fault strand (P611786). BAS Basal Andesite Sill-complex; KHT Kingshouse Tuffs; USA Upper Streaky Andesites

(Plate 14b) North face of Aonach Dubh [NN 15 56] and northern side of Glen Coe [NN 16 57]. b. Traces of the Northeastern Graben Fault (NEGF) and its footwall scarp along the north side of the Pass of Glencoe [NN 16 57]. The scarp, composed of Basal Andesite Sill-complex (BAS), formed a volcanotectonic topographical barrier during emplacement of the Upper Etive Rhyolite (UER), which is ponded against it, as well as forming a subterranean barrier during intrusion of the Lower Streaky Andesites sill (LSA-sill) within the Glencoe Graben. Lower Streaky Andesites lavas, which form much of the ridge crest from Am Bodach to the Aonach Eagach, were extruded onto the footwall block outside of the graben. The talus cone in the lower left of the view is the largest and most active in the vicinity (P611787). CRT Crowberry Ridge Tuffs; OF Ossian Fault

(Plate 15a) Lower Streaky Andesites on Gearr Aonach and Aonach Dubh. A. Typical exposure of the Lower Streaky Andesites showing purple andesite streaked with rhyolite. Both rock types contain up to 5 per cent of plagioclase phenocrysts; Gearr Aonach [NN 167 561] (P611788).

(Plate 15b) Lower Streaky Andesites on Gearr Aonach and Aonach Dubh. b. Two of the Three Sisters, Gearr Aonach (left) and Aonach Dubh (right) showing dramatic thickness variation of a Lower Streaky Andesites sill: viewed towards the west. The sill is about 100 m thick on Gearr Aonach and thins to only a few metres over a distance of less than 750 m away from the viewer, adjacent to the Upper Streaky Andesites vent on Aonach Dubh. The sill is the same intrusion as seen in (Plate 14b) (P611789). BAS Basal Andesite Sill-complex; CRT Crowberry Ridge Tuffs; KHT Kingshouse Tuffs; LER Lower Etive Rhyolite; LSA Lower Streaky Andesites; LTS Lower Three Sisters Ignimbrite; UER Upper Etive Rhyolite; USA Upper Streaky Andesites; UTS Upper Three Sisters Ignimbrite

(Plate 16a) Lower Three Sisters Ignimbrite showing well-developed eutaxitic texture (welding), with moderately abundant fiamme and small lithic clasts. The contrast between the non-silicified matrix (a) and the silicified matrix (P611790)

(Plate 16b) Lower Three Sisters Ignimbrite showing well-developed eutaxitic texture (welding), with moderately abundant fiamme and small lithic clasts. The contrast between the non-silicified matrix (b) records variable hydrothermal alteration; Sròn na Creise [NN 239 521] (P611791).

(Plate 17) East flanks of Sròn na Creise and Stob a’ Ghlais Choire showing slumping of the Upper Etive Rhyolite (UER) towards the Chasm step-fault system and related thickening of the Lower Three Sisters Ignimbrite (LTS) towards the axis of the Glencoe Graben. Also shown are breccia dykes in the Dalradian metamorphic basement (DA) along the trace of Northeastern Graben Fault, the faulted margin of the lacustrine facies of the Kingshouse Tuffs (KHT; extreme right), a topographical depression eroded deeply into Lower Etive Rhyolite (LER) and partially draped by Crowberry Ridge Tuffs (CRT), the Glas Choire Sandstone Member (GCS) in the vicinity of its type locality, and the overlying Bidean nam Bian Andesite Member (BBA) (P611792).

(Plate 18) Gearr Aonach viewed from the east across Coire Gabhail [NN 16 55] showing ponding of the Lower Three Sisters Ignimbrite (LTS) within a downsag north-west of the Queen’s Cairn Fault. The ignimbrite is up to 50 m thick along the downsag axis (which trends north-west along the Glencoe Graben) and it thins progressively towards the south-western graben hinge, as does the overlying sill of the Upper Streaky Andesites (USA). The Upper Three Sisters Ignimbrite (UTS) shows similar thinning relationships, but overlaps the hinge line. The large composite debris cone in the foreground was formed by catastrophic rockfall following deglaciation (see p.109) (P611793). LSA Lower Streaky Andesites; UER Upper Etive Rhyolite

(Plate 19a) Upper Three Sisters Ignimbrite in Coire nam Beitheach [NN 142 548]. a. Stratification and inverse grading of lithic tuff at the base of the ignimbrite; the substrate is Lower Etive Rhyolite (right-hand side). Scale intervals are 5 cm (P611794).

(Plate 19b) Upper Three Sisters Ignimbrite in Coire nam Beitheach [NN 142 548]. b. Mesobreccia layer including large matrix-supported blocks derived from the Basal Andesite Sill- complex (located with arrows) and Lower Etive Rhyolite. (See also (Figure 14)) and (Figure 21) (P611795).

(Plate 20) Upper Streaky Andesites agglomerate forming part of the infill of the vent exposed in the north-eastern shoulder of Aonach Dubh (see (Plate 14a) and (Plate 15b). The various angular lithic fragments are intensely altered (P611796).

(Plate 21a) Glas Choire palaeocanyon, infills of sandstone and conglomerate, with overbank siltstones. a Undulose-stratified pebbly sandstone within the palaeocanyon [NN 241 514] (P611797).

(Plate 21b) Glas Choire palaeocanyon, infills of sandstone and conglomerate, with overbank siltstones. b Cobbles and boulders of metamorphic basement and Rannoch Moor granite in poorly sorted conglomerate that infills channels within the palaeocanyon [NN 241 514] (P611798).

(Plate 21c) Glas Choire palaeocanyon, infills of sandstone and conglomerate, with overbank siltstones. c Planar-bedded and laminated siltstone and fine-grained sandstone showing normal grading and loading-related soft-sediment deformation, interpreted as overbank deposits; Stob a’ Ghlais Choire [NN 2418 5168] (P611799).

(Plate 22a) and b Bidean nam Bian Andesite Member at Stob Coire nan Lochan [NN 148 549]; this columnar jointing persists through more than 200 m thickness with no apparent discontinuity, so that the sheet appears to be a single cooling unit (P611800) and (P611801).

(Plate 22b) a and b Bidean nam Bian Andesite Member at Stob Coire nan Lochan [NN 148 549]; this columnar jointing persists through more than 200 m thickness with no apparent discontinuity, so that the sheet appears to be a single cooling unit (P611800) and (P611801).

(Plate 23) View (towards the north-west) illustrating the trace of the ring-fault and fault-intrusions between Cam Ghleann (foreground) and Stob Mhic Mhartuin. The western outcrop of the Rannoch Moor Pluton is also seen, and part of the Northeastern Graben Fault-zone is represented by the breccia dykes that cut the Dalradian metamorphic basement. Pleistocene till with patches of peat and alluvium cover much of the lower ground, which lies about 250 m above sea level (P611802).

(Plate 24) The ring-fault at An t-Sròn and on Stob Coire nam Beith (background) (viewed towards the south) is traceable in 1 km of vertical relief and shows at least 500 m of vertical displacement of the volcanic succession and Basal Andesite Sill-complex. The deep gully formed along the fault is known as The Chasm of An t-Sròn; the other prominent gully to the right marks a related fracture that lies along a planar projection (dotted line) of the ring-fault plane that traces through the lower ground. (A complementary fracture that is a planar northwards continuation of ring-fault plane in An t-Sròn occurs on the valley side behind and to the right of the viewer) (P611803).

(Plate 25a) Main Fault at Stob Mhic Mhartuin [NN 2082 5742]. a The fault-plane exposed here dips outwards (away from the volcano complex) at about 63°; view is towards the south-east (P611804).

(Plate 25b) Main Fault at Stob Mhic Mhartuin [NN 2082 5742]. b Detail of the Main Fault zone (see a). The banded breccias are some 25 cm thick and show a general increase in microbrecciation and streaking with flinty crush-rock towards the main band of crush-rock and the porphyritic rhyolite. Despite the seemingly straightforward succession of zones, detailed study shows that there has been substantial mixing of components (P611805).

(Plate 26a) Photomicrographs of polished thin sections of flinty crush-rock and porphyritic rhyolite in the ring-fault zone at Stob Mhic Mhartuin [NN 2082 5742]. Fragments of quartz appear white and feldspar phenocrysts (f ) are indicated. a. Textures recording intimate fluid-state interlamination and cross-mixing of solids between melts that formed flinty crush-rock (dark and prevalent on right-hand side) and porphyritic rhyolite (pale and prevalent on left-hand side). Fragments of quartz, appearing white and with smaller grains showing rounding, occur mainly in the crush-rock component but are also embedded in the rhyolite (far left). Feldspar phenocrysts of the rhyolite are heavily altered and broken; an original cluster appears to have been attenuated into the flinty crush-rock by laminar flow (middle). Field of view is 3 mm wide: plane-polarised light (P612385).

(Plate 26b) Photomicrographs of polished thin sections of flinty crush-rock and porphyritic rhyolite in the ring-fault zone at Stob Mhic Mhartuin [NN 2082 5742]. Fragments of quartz appear white and feldspar phenocrysts (f ) are indicated. b. Textures recording various degrees of mingling of original melts. The flinty crush-rock component predominates on the left-hand side (dark with numerous quartz fragments appearing white) and contains isolated feldspar crystals of uncertain origin. Rhyolite forms the pale streak in the middle, which is flanked by intimately mingled (finely interlaminated) rhyolite and crush-rock. Fragmented quartzite forms the bright band on the right-hand side. Field of view is 3 mm wide: plane-polarised light (P612386).

(Plate 26c) Photomicrographs of polished thin sections of flinty crush-rock and porphyritic rhyolite in the ring-fault zone at Stob Mhic Mhartuin [NN 2082 5742]. Fragments of quartz appear white and feldspar phenocrysts (f ) are indicated. c Two lithic fragments (arrowed) of granophyric quartz–K-feldspar intergrowths contained in groundmass of (porphyritic) rhyolite; closely similar granophyric textures occur in a nearby xenolith of granite enclosed in the fault-intrusion. Field of view is 3 mm wide: cross-polarised light (P612387).

(Plate 27) Glen Coe, looking westwards. Note the U-shaped cross profile of this classic glacial trough (P000731).

(Rear cover)

(Front cover) Glen Coe viewed towards the south-west from The Study (Photographer: B P Kokelaar) (P611763).

(Frontispiece)Viewed due east from the summit of Bidean nam Bian (1150 m), the successive ridges of Beinn Fhada, Buachaille Etive Beag and Buachaille Etive Mòr provide serial sections through the volcanic succession of the Glencoe caldera volcano. The prominent summit in the distance (middle) is of Stob Dearg (1022 m), which presents remarkable exposures of three successive volcanic cones formed during powerful explosive interactions of magma with water. Beyond Stob Dearg lies the desolate Rannoch Moor, mostly underlain by a granitic pluton that was unroofed just before the volcanism at Glen Coe (Photographer: B P Kokelaar) (P611764).

Tables

(Table 1) Stratigraphy of the Neoproterozoic Dalradian Supergroup in the area of Glen Coe (after Bailey, 1960; Treagus, 1974; Hickman, 1975). Thickness data, from Hickman, are maxima for the area.

(Table 2) Lithostratigraphical and lithodemic nomenclature used in the Glencoe Caldera-volcano Complex.

Tables

(Table 1) Stratigraphy of the Neoproterozoic Dalradian Supergroup in the area of Glen Coe (after Bailey, 1960; Treagus, 1974; Hickman, 1975).

Thickness data, from Hickman, are maxima for the area.

References