Grampian Highlands Field Guide: Day 4 - Tomintoul area
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Author: J R Mendum, BGS
Day 4 – Tomintoul Area
Aims: to take a brief tour around the geology of the eastern Grampian Highlands; to examine gneissose pelitic rocks of the Grantown Formation (Grampian Group) and elements of the Appin Group succession in the Bridge of Brown–Tomintoul area; to view the erosional and depositional glacial features of Glen Avon; to visit the ‘Boulder Bed’ in the Muckle Fergie Burn section.
In response to the request for a more relaxed but scenic day as part of the week’s excursion a coach-based trip to view elements of the geology and scenery of the Tomintoul area was organised. The route follows the A9 up to Aviemore, and thence takes the A95 to Grantown on Spey via Dulnain Bridge. From Grantown we proceed along the A939 to Tomintoul, stopping at several geological localities. From Tomintoul the plan is to walk up part of Glen Avon. The return trip is by the A939 via the Lecht, Gairnshiel Bridge, then over to Deeside and Braemar, and over the Cairnwell to Glen Shee (A93). This itinerary takes us across the southwest part of the Northeast Grampian Highlands where aspects of the geology differ considerably from the central part (Figure 4.1). For a recent summary of the geology and descriptions of the designated GCR sites in this area see Stephenson et al. (2013b).
Geologically, the A9/A95 between Blair Atholl and Grantown passes through a long section of the Grampian Group (the Strathtummel and Cromdale basin successions), sadly much of which is poorly exposed, and scenically and geologically somewhat boring. We will visit one of the more exciting parts by Dulnain Bridge, and then move on to look at Grampian-Appin Group transition rocks around Bridge of Brown, and parts of the Appin Group sequence at Bridge of Avon and the Creag Chalcaidh Quarry. Around Tomintoul the Appin Group is well represented (Figure 4.2), although parts of the sequence are condensed. Note that in this part of the East Grampians bedrock is mainly exposed in burn and river sections, or in artificial excavations (e.g. quarries, road cuts). The regional outcrop pattern defined by the Grampian, Appin and Argyll group rocks here forms a large-scale ‘kink’, marked by a change in strike from northwest to northeast, a structure informally termed the ‘Knee bend’ (Figure 4.1). It extends from Speyside to Deeside and is reflected in both the stratigraphy and structure; its nature and effects will be discussed. The Dalradian sequence is largely right way up, and regionally it dips and youngs eastwards (Figure 4.3). Note that we are on the western fringe of the Buchan Block, an area underlain by different basement rocks to the rest of the Highlands, which during the Grampian Event experienced a different structural history and high temperature but low pressure metamorphism (‘Buchan’ type). In addition the area was intruded by numerous Early Ordovician mafic-ultramafic plutons and crustal-derived S-type granitic plutons. These features impinge in part on the nature of the Dalradian sequence, and the structural and metamorphic patterns in the Tomintoul–Dufftown–Keith area.
From near Tomintoul we will walk up part of Glen Avon through conglomerates of the Devonian outlier to the Muckle Fergie Burn where the ‘Bounder Bed’ lies within a sequence of psammites, semipelites, metalimestones and metadolostones (termed the Auchnahyle Formation), interbedded with both intrusive and extrusive mafic rocks. This mixed and variably developed sequence at the base of the Islay Subgroup is overlain by the Kymah Quartzite Formation, a unit that can be traced along most of the Dalradian outcrop. It is the lateral equivalent of the Slieve Tooey, Jura, Schiehallion, Creag Leacach, and Durn Hill quartzites. It passes up into a more mixed Easdale Subgroup sequence, poorly exposed east of Tomintoul, but which on upper Donside contains psammite and semipelite units with locally prominent metalimestones. A graphitic pelite unit, termed the Glenbuchat Graphitic Schist Formation, forms a prominent marker that can readily be correlated with the Easdale Slate, Ben Eagach Schist, and Glas Maol Schist formations. Further east on Donside, the Dalradian succession is disrupted by the Portsoy Shear Zone, here coincident with the western edge of the Morven-Cabrach Gabbroic Pluton. This large layered mafic-ultramafic intrusion is Early Ordovician in age, and part of the Northeast Grampian Basic Subsuite. The pluton has intruded the surrounding partly gneissose upper Argyll Group rocks. The sequence passes eastwards up into the turbiditic Southern Highland Group rocks in mid-Aberdeenshire (Figure 4.1).
A short stop will be made to inspect the roche moutonées on the eastern fringe of Dulnain Bridge at [NJ 0020 2500]. These glaciated ‘tumps’, which clearly show that ice flow was northeastwards (towards 054°), are composed of gneissose semipelite with abundant quartzofeldspathic segregation pods and veins and some pegmatitic leucotonalite and granite veins. The semipelite unit is part of the Grantown Formation, a mixed sequence of gneissose semipelites and pelites, metalimestones, calc-silicate rocks, and subsidiary psammites and quartzites (Highton, 1999). In parts, notably farther north on Laggan Hill [NJ 004 267], the pelites are notably kyanite-bearing, testifying to the aluminous nature of the original mudstone. The Grantown Formation is the lowest unit of the Grampian Group exposed in this area. It is overlain by the Nethybridge Formation, a predominantly psammite unit with subsidiary semipelites, and calc-silicate ribs. The Grantown Formation is also recognised at Ord Ban, 3 km south of Aviemore, and may be laterally equivalent to the lithologically similar Kincraig Formation, which forms the basal unit of the Grampian Group farther southwest. However, its relative stratigraphical position and links to the Corrieyairack Basin succession (Day 5) are unclear.
Bridge of Brown
Just beyond Grantown we turn southeast onto the A939, the course of which mostly follows the old Wade military road that linked Blairgowrie to Fort George via Braemar and Grantown. It was one of the last major ‘Wade’ roads constructed, being built between 1752 and 1755. Construction was overseen by Major William Caulfeild (1698–1767), who succeeded General George Wade (1673–1748) in 1740. Caulfeild, of Irish birth like Wade, became Inspector of Roads in 1732 and carried on in this position until his death. Whereas Wade supervised the building some 250 miles of Highland roads (+ 40 bridges and 2 forts), Caulfeild was responsible for the construction of some 800 miles (+600 bridges).
The Hills of Cromdale to the north are underlain by psammites and quartzites of the Tormore and Nethybridge formations (Grampian Group). The presence of thick quartzite units has enabled a more detailed local stratigraphy to be erected. These more competent units have also acted as a locus for the development of kilometre-scale WNW-vergent folds (F2), which dominate the structure, both here, and to the south in the Braes of Abernethy.
By the Bridge of Brown [NJ 1241 2063] a section that spans the Grampian-Appin Group boundary is exposed in the Burn of Brown (Figures 4.2, 4.4), which here is incised into the bedrock. The beds dip SSE at 30° to 40°. The presence of sedimentary structures (cross-bedding, slump structures) in the Grampian Group psammites, the overall lithostratigraphy, and the lack of any significant folding, all suggest that the sequence is non-inverted and youngs southeastwards. The Grampian Group rocks, exposed north of the Bridge of Brown, consist mainly of feldspathic psammites with subsidiary micaceous psammite, semipelite and some quartzite units. They belong to the Strathavon Psammite member, the youngest unit of the Tormore Psammite Formation, which shows marked facies variations in this area. Bed thicknesses decrease and the semipelitic component increases upwards, and just north of the Bridge of Brown we pass up into a flaggy, thin-bedded to laminated, dominantly semipelite and micaceous psammite unit. This is the Dalvrecht Slate Formation, the basal unit of the Lochaber Subgroup. Deformation has been focussed along the Grampian Group-Appin Group boundary here, resulting in some attenuation and increased flagginess. In thin section up to three separate cleavages can be recognized. However, there is no evidence for a major dislocation. The semipelitic beds show some development of garnet and biotite porphyroblasts. Thin lenticular bands of calc-silicate rock can be found upstream of the present road bridge and these become more abundant around the old Wade Bridge, where green-grey micaceous psammites and semipelites crop out. Upstream from the Wade Bridge we pass into more uniform pale to mid grey-green, fine-grained calcareous semipelite and highly micaceous psammite, with abundant white and green calc-quartz pods (Figure 4.4). Calc-silicate pods overgrow bedding features indicating their diagenetic (+ metamorphic) origin. Darker green amphibolitic units, probably originally marl beds, are also present. These tremolite-rich lithologies, here termed the Fodderletter Calcareous Flag Formation, can be traced from the upper part of Glen Avon, northwards to the Banffshire coast.
Upstream at [NJ 1266 2034] a c.5 m thick massive gneissose semipelite unit, the Fireach Beag Kyanite Gneiss Member (Figures 4.2, 4.4), forms a small waterfall. This distinctive lithology, which contains contains blue-grey kyanite laths up to 2 cm long and abundant quartz and quartzofeldspathic segregations veins and pods, lies within the calcareous flag sequence. In thin section well-formed muscovite laths, intergrown with more ragged biotites, enclose coarse-grained aggregates of quartz and plagioclase (oligoclase). Kyanite occurs in laths up to 5 mm long, partly altered to fine-grained muscovite, and fractured staurolites up to 1mm long are present. However, the garnets have been altered here to quartz-feldspar-biotite-ilmenite aggregates. Ilmenite is an abundant accessory mineral, with rutile, apatite and tourmaline also present. Retrograde chlorite is locally developed. This distinctive unit can recognized throughout the Glenlivet district; a similar lithology is also present on the Banffshire coast.
Bridge of Avon
From the Bridge of Brown the A939 traverses eastwards up over the shoulder of Tom nam Marbh before turning southeast and descending to cross the River Avon at a new bridge (built 1990) by Kylnadrochit Lodge. We turn back to park by the Old Bridge (built 1754) at [NJ 1501 2013]. The sequence in the River Avon here looks relatively simple with the bedding dipping moderately to steeply southwards (upstream). However, there is evidence from the mapped outcrop pattern and detailed structures that the Appin Group succession has been folded into kilometre-scale NW-vergent folds and then subjected to refolding (Figures 4.3, 4.5). In addition, the trace of a N–S trending sub-vertical fault is coincident with the line of the river directly beneath the bridge. The fault offsets the sequence sinistrally by about a hundred metres. The stratigraphy is condensed, probably representing deposition over an original basin high. The Ballachulish Subgroup sequence comprises the graphitic pelite of the Mortlach Graphitic Schist Formation, overlain by the Corriehabbie Quartzite Formation and the Ailnack Phyllite and Limestone Formation (Figure 4.2). The overlying Inchrory Limestone Formation belongs to the Blair Atholl Subgroup. Note that although the lithologies are folded and have been metamorphosed to kyanite grade (lower amphibolite facies), locally they still show sedimentary features, e.g. cross-bedding in the quartzite and grading in some of the semipelites.
The Mortlach Graphitic Schist Formation lies in the core of a major antiform here (Figures 4.3, 4.5). It is poorly exposed in the river section downstream of the bridge, mostly on the west bank. It consists of dark grey to black, schistose graphitic pelite and semipelite, with small garnets locally abundant. Pyrite and quartz veining are common. In thin section kyanite and staurolite porphyroblasts can be seen. There is a rapid transition up into the Corryhabbie Quartzite Formation, a white to fawn, blocky, commonly indurated, fine- to coarse-grained quartzite and psammite unit. Typically, it is thin- to medium-bedded with some gritty feldspathic basal zones in individual sand packets. Cross bedding is present locally, typically defined by heavy-mineral streaks (magnetite/hematite). Around Bridge of Avon the quartzite formation is only about 55 m thick, but it thickens to over 250 m a few kilometres to the north-east and farther south in the Water of Ailnack section. The quartzite is a succeeded by the Ailnack Phyllite and Limestone Formation, represented here mainly by semipelites. Note that elsewhere in the Glenlivet district this formation is considerably thicker with psammite, calc- silicate and metalimestone units that have been allocated member status. Here, in its basal part is a c.5 m-thick almost pure white metalimestone, the Torulian Limestone Member, which forms a prominent feature, both in the river and on the east bank section. This calcitic metalimestone unit serves as a marker to illustrate the fault offset. It shows poorly defined thin banding and some pyrite-rich laminae. Where etched by the river, small-scale tight to isoclinal minor folding can be seen. As the fold axial planes are bedding-parallel, and folds are confined to individual layers, they have been interpreted as slump folds, but may be F1 structures. The unit was probably deposited as a chemical precipitate, rather than as a biochemical deposit. The metalimestone is succeeded by phyllitic semipelite and pelite, in parts graphitic, and by thinly banded calcareous semipelite, calc-silicate-rock and impure metalimestones. These latter lithologies show good examples of F3 minor folding (Figure 4.6), well-exposed directly beneath the Old Bridge. Up-river the sequence is dominated by semipelites that constitute the upper part of the Ailnack Phyllite and Limestone Formation, which here is distinctive enough to be termed the Kylnadrochit Semipelite Member.
Creag Chalcaigh Quarry
Returning to the A939 we traverse southeast for a mere 2 km to our next stop, the long-disused Creag Chalcaigh Quarry (parking at [NJ 1558 1937]). Geologically, we have moved up-sequence from the Kynadrochit Semipelite Member into the overlying Inchrory Limestone Formation, the main metalimestone unit of the Blair Atholl Subgroup (Figure 4.5). Unlike other metalimestone units, this formation retains its lithological characteristics over most of its extensive Dalradian outcrop. In this Campdalmore area, the formation appears to be over 400 m thick, but in the quarry tight to isoclinal minor and medium-scale F2 folds and more open F3 folds are discernable. The overall outcrop pattern also suggests some fold repetition; thus, its true stratigraphical thickness is estimated to be closer to 150 m. The formation consists of flaggy to massive, fine- to coarse-grained (typically 2 mm grain size), crystalline metalimestone, normally thinly to thickly bedded or banded. Finer grained variants are mid- to dark grey, whereas the coarser grained metalimestones are pale bluish grey, commonly almost translucent. Laminae and thin interbeds of graphitic pelite are common and pyrite is also abundant. Minor thin siliceous, locally cherty, bands are present and thicker calcareous semipelite interbeds are also seen. Calcite veining is common, and adjacent to faults the metalimestone is typically recrystallized. In this area dark grey-green amphibolite lenses and pods are present in the Inchrory Limestone Formation and underlying Kylnadrochit Semipelite Member. They appear to cross cut bedding locally but are strongly deformed and typically boudinaged. They represent original dolerite or basalt intrusions, but many show metamorphic reaction rims with the adjacent metalimestone. It is unclear as to whether they were intruded early in the geological history, possible coeval with volcanic units in the Argyll Group, or whether they link to the Early Ordovician Morven–Cabrach Pluton. Pods are present on the north-west face of the quarry at [NJ 1554 1944]. A view up Glen Avon can be obtained by walking up a ramp on the southwest edge of the quarry.
From the quarry we pass on through Tomintoul, the highest village in the Highlands (345 m above OD), laid out in grid pattern by Alexander, 4th Duke of Gordon in 1775. At the southeast end of the village a single-track road leads southwest into Glen Avon with an off-road parking area at [NJ 1648 1766]. The intention is take the good track southwards from here that descends gently into Glen Avon, meeting the main tarmac road (private access) by Delavorar. We then continue up the glen to Birchfield and the Muckle Fergie Burn, a significant eastern tributary of the River Avon (in total 4 km walk). In the lower steeply incised part of the burn section are outcrops of pillowed mafic lavas, overlain upstream by interbedded psammites and semipelites with metalimestones, metadolostones and including the ‘Boulder Bed’ (Figure 4.7). After viewing these outcrops, a shorter return route is via Auchnahyle, back to Birchfield, and thence reversing our route back to the car park. Note that the brief detour to the Queen’s Cairn, only a short distance from the car park is also recommended (Figure 4.8).
In the first section the track traverses through rather poorly exposed Old Red Sandstone conglomerates that dip some 40° to 50° to the WSW. The conglomerates, termed the Delnabo Conglomerate Formation, form the main part of the Tomintoul Outlier, a sizeable remnant of a formerly more widespread, post-orogenic, red-bed cover of either Late Silurian or Early Devonian age (Figure 4.7). These coarse clastic sediments were deposited in a fault- controlled basin in large alluvial fans on the eroded remnants of the Grampian orogenic belt. The higher sandstone and siltstone units are fluvial and lacustrine. The unconformable base of the sequence is well exposed in the lower part of the gorge of the Water of Ailnack, above Delnabo. The lithologies, position, exact age, paleogeography and significance of the outlier all raise questions. Note that its western margin now lies in the ‘crook’ of the ‘Kneebend’.
The valley of the Avon is partly controlled by a series of intersecting faults that extend northwards from the eastern margin of the Cairngorm Pluton. However, although individual faults show variable offsets, the mapped fault system does not significantly displace the Dalradian stratigraphy or the Old Red Sandstone cover either laterally or vertically (Figure 4.7). The Quaternary history of the area has exerted perhaps the major control on the present day geography and geomorphology of this area. The present course of the River Avon is a product of river capture with the former headwaters of the River Don (above Inchrory) having been diverted northwards. However, Linton (1954) argued that breaching of the Don- Avon watershed was initially achieved by ice flowing northwards from the Cairngorms ice sheet (notably from Ben Avon and Beinn a’ Bhuird) and that the topography has been modified subsequently by glacial meltwater activity linked to overall eastward drainage from ice-dammed lakes that developed in the Spey valley and adjacent catchments during the later phases of Late Devensian glaciation. In contrast, the 7.5 km-long gorge of the Water of Ailnack is a spectacular example of the power of glacial meltwater alone — note that Linton (1954) recorded possible glacial lake strandline features in Glen Loin at c.580 m above OD. Linton also noted the presence of sands and gravels perched high (up to 75 m) above the floor of the present valley of the Avon near Dalestie at around [NJ 166 112]. He linked these to a small ice-dammed lake in the valley of the Burn of Little Fergie, at the head of which lies a further meltwater channel (‘The Eag’). Patently, at this time the Avon valley was still occupied by ice. The 200 m high cliffs in Glen Avon that extend for 3.5 km north of Inchrory [NJ 179 081] testify to the influence of ice breaching, and subsequent glacial meltwater activity and fluvial incision. The present valley of the Avon around Delavorar [NJ 1672 1586] and Birchfield [NJ 1661 1489] has a wide floodplain, and it is only when the profiles of the tributary burns and the terraced areas some 50 m above the valley floor are considered that the amounts of glacial and recent down-cutting become apparent.
Our next geological stop is in the lower part of the Muckle Fergie Burn section. The burn cuts across the strike of bedding in the Auchnahyle and overlying Kymah Quartzite formations, the lowest units of the Islay Subgroup (Argyll Group), which here dip eastwards at 30° to 70° eastwards (Figures 4.7) The underlying Glenfiddich Pelite Formation (Blair Atholl Subgroup), composed dominantly of graphitic pelite and semipelite, in part calcareous, appears to pass up rapidly into the Auchnahyle Formation, which consists of psammites and semipelites with thin quartzite, metalimestone and metadolostone units (Figure 4.9). Within this sequence lies the ‘Boulder Bed’, interpreted as a metadiamictite of glacial origin. Amphibolite units occur at the base of the Auchnahyle Formation and also at several places higher in the sequence; most appear to be metadolerite sheets (commonly sills) of intrusive origin. However, at [NJ 1657 1401] a prominent coarse- to medium-grained amphibolite shows internal structures resembling pillows with radial cracks and a crude textural concentric zonation (Figure 4.10). Chew et al. (2010) presented geochemical plots that show it is a tholeiitic sub-alkali basalt, enriched in Fe, Ti and high field strength elements such as Nb, Zr and Y compared to the younger metavolcanic mafic rocks of the Delnadamph Volcanic Formation (Easdale Subgroup), which crop out higher up the burn.
The succession in the Muckle Fergie Burn above the ‘pillowed’ mafic lavas is significant and hence is documented in detail below. However, as access along the burn is awkward in places (particularly for a larger party), I intend to skip up the sequence to merely access outcrops of the ‘Boulder Bed’, firstly by skirting above the wooded gorge, and then by descending a short but steep grassy bank back down to the burn higher up.
The ‘pillowed’ metavolcanic unit is succeeded by micaceous and feldspathic psammites with some siliceous psammite beds. Cut-offs and grading imply that the sequence youngs eastwards. They are intruded locally by a 4m-thick sheet of foliated metadiorite. At [NJ 1655 1399] a 4m-thick fine-grained metalimestone crops out and in the following psammite-semipelite section (c.15 m thick) graphitic pelite beds and thin metalimestones are present. A c.6 m-thick, cream to fawn weathering metadolostone unit follows, overlain by a c.5 m-thick grey-green, unbedded, chloritic and locally amphibole-bearing highly micaceous psammite containing sub-angular to sub-rounded, rusty brown stained, dolostone clasts. A thin psammite bed separates this lower diamictite unit from a 5–7 m thick upper unit, which contains moderately abundant granitoid and dolostone clasts in a more purplish-grey matrix (Figure 4.11). Pyrite is common in the metadiamictite (‘tillite’) units. Upstream, in the psammite-dominated sequence is a hard, indurated, almost glassy quartzite, again containing significant pyrite. These lithologies, interpreted as glacial in origin, lie in a similar stratigraphical position to the Port Askaig Tillite Formation and its correlative units. The unit passes up into the Kymah Quartzite Formation (Figure 4.9). Interestingly, elsewhere, for example in Upper Donside, metadiamictite units lie within the basal part of the Kymah Quartzite. Note that in many parts of the Glenlivet and Glenbuchat districts the Auchnahyle Formation is absent.
From Tomintoul we take the A939 southeast up the Conglass Water and to the Lecht. We pass the old manganese mine by the Well of the Lecht, and the ski areas accessed from the summit that look somewhat more attractive in wintertime. Although we have now passed through into the Easdale Subgroup (here very poorly exposed) the lithologies and stratigraphy that can be mapped does not correlate well with the typical succession found in Perthshire or even along the Banffshire coast. Here, the sequence is again condensed and lithologies atypical. There is some evidence of both vein-hosted and sedimentary-hosted mineralisation. The route of the A939 over the Lecht also corresponds to a NNW-trending zone of faulting, informally known as the Lecht Lineament. The faulting appears to have occurred mainly during the later stages of Caledonian orogenesis. Nevertheless, the ‘Lineament’ may well reflect earlier fault or shear zones in the basement to the Dalradian sequence, which in turn influenced sedimentation, structural development, mineralization, etc.
We will make a short stop at the parking and viewpoint [NJ 254 098] on the Hill of Allargue above Cockbridge to view the upper part of Donside (+ Corgarff Castle) and the Cairngorms, notably Ben Avon and Beinn a’ Bhuird. The influence of the bedrock geology is apparent in places, notably in areas underlain by Appin Group metalimestones or metacarbonate rocks, which are greener and used as pasture or grazing. Our return southwards will cross parts of the late-Silurian Glengairn and Lochnagar Granite Plutons. These granitoid intrusions form part of a concentration that stretches along Deeside from the Crathes Pluton in the east to the Cairngorm Pluton in the west. Farther south between Braemar and Glen Shee, the road again traverses across the Appin and Argyll Group successions, which here show greater degrees of deformation, tight folding, and shear zone development, compared to the Tomintoul region. Again, sadly, the exposure does not do justice to the geology.