2 The physiography, Cainozoic landscape evolution and Quaternary geology of the district

2.1 Physiography

The district embraces the south-western sector of the Cairngorms, which include some of the highest mountains in Britain, notably Ben Macdui (1309 m), Braeriach (1296 m) and Cairn Toul (1213 m) (Figure 8).

The huge corries and troughs of the Cairngorms contrast with the relatively less glacially modified ground to the south. Networks of subparallel, glacial drainage channels are prominent on the west-facing slopes of the Cairngorms and Glen Tromie where the margin of the last ice sheet retreated towards the western Highlands. The broad valley of the Geldie Water once contained the headwaters of the River Dee, but the catchment has been captured by the northward flowing River Feshie. Likewise, the valley of the Tarf Water to the south now flows into Glen Tilt rather than the Dee. The southern half of the district is dominated by the Gaick Plateau, which is dissected by small corries and a major glacial breach that links Glen Tromie, in the north, with the valley of the Edendon Water in the south. The limited glacial erosion here contrasts starkly with that to the north-west, where powerful, topographically-constrained ice streams following Glen Truim and the Spey Valley have deeply etched and scoured the landscape, forming several prominent roche moutonée.

These mountaintops represent high points on the extensive, deeply dissected Cairngorm Plateau (Hall, 1986; Hall and Sugden, 1987). Running approximately north–south through the plateau is the Lairig Ghru (Figure 9), a deep glacial breach that connects Speyside in the north to Deeside in the south-east. Another near-parallel breach lies to the east linking Strathnethy with Glen Derry to the south.

The Cairngorms are an area of outstanding geomorphological interest (Shaw and Thompson, 2006), comprising an exceptional assemblage of pre-glacial, glacial, glaciofluvial and periglacial landforms and deposits (Gordon, 1993; Glasser and Bennett, 1996). These include planation surfaces, tors and pockets of deeply weathered bedrock that have survived several periods of glaciation, illustrating aspects of longer-term landscape development. It contains a striking assemblage of landforms created by ‘selective glacial erosion’, including vast corries, arêtes and breaches, together with those related to the retreat and decay of glacier ice, including moraines, deep drainage channels and deposits formed in temporary ice-dammed lakes. In addition, the Cairngorms include a wide range of periglacial phenomena, both active and relict, that result from repeated freezing and thawing of rocks and soil under the influence of gravity.

The north-west of the district extends almost to the rim of the extensive Monadhliath Plateau. Between this massif and the Cairngorms lies the heavily glaciated Spey Valley, through which relatively fast-flowing corridors of ice (ice streams) have flowed during several glaciations, scouring away most weathered rocks and laying down extensive blankets of till on the valleysides. The south-facing slopes of the Monadhliath Plateau are characterized by knobbly, mammillated, heavily ice-plucked rock surfaces whereas several large, highly elongate, streamlined roche moutonée features lie in the centre of the vale (Figure 10).

The high ground in the south of the district takes the form of another extensive, gently undulating plateau with isolated higher mountaintops, the altitudes of which range from 800 m to the highest point, Carn na Caim, at 941 m OD. This area includes the Gaick Forest and northern parts of the Forest of Atholl, but is generally referred to as the Gaick plateau or the ‘Gaick’ (Sissons 1974; Hall and Mellor, 1988). The plateau is dissected by a major north-south glacial breach that links Glen Tromie, in the north, with the valley of the Edendon Water, to the south, and is referred to generally as the ‘Gaick Pass’. Though slightly lower (400 m) than the Pass of Drumochter to the west, and a more direct route, the valley was apparently too narrow and tortuous for the railway and main road to follow with the result that it now is a splendid wild, rugged and relatively unspoilt glen. The Gaick Pass contains three lochs, Loch an t-Seilich, in the north, Loch Bhrodainn, in the centre, and Loch an Dùin in the south (Plate 21).

The plateau is best developed between the Gaick and Drumochter passes where it is bounded by steep grassy slopes with 200 to 450 m drops to lower ground. The plateau surface is largely peat covered (Plate 22), but there are also extensive areas of short, grassy turf grazed by deer in the summer months, that have developed on the relatively free-draining regolith.

The plateau is generally lower to the east of the Gaick Pass, where it is bounded to the north by the valleys of the ‘Upper Feshie’ and the Geldie Burn, and to the south by the steeply incised Glen Tilt. The highest point is Beinn Dearg (1008 m), which is capped by a magnificent blockfield formed of granite belonging to the Glen Tilt Pluton (see section 2.6.6.3).

Glen Tilt is a long, straight, fault-guided valley that is unlike most in the region. It is a classical v-shaped valley that displays very little evidence of glacial modification. The steep-sided slopes of the valley are underlain by extensive deposits of talus, but little till. The absence of glacial erosion is partly the result of the area having been protected within a local centre of ice dispersion, and partly because ice flowed across the valley, not through it (Sutherland, 1984). The headwaters of the River Tilt have become deeply incised to form almost impenetrable gorges. Glen Tilt is overshadowed by the towering northern flanks of the Beinn a’ Ghlo massif, which are truncated to the east by Glen Loch, an impressive glacial breach. The steep slopes in this part of the district have foundered extensively.

2.2 Landscape evolution during the Cainozoic era

The geological record during the Tertiary is largely one of slow landscape evolution by subaerial erosion, intermittent regional uplift and the development of deep weathering profiles (see Hall and Sugden, 1987; Hall, 2004 for summary). The Paleocene Epoch was marked by considerable volcanic activity along the western seaboard of Scotland. Uplift of as much as 1.5 km was associated with this event, causing an eastward tilting of peneplains formed during the Upper Cretaceous and earlier. Some valleys in the district, such as the Dee-Geldie Water and Tarf Water, may have become first established on these tilted peneplains. A warm, humid climate prevailed until the late Miocene, during which time the effects of chemical weathering penetrated deeply below the land surface forming clayey saprolites, pockets of which remain in the district. Mechanical weathering became increasingly intense during the Pliocene and early Pleistocene epochs as the climate deteriorated and granular disintegration occurred, particularly of coarse-grained igneous rocks, to form granular saprolites. Subsequent glacial erosion has failed to remove these ancient regoliths over the plateaux of the district, where they locally extend to depths of 10 m or more (Hall, 2004).

Note the tie lines from the isotope and polarity records to the chronostratigraphy.

Many valleys in the district are long-established elements of the drainage system (Bremner, 1942; Hall, 1991, 2004; Linton, 1951). For example, eastward directed valleys such as the upper reaches of the Spey and the Geldie Water/Dee appear discordant to regional structures, implying superimposition. The River Feshie has captured the former upper reaches of the Geldie Burn where it changes abruptly in direction at a point close to the watershed (and county boundary) separating the principal catchments of the rivers Spey and Dee (Linton, 1949). The eastern part of the plateau is drained by the Tarf Water, which was also a former tributary of the River Dee, but has been captured by the River Tilt at the Falls of Tarf, and now forms part of the catchment of the River Tay.

The repeated glaciations of the Quaternary have considerably modified the pre-glacial topography of the district by widening, straightening and deepening pre-existing river valleys, breaching watersheds and carving out corries (Hall and Jarman, 2004). The imprint of the last major glaciation (Main Late Devensian, MLD) (Figure 11) is the strongest, but many features of glacial erosion must partially owe their existence to earlier events and to the ‘average’ glacial conditions that pertained for most of the Quaternary Period (Porter, 1989; Sutherland and Gordon, 1993). Despite considerable Pleistocene erosion, large expanses of high, gently undulating, pre-Pleistocene erosion surfaces remain relatively intact, both on the Gaick and the Cairngorm plateaux (Hall, 1986; Hall and Sugden, 1987). During the major glaciations relatively fast-flowing, warm-based ice streams flowed around the Cairngorm and Gaick plateaux causing considerable subglacial erosion in Strathspey, Glen Truim and Glen Garry, whereas sluggish cold-based ice on the adjoining plateaux had little erosive power (Sugden, 1968).

2.3 The Quaternary record

2.3.1 Pre-Late Devensian events

Some fourteen climato-stratigraphical stages of alternating glacial (cold) and interglacial (temperate) conditions are recognised in the British Isles (Gibbard et al., 2005) (Figure 11), but no deposits of the last (Ipswichian) interglacial or older periods have been positively identified in the district, although weathered remnants of a formerly more extensive till sheet that has been largely eroded away during the MLD glaciation have been found (Pattack Till Formation; Ailleag Diamicton Member). When compared to the longer sequences found in the Inverness area, where intervening interglacial and interstadial deposits have been located (Fletcher et al., 1996), the colour, iron staining and weathered condition of many clasts in these remnants suggests that they have been exposed to full interglacial conditions and are pre-Devensian in age. No Early or Mid-Devensian deposits have been identified in the district, which might indicate that the last ice sheet built up relatively early.

2.3.2 Late Devensian

Most of the glacigenic deposits in the district were laid down during the Main Late Devensian (MLD) glaciation which began between 32 and 28 ka BP. There is growing evidence from north- west Europe that the Last Glacial Maximum (LGM) occurred relatively early in this glaciation, before 22 ka BP (Bowen et al., 2002: Bradwell et al., 2008), when the region was overwhelmed entirely by ice (Sutherland and Gordon, 1993; Boulton et al., 2002). A period of glacial retreat followed before significant readvances occurred after 18.4 ka BP, particularly involving coastal ice streams. In north-eastern Ireland, high-precision radiocarbon dating on foraminiferids contained within glaciomarine muds indicates that a period of ice retreat between 16.7 and 14.7 ka BP preceded another significant readvance at c.14 ka BP (McCabe et al., 1998). The latter event was possibly accompanied by significant readvances in the district (Everest and Kubik, 2006).

The MLD glaciation probably first witnessed the expansion of ice fields and glaciers in the mountains of the Western Highlands. As the ice advanced eastwards, it blocked some suitably orientated valleys causing ponding, such as in the upper reaches of the rivers Pattack and Mashie (Merritt, 1999). Eventually the deltaic and glaciolacustrine sediments (Ceardaich Sand and Gravel and Linn of Pattack Silt formations respectively) deposited in these lakes were overridden and largely reworked into diamictons of the Ardverikie Till Formation beneath the advancing ice. A similar passage of events occurred to the south of the Gaick plateau, where ice first blocked the lower reaches of southward draining valleys causing ponding before it overrode the deposits and impinged against the plateau (Merritt, 2004d). The absence of far-travelled erratics within the Cairngorms and eastern Gaick indicates that local ice caps had developed there, at least, before the district became overwhelmed by ice sourced from the west (Bremner, 1929; Sissons, 1976). Strathspey ice carried erratics of psammite to elevations of up to 800 m on the flanks of the northern Cairngorms (Sugden, 1970).

The entire district was probably overtopped by ice by the LGM (Figure 12), judging from the evidence of lee-side joint-block removal, glacially transported tor blocks and stripping of blockfields at elevations of up to 1200 m on Ben Macdui, Beinn Mheadhoin and Derry Cairngorm (Hall and Glasser, 2003). A powerful topographically controlled ice stream was centred on the Spey Valley, as evidenced by severely ice plucked and mammillated rocks on the western valley sides overlooking Newtonmore and Kingussie, and the mega-roche moutonnée features of Creagan a’ Choin and Ordan Shios, south of Newtonmore (Plate 23), which have tails of drift stretching in the former up-ice direction. Strathspey ice extended eastwards against the Cairngorm massif and probably penetrated into the catchment of the River Dee via the valley of the Geldie. The Strathspey ice stream was mainly sourced to the west of the district with a feeder draining ice from the Loch Ericht depression via the valley of the River Truim. Ice flowing through the Loch Garry depression impinged on the southern slopes of the Gaick plateau.

The contrast in intensity of glacial erosion between deeply incised troughs and undulating high plateau is exemplified in the Cairngorms (Figure 9), which represent a classic example of such ‘selective linear erosion’ (Sugden, 1968; Rea, 1968; Hall and Glasser, 2003). Ice on the plateaux is believed to have been relatively thin, dry, cold-based and to have flowed predominantly by internal deformation. In contrast, convergent flow within valleys and across cols was wet based, promoting basal sliding and enhancing rates of glacial erosion (Sugden, 1968; Gordon, 1993). Similar contrasts in erosion occur in the Gaick, for example, the deep ‘U-shaped’ glaciated valley of Chama Choire that cuts into the plateau to the west of the Gaick Pass. The mouth of the valley of the Allt Gharbh Ghaig, east of Gaick Lodge, is also ‘U-shaped’, similar to the main glacial breach forming the Gaick Pass. However, it soon becomes ‘V-shaped’ eastwards with interlocking spurs, suggesting that these upper ‘canyons of adjustment’ have experienced minimal glacial erosion (Hall, 2004). This observation is supported by the particularly widespread development of deep weathering profiles hereabout.

2.3.3 The pattern of deglaciation

The general pattern of deglaciation across the district has been established from the distribution and cross-cutting relationships of ice-marginal glacial drainage channels and associated low, sub-parallel moraine ridges, together with lee-side and humped-profile channels on cols, and eskers. The reconstruction shown in (Figure 13) also takes into account sedimentological evidence, clast provenance and the distribution of ice-marginal outwash fans. There is, however, very little chronological control and, despite many years of research, the timing and extent of glacial readvances in the district causes controversy.

There is converging evidence from the Cairngorms that locally sourced ice began to decouple from ice flowing from the main Western Highlands divide via the Strathspey outlet glacier following the LGM, but before the sudden climatic amelioration at the beginning of the Windermere Interstadial (Everest and Golledge, 2004; Everest and Kubik, 2005). An independent plateau ice cap supporting a radial pattern of outlet glaciers developed on the western Cairngorms for at least one millennium. These glaciers retreated actively to form ‘classical’ hummocky moraine, such as in Glen Geusachan. Sissons (1979a), however, had attributed these landforms to the LLR (Figure 14). If Everest and Kubik are correct the Cairngorms may have contained relatively little glacier ice during the LLS, as concluded by Sugden (1970, 1980).

There is abundant evidence in the north-western part of the district for the sequential retreat of the Strathspey outlet glacier, following its evacuation of Rothiemurchus. For example, there is a series of ice-marginal drainage channels, benches, kame terraces and associated lateral moraines that stretch along the northern side of the Spey Valley between Kincraig and Glen Banchor. The features occurring in the vicinity of Loch Gynack, north of Kingussie, are particularly fine examples. The channels descend eastwards at successively lower elevations and were clearly formed at the margin of the retreating glacier. Short-lived ice-marginal lakes formed locally into which meltwater streams laid down deltas; these deposits of sand and gravel now form flat- topped mounds with former ice-contact slopes facing down the valleyside. Evidence from Raitts Burn, 4 km north-east of Kingussie (Phillips and Auton, 2000; Phillips et al., 2007), indicate that the glacier remained active during its retreat westwards.

Following its retreat from Glen Banchor, the snout of the Strathspey glacier appears to have stabilised for a while in the vicinity of the Woods of Glentruim, 6 km east of Laggan, where an assemblage of moraines, kame terraces, eskers and other ice-contact features are preserved. The presence of Strathspey ice hereabout caused meltwaters emanating from a contemporaneous outlet glacier occupying Glen Truim to be diverted via the site of Loch Etteridge. Once the Strathspey glacier had retreated westwards towards Laggan, the Truim was able to take its present course, abandoning the Etteridge route, and in doing so saving the remarkable set of glaciofluvial features preserved there from subsequent fluvial erosion (Walker, 1993).

There is also abundant evidence in the form of ice-marginal drainage channels, benches, kame terraces and associated lateral moraines for the retreat of the south-eastern margin of Strathspey ice, firstly along the eastern slopes of the valley of the River Feshie downstream of Glenfeshie Lodge (Werrity and McEwen, 1993), then subsequently along the eastern slopes of Gleann Chomhraig and the valley of the Allt an Dubh-chadha (Golledge, 2000), to the west of the lodge.

The presence of suites of ice-marginal meltwater channels along the south-facing slopes of the Cairngorms in Glenfeshie Forest suggests that ice sourced from an ice divide over the Gaick had held Cairngorm ice at bay. Ice from both centres had funnelled into the valley of the River Dee in the vicinity of White Bridge. A series of ice-marginal channels that cross the southern slopes of Glen Luibeg most probably mark lateral positions of the combined Dee/Luibeg outlet glacier following its separation from an entirely Cairngorm-sourced glacier in Glen Derry during ice- sheet deglaciation (Golledge, 2003).

It is clear that the Gaick was affected by three discrete masses of ice (Merritt (2004a). The eastern part was covered by a local ice cap that extended south-eastwards into Glen Loch and eastwards towards Glen Shee. North-eastern parts of the Gaick were affected by ice sourced in the Cairngorms, whereas the remainder was over-ridden by ice sourced to the west. A boundary along the north-western margin of the Gaick separated ice flowing from Drumochter and the Loch Ericht basin around the northern rim of the plateau (‘Strathspey’ ice), from ice flowing directly from the West Drumochter Hills and the Loch Garry basin around the south of the plateau (‘Perthshire’ ice). The configuration at the Last Glacial Maximum (LGM) (Figure 12) has been extrapolated taking into account the position and orientation of prominent glaciated valleys, valley-side prisms of till and large-scale glacial gouges.

The eastern Gaick was occupied by a locally sourced ice cap that was centred on some of the lowest ground and extended to the south-west. Locally sourced ice converged with ice flowing from afar, especially through the deep valleys to the west and south-west. As suggested by Sutherland (1984), ice flowed across Glen Tilt, not through it, which explains why this classical ‘V-shaped’ valley shows little evidence of glaciation. Glen Loch and valleys to the south-west of the area were major outlets. There is no evidence that the eastern Gaick was ever over-ridden by ice sourced from outwith the area.

The initial retreat of ice over the eastern Gaick was probably restricted to outlet glaciers within Glen Loch and Gleann Mhor, where there are excellent retreat moraines and associated features. It is speculated that once ice sourced from outwith the eastern Gaick began to retreat to the north, west and south, local ice was able to expand. It funnelled down the main valleys around the periphery and then retreated actively to form the suites of moraines and ice-marginal channels. No distinct terminal moraines have been identified suggesting that these outlet glaciers were short lived. The distribution of ice-marginal drainage channels and associated low moraine ridges of the Gaick plateau Moraine Formation (Plate 24) indicate that the ice continued to retreat concentrically inwards towards the lowest ground in the valleys of the Tarf Water and Bruar, where it finally decayed. It is unlikely that the shrinking mass of ice remained ‘active’, suggesting that the concentric features formed by seasonal ice melt and paraglacial processes around the periphery of cold-based ice, rather than seasonal ice advance. The features are best developed where bedrock is thoroughly decomposed and hence prone to glaciofluvial erosion and slope processes (Merritt, 2004e).

The rest of the Gaick was over-ridden by ice sourced from the west-south-west of the district. It flowed across the western plateau into all the valleys draining into the Gaick Pass, including the lower valley of the Allt Garbh Gaig. Strath Spey ice shrank away from the northern rim of the plateau leaving behind the extensive sets of low benches and shallow ice-marginal drainage channels that are particular apparent in the spring when snow melts on these slopes (Merritt, 2004e, 2004f). As the two ice masses ‘unzipped’, ice flowed from the plateau into the vacated northern corries, firstly into Coire Bhran [NN 805 855], then into the mouth of the central breach. Here an outwash fan was laid down at the northern end of Loch an t-Seilich [NN 762 875] (Merritt, 2004b). Subsequently, ice flowed into Coire Chuaich [NN 717 864] and finally into Choire Chais [NN 695 825]. A continuous series of recessional features tracks the retreat of ice from the Loch an t-Seilich basin south-westwards through the Gaick Pass, along the valley of the Allt na Craoibhe and into Cama Choire (Figure 13).

At a relatively early stage in the deglaciation, Perthshire ice began to retreat from Glen Tilt and the southern slopes of the eastern Gaick lying to the east of the Bruar Valley. This allowed ice to funnel down the deep, parallel valleys of Gleann Diridh and Gleann Mhairc from the eastern Gaick ice cap. However, the absence of clearly defined terminal moraines and associated outwash fans in the Gaick Forest suggests that the readvances were short lived. As Perthshire ice continued to withdraw it allowed ice from the western Gaick to advance into the valley of the Edendon Water and neighbouring valleys where it subsequently retreated to form splendid sets of ice-marginal moraine ridges and associated drainage channels (Merritt, 2004c).

Strathspey ice retreated westwards from the valley of the Allt Bhran at an early stage allowing ice sourced from the south to flow into Coire Bhran. A massive outwash fan [NN 7675 8825] (Bruach Dhubh Fan) was formed at the mouth of the central breach by meltwaters flowing from the south (Merritt, 2004b), whilst Strathspey ice retreated westwards towards Meall Chuaich. The latter ice subsequently pulled back from the breaches on either side of Meall Chuaich and retreated down the valley of the Allt Cuaich towards Dalwhinnie. There is evidence of both ponding and minor readvances within the valley of the Allt Cuaich (Merritt, 2004f).

The final stage in the deglaciation of the Gaick witnessed the retreat of ice from Coire Mhic-sith, in the south-west corner of the district. This was accompanied by the formation of an ice- dammed lake in the valley, which drained initially via spillways and later along the ice margin (Lukas and Merritt, 2004; Lukas, 2004).

2.3.4 The Windermere (Lateglacial) Interstadial

Deglaciation commenced in a cold, arid environment and parts of the Cairngorms at least probably witnessed several thousand years of ice-free, periglacial conditions before the onset of rapid warming at 14 692 BP, the beginning of the Windermere Interstadial (WI) (Lowe et al., 2008). The abrupt amelioration in climate occurred when temperate waters of the Gulf Stream returned to the sea off the western coasts of the British Isles. River terraces and alluvial fans formed across the district by paraglacial processes, sweeping away loose glacial debris before soils became stabilized by vegetation (Ballantyne, 2002). Reported evidence of the WI is restricted to one site in the district, Loch Etteridge, where buried organic deposits have been cored (Walker, 1993; Lowe et al., 2008). An oscillatory climatic deterioration occurred during the WI and it is likely that glaciers had already started to build up in the mountains before more sustained cooling began at 12 896 BP, the start of the Loch Lomond Stadial (LLS).

2.3.5 The Loch Lomond Stadial

Glaciers undoubtedly existed in the district during the LLS, but controversy remains as to their extent (Figure 14) and whether any remnants of the MLD ice sheet survived during the WI (Merritt et al., 2004a). Many periglacial deposits and features formed during this very cold period.

There are four possibilities for when the high ground in the southern half of the district was last glaciated (Merritt et al., 2004b). The generally accepted paradigm is that renewed glaciation occurred in the Loch Lomond Stadial (LLS) following the complete decay of the Main Late Devensian ice sheet (Sissons, 1979b; Sutherland and Gordon, 1993). The second option is that most of the glacial landforms and deposits on and around the Gaick plateau formed during the progressive and substantially uninterrupted retreat of the MLD ice sheet: any LLS ice was restricted to a few small corrie and niche glaciers. The third option is that the Cairngorm model provides a paradigm for the Gaick in that at least some of the evidence previously linked with the LLR relates to independent plateau ice sheets that developed during ice sheet deglaciation prior to the Windermere Interstadial. A fourth option is that deglaciation of the MLD ice sheet had not been complete before cooling associated with a regional readvance of ice masses occurred during latter stages of the Windermere Interstadial and through the LLS.

The glacial reconstruction of a single, independent, late-glacial ice cap over the Gaick plateau is clearly at odds with the account of deglaciation given above, which suggests that as the underlying relief was uncovered topography gained an increasing control on the ice margin. Ice funnelled into valleys and corries locally to form hummocky moraines and related features diachronously, not contemporaneously as proposed by Sissons. Besides the absence of direct dating control, the main problem is the lack of coherent, identifiably younger sets of terminal or lateral moraines in the area.

Benn and Ballantyne (2005) offered an alternative reconstruction to Sissons of an independent LLS ice cap sourced in the West Drumochter Hills and it is possible that a similar, highly asymmetrical ice cap formed over the western part of the Gaick plateau with outlet glaciers flowing eastwards into the central breach and the Edendon valley. The eastern Gaick possibly supported a contemporaneous ice cap not too dissimilar to that modelled by Sissons (1974), but with an outlet glacier flowing westwards into the central breach. The best terminal moraine in the area occurs within the corrie situated to the east of Carn nan Gabhar in the Beinn a’ Ghlo massif; its LLS age is not disputed.

Benn and Ballantyne (2005) used well-practised and reported procedures to support the conclusion that the West Drumochter Hills and Pass of Drumochter were last glaciated during the LLS following the complete decay of the MLD ice sheet. However, they were unable to rule out the possibility that remnants of the MLD ice sheet remained in the area throughout the Windermere Interstadial as has been debated forcefully, for example, regarding the Cairngorm Mountains, some 25 km to the north-east of Drumochter (Sugden, 1980). Indeed, Ballantyne et al. (1987) and Stone and Ballantyne (2006) concluded that there is a distinct possibility that ice survived the Interstadial on high ground elsewhere in Scotland, and Clapperton (1997) argued that the MLD ice sheet was too large to have melted completely before the end of the interstadial. The survival of ice-sheet remnants would explain why no deposits of Lateglacial Interstadial age have been found hitherto within the Drumochter-Gaick area, why no coherent set of laterally extensive end moraines or stratigraphical evidence has been found that supports a separate LLS glaciation of the extent reconstructed by Sissons (1974; 1980) or Benn and Ballantyne (2005), and why the last ice to cover the eastern Gaick was centred over the lowest ground in the valley of the River Tarf (Merritt, 2004).

Ballantyne (2002) concluded that hummocky moraine is likely to have formed in particular climatic and glaciological settings following the LLR when there was a large reservoir of paraglacial material available for reworking, but it has yet to be demonstrated beyond doubt that these conditions only occurred towards the end of the LLS. For example, recent cosmogenic isotope ages obtained from boulders on ‘classical hummocky moraine’ in Glen Geusachan (Everest & Kubik, 2006) indicate that this particular example formed during the retreat of a local ice cap during an earlier readvance (c.16 ka BP).

The most recent sophisticated thermomechanical ice sheet modelling of the glacial configuration during the Loch Lomond Stadial by Golledge et al. (2008) suggests that mostly thin, cold-based plateau ice fields developed on the Cairngorm, Monadhliath and Gaick plateaux relatively early in the stadial (12.7 ka) (Figure 15). If correct, much of the evidence illustrated in (Figure 13) for deglaciation across the Gaick is palimpsest and is indeed unrelated to LLS ice.

2.3.6 Holocene

The Holocene began abruptly at 11 703 BP when the warm Gulf Stream current became re- established, providing an ameliorating influence on the climate of the British Isles (Lowe et al., 2008a). At first the widespread occurrence of bare, unstable soils must have led to a period of intense fluvial erosion and deposition with enhanced debris-flow activity on mountainsides and land slipping (Ballantyne, 2002). Soils gradually became more stable following the establishment of vegetation, firstly of shrubs and scrub communities, as during the early part of the Windermere Interstadial, and later by woodland (see Sutherland, 1993; Gordon, 1993; Bennett, 1996 for summary of research in the district). Sites at low altitude indicate that, following a distinct phase of juniper dominance, forests of birch and hazel with subsidiary oak and elm had developed early in the Holocene. Pine spread into the district at about 8800 yrs BP and was the dominant forest for the next four millenia (Bennett, 1989; 1996). Alder appears to have arrived after the arrival of pine, but was never abundant. Humans had begun to have an impact on the landscape around Loch Garten by 3,900 yrs BP, as reflected in the decrease in the number of trees, especially pine, and the concomitant increase in abundance of heather.

Climatic deterioration appears to have begun soon after the beginning of the Holocene. Distinct layers of pine and birch stumps preserved quite widely in blanket peat deposits indicate that the forest locally succumbed to the spread of Sphagnum moss when colder and wetter conditions pertained. The inception of blanket bog has been dated to between about 8,000 and 4,400 BP in the Cairngorms (Pears, 1970, 1988), where the treeline has fallen from about 790 m OD (recorded from the highest stumps of birch), to its present natural level of about 680 m OD (Pears, 1988).

Although the imprint of glaciation remains dominant, post-glacial processes have superimposed subtle, but distinctive, modifications on the landscape. Steep mountain sides have been modified by rockfalls, soil creep and debris flows, whereas valley floors have been affected by rivers forming spreads of alluvium and river-terrace deposits and by the accumulation of alluvial fans at the mouths of tributary streams. It is apparent that accelerated erosion, land slippage and debris flow have occurred within the past few centuries, but whether the increase is related to the destabilisation of mountain soils by overgrazing, or by a change to a more stormy climate, is debatable.

2.4 Quaternary geology of the eastern Grampian Highlands

The history of Quaternary research in the Eastern Grampian Highlands has been summarised by Sutherland (1993). The primary survey of the region supported the prevailing opinion that deglaciation followed an orderly sequence of events comprising ‘ice cap’, ‘confluent glacier’, ‘valley glacier’ and ‘corrie glacier’ stages (Barrow et al., 1913; Hinxman and Anderson, 1915; Hinxman, et al., 1923). Former flow directions of ice at the ‘maximum glaciation’ were determined from the distribution of erratics, particularly granites and granodiorites from the south-west of the district, and the Cairngorms. This evidence suggested that regional ice accumulation areas were centred over the Ben Alder/ Rannoch Moor area and the Cairngorm massif. NNW-trending striae on the bedrock exposed at the head of Gleann Einich are cited as evidence for this western-sourced ice overtopping the Cairngorm Plateau. On the contrary, the absence of erratics of metamorphic rock in the core area of the mountains suggests that this ice was diverted around the Cairngorms, down Strathspey and through the valley of the Geldie Water towards Glen Dee (Brazier et al., 1996b). Barrow et al. (1913) concluded that deposits of the ‘maximum glaciation’ were chiefly restricted to the higher ground and cols, because till deposited in the valleys had been removed during the subsequent ‘valley glaciation’, when hummocky morainic deposits were laid down.

Granodiorites from the Rannoch area were found only on the western margins of the Gaick plateau and no Cairngorm granite was found within the Gaick. This indicated that the Gaick lay beneath an ice divide (‘ice shed’) and that ice sourced locally had been sufficiently strong to fend off ice flowing from afar. Barrow et al. (1913) concluded that the local ice later became dominant in parts of the Gaick when the exotic ice became less powerful. Ice flowing from the south-west via the Loch Garry basin split at about the Pass of Drumochter, some flowing north- eastwards toward the valley of the River Spey, the rest flowing south-eastwards toward Strathmore, encroaching only on the southernmost part of the district (Hinxman, et al., 1923).

The pattern of ice flow thus deduced at the ‘maximum glaciation’ has been accepted generally (Charlesworth, 1955, 1957; Sissons, 1967; Sutherland, 1984; Sutherland and Gordon, 1993). Charlesworth (1955, 1957) concluded that a distinct ‘Highland Readvance’ had affected the district, but his reconstructed ice margins were largely unsupported by detailed field evidence. Sissons (1967, 1974, 1979, 1981) subsequently used limits of ‘hummocky moraines’ and other evidence largely derived from air photographs to delineate areas affected by his more restricted ‘Loch Lomond Readvance (LLR)’. He reconstructed 17 glaciers in the Cairngorms, most of them in corries, but including larger valley glaciers in Glen Eidart, Garbh Choire and Glen Geusachan. Sugden (1970, 1980) interpreted many of the moraines in the Cairngorms as sub-glacial, glaciofluvial landforms, and, like Sissons, proposed that deglaciation took place by in-situ ice stagnation. This concept was endorsed by Young (1975, 1976, 1978) in his detailed geomorphological mapping from air photos of Glen Feshie, Rothiemurchus and adjoining areas. Sissons (1974) reconstructed an ice cap over the Gaick plateau with outlet glaciers radiating into the surrounding valleys, including the Pass of Drumochter, but could locate no terminal or lateral moraine ridges.

The LLR limits established by Sissons have been dated indirectly on pollen-stratigraphical evidence by comparing cores taken from organic sediment filling kettleholes both within and outwith those limits. For example, cores taken from Loch Etteridge [NN 688 929], some 19 km to the NNE of the Pass of Drumochter, reveal a classic ‘tripartite sequence’ including sediment deposited during the Loch Lomond Stadial and Windermere Interstadial (Sissons & Walker, 1974; Walker, 1975a, Walker 1993). The base of the organic sequence at Etteridge has yielded a new AMS uncalibrated radiocarbon date of 12 930 ±40 BP, indicating that the area around the loch has not been glaciated during the Loch Lomond Stadial (Everest and Golledge, 2004). This is corroborated by the recent identification towards the base of the sequence of microtephra correlated with the main Borrobol event at 14,400 cal BP (Lowe et al., 2008). In contrast, the basal pollen assemblage in cores taken from a kettlehole located on the col of the pass [NN 629 762] has been correlated with a similar assemblage at Etteridge that yielded an early Holocene radiocarbon date of 9405 BP (Sissons & Walker, 1974; Walker, 1975a, b; Walker, 2008). Furthermore, a series of radiocarbon dates from basal sediment in high corries on Braeriach were found to be no older than about 7 ka BP (Rapsom, 1985).

Both the general pattern of ice flow across the district and the series of events has been accepted generally (Jamieson, 1908; Hinxman and Anderson, 1915; Charlesworth, 1956; Sugden, 1970, 1974; Young, 1974; Sutherland, 1984, 1993; Hall and Mellor, 1988; Brazier et al., 1996a, 1996b, 1998), although modern three-dimensional thermomechanical ice-sheet modelling suggests that the ice sheets that covered the Central Highlands were more dynamic than thought hitherto (Boulton and Hagdorn, 2006). During deglaciation, ice sourced in the western Highlands (Strathspey ice) retreated in a general westerly direction exposing peaks and valleys in the east before those in the west. There were complex interactions with ice sourced locally in the Cairngorms and Gaick. Retreat was punctuated by at least one major stillstand involving Strathspey ice, evidenced by extensive lateral moraines in Glen More and at the mouths of Gleann Einich and the Lairig Ghru, just to the north of the district. Ice-marginal lakes were ponded up at this time within Gleann Einich and the Lairig Ghru, between active Strathspey ice occupying Rothiemurchus, and local glaciers (Brazier et al.,1998; Golledge, 2002).

Some of Sissons’ (1979) LLR limits in the Cairngorms and his interpretation of the style of deglaciation have been challenged, especially in Glen Geusachan and Garbh Coire (Bennett and Glasser 1991; Bennett, 1996; Brazier et al., 1996b), also around the Gaick plateau and in the Pass of Drumochter (Lukas et al., 2004). Contrary to earlier views of widespread ice stagnation, deglaciation generally involved ‘active retreat’ of topographically controlled glaciers that produced hummocky moraine together with series of lateral moraines and associated drainage channels (Bennett and Boulton, 1993). Furthermore, it now appears from the cosmogenic dating of boulders that in the western Cairngorms the larger valley glaciers reconstructed by Sissons, such as Glen Geusachan and Garbh Coire, were outlet glaciers of a plateau glacier centred on Moine Mhor that became established during a prolonged stillstand in deglaciation between 16 and 14 kyr BP, prior to the Loch Lomond Stadial (Everest and Kubik, 2006) (Figure 14). These dates have been questioned by Ballantyne et al. (2009), but independent evidence from laminated sediments in Gleann Einich also suggest that the stillstand lasted at least 1000 years (Brazier et al.,1998).

On present evidence the stillstand cannot be correlated with a specific northern hemispheric cooling event. It occurred shortly after Heinrich Event 1 and before the LLS, probably mainly as a result of a local glacial reorganization brought about as topography increasingly affected local glacidynamic conditions during ice-sheet thinning and retreat. Recent, minimal cosmogenic 10Be ages of 16.2 and 15.4 kyr BP on rock glacier deposits in Strath Nethy and the Lairig Ghru respectively confirm that these parts of the Cairngorms escaped glaciation during the Loch Lomond Stadial (Ballantyne et al, 2009) and luminescence ages of glaciolacustrine sediments within Gleann Einich and the Lairig Ghru suggest that Cairngorm ice may have retreated from these glens as early as between 23 and 17 ka BP (Golledge et al., 2002). This supports the conclusion of Sutherland (1982) that precipitation starvation, rather than climatic warming resulted in negative mass balance of eastern parts of the last ice sheet, particularly where accumulation areas were small, such as in Glen Luibeg. The margin of the Strath More ice lobe at the mouth of Gleann Einich was dynamic (Golledge, 2002) and other evidence of oscillatory of ice-lobe margins disturbing sediments deposited in temporary ice dammed lakes during deglaciation has also been reported in Glen Luibeg (Golledge, 2003), between the Lairig Ghru and Glen Derry, and at Raitts Burn in the Spey Valley north-east of Kingussie (Phillips and Auton, 2000; Phillips et al., 2007).

2.5 Lithostratigraphy of the Newtonmore–Ben Macdui district

The lithostratigraphical framework adopted for classifying the Quaternary deposits on sheets 64E and 64W is based on the ‘top-down’ scheme outlined by McMillan et al. (2004, 2011). Most litho-morphogenetic and formally defined units established for the glacigenic deposits (glacial, glaciofluvial, glaciolacustrine) are thought to be the product of Devensian stage glaciations. They are assigned to two geographically defined sub-groups of the Caledonia Glacigenic Group, namely the Central Highland Glacigenic Subgroup and the Cairngorm-East Grampian Glacigenic Subgroup. The boundary between the subgroups divides those areas affected by ice flowing from the main ice divide of the MLD ice sheet, to the west of the district, from areas affected mainly by locally sourced ice, namely the Cairngorms and eastern Gaick (Figure 16). Pre-Devensian units are assigned to similar subgroups of the Albion Glacigenic Group. Late-glacial and Holocene fluvial and lacustrine deposits together with peat, head and regolith are assigned informally to the Grampian Catchments Subgroup of the Britannia Catchments Group.

Few formal lithostratigraphical units have been identified in the district apart from within the older glacigenic sequence (Table 4), for which the lithostratigraphy established on Sheet 63E (Dalwhinnie) is adopted (Merritt, 1999). These older units have been found mainly to the south of the Gaick plateau in the valleys of the Allt Shuas Chulaibh, Edendon Water, Allt Glas Choire, Allt a’ Mhuilinn, Allt a’ Chireachain, Allt a’ Choire Bhig and Allt a’ Choire Mhor (Merritt, 2004d).

Table 4 : Lithostratigraphy of some glacigenic deposits in the district.

The lowest unit in the succession, the Pattack Till Formation is a pale to moderate yellowish brown or light olive-brown, very compact, stony, very sandy, clayey diamicton with wisps of fine-grained sand. It contains angular to subrounded clasts of micaceous psammite, quartzitic psammite, reddish brown porphyritic felsite and sparse rounded to well-rounded cobbles and boulders of grey porphyritic granodiorite probably sourced from the Rannoch Moor area to the south-west. Many clasts are unsound and iron-stained suggesting moderate weathering. The till is at least 3.5 m thick locally and is mainly of ‘lodgement’ type (cf. Benn and Evans, 1998).

The Ailleag Diamicton Member of the Pattack Till Formation is a very compact, ferruginous, clast-supported diamicton resembling regolith that crops out from beneath till within the deeply incised valley of the Fèith Gorm Ailleag [NN 8029 7984], to the south of the Gaick plateau. This unit is probably a head deposit that predates the last regional glaciation of the area.

The Pattack Till is overlain locally by heavily iron-stained, poorly sorted gravely deposits that were probably laid down as moraines and ice-proximal fans during the retreat of the pre- Devensian ice sheet. More commonly the till is overlain by up to at least 12 m of sand and gravel assigned to the Ceardaich Sand and Gravel Formation. This unit generally coarsens upwards and its base is a sharp, uneven erosion surface. The subangular to well rounded gravel is similar in composition to the underlying till, although less weathered. The unit is either trough cross stratified (Plate 25), in which case it was laid down as glaciofluvial outwash, or displays larger scale planar cross stratification indicative of deltaic deposition (Plate 26). The deltaic sequences include ripple-drift cross-laminated sand overlying pale yellowish brown, laminated clay and silt (bottom sets) or thinly interlaminated, very fine-grained sand, silt and clay with sparse pebbles (dropstones). The basal, fine-grained part of this composite unit, up to at least 5 m thick, is assigned to the Linn of Pattack Silt Formation.

The full sequence is rarely found as it pinches out laterally as a result of subglacial erosion and glacitectonism. The sand and laminated silt and clay has commonly been sliced-up, intercalated tectonically with beds of sand and gravel, or severely compacted, sheared, boudinaged and deformed into a glacitectonite (sensu Benn and Evans, 1998). The shearing and deformation increases upwards, with prominent slickensiding.

The deformed sediments are commonly bounded by sharp, subhorizontal, planar contacts at the base of overlying units of diamicton assigned to the Ardverikie Till Formation. The latter is generally a pale yellowish brown to light olive-grey, extremely compact, stony, silty, sandy diamicton containing angular to subrounded clasts mainly of micaceous and quartzitic psammite, with some reddish brown porphyritic felsite, mica-schist, and sparse pink granitic vein rock. Significantly, it also contains sparse rounded to well-rounded boulders of grey, porphyritic granodiorite (Rannoch?). The till locally contains wispy, subhorizontal laminae of fine-grained sand, some of which pass laterally into augen-shaped lenses (up to 30 cm thick) within which bedding is folded, microfaulted and sheared. These features are typical of deformation tills (Benn and Evans, 1998). Elsewhere the unit is generally of ‘lodgement’ type, being massive apart for subhorizontal fissures and concavo-convex discontinuities. A good fabric is commonly developed from which palaeoflow may be deduced.

2.6 Superficial deposits and Quaternary landforms

2.6.1 Till

Till underlies much of the lower-lying, relatively featureless and poorly-drained parts of the district, commonly concealed beneath younger superficial deposits such as peat. It generally comprises extremely compact, pale yellowish brown, silty, clayey, sandy, stony diamicton. The uppermost metre or so is generally less compact, crudely stratified and sandy. The colour and clast composition of till varies locally, governed by the type of underlying bedrock. However, those assigned to the Ardverikie Till Formation of the Central Grampian Glacigenic Subgroup are dominated by clasts of micaceous psammite and schistose semipelite with subordinate porphyry, granodiorite, and granite. Those assigned to the East Grampian Glacigenic Subgroup (Banchory Till Formation) are dominated by granite in the Cairngorms, micaceous psammite and porphyry in the Gaick and by calcsilicate rocks in Gleann Mòr and Glen Loch. Prisms of till tens of metres thick have been plastered on hillsides in the north-west of the district and to the south of the Gaick plateau; elsewhere it is generally thin (<5 m) and patchy.

Till mainly consists of ice-transported material laid down subglacially, but it also includes deposits formed as cohesive debris flows at the margins of retreating ice sheets and shortly after the ground was laid bare by paraglacial processes. Deposits formed in the last two environments commonly occur towards the surface, and comprise a metre or so of heterogeneous, very poorly sorted, crudely stratified, gravelly diamicton intercalated with gravel, silty sand, silt and clay. Sediments that accumulated at the ice front have been modified and partially redeposited by ephemeral meltwater streams and sheet-wash; they are generally permeable and include large boulders, locally up to several metres in diameter (Plate 27).

In contrast, tills formed in the subglacial environment are generally more homogeneous, much more compact, clayey, fissile and are relatively impermeable. In general, it is not practical to map the different types of till separately, even though the boundary between them is commonly unconformable. Locally, however, the supraglacial deposits are several metres or more thick, form constructional mounds and have been mapped out as ‘morainic deposits’ (see below).

The subglacially formed diamictons generally take the form of ‘lodgement’ till (Plate 28). This type was deposited beneath actively moving glaciers as a result of frictional retardation of debris particles and debris-rich ice masses against the glacier bed. Such tills have also undergone intense subglacial deformation and are typically very stiff, stony, sandy, clayey diamictons with matrix support and little stratification. Boulders are generally not as large as those occurring in associated ice-marginal deposits, but they typically have bevelled and striated surfaces. Subhorizontal fissures resulting from shearing and subsequent pressure release are common towards the upper surface of lodgement tills, imparting a platy structure. Concavo-convex discontinuities formed by subglacial erosion and shearing are common throughout. All types of discontinuity may be lined with silt, clay and silty fine-grained sand, the latter commonly indurated and ferruginous.

Some units of till formed within the deforming bed of the ice as it overrode unconsolidated deposits or decomposed bedrock. These ‘deformation’ tills (sensu Benn and Evans, 1998) commonly comprise extremely compact, sandy, silty, clayey diamictons with a prominent, gently undulating subhorizontal fissility. Gravel clasts are relatively well dispersed in the matrix compared to lodgement tills, but boulder pavements and clusters of cobbles also occur. They exhibit considerable lateral and vertical variation in lithology and commonly include sheared lenses of sand, gravel, silt and clay (Plate 29).

The latter have locally been severely compacted, sheared, boudinaged and deformed into planar to gently undulating, subhorizontal beds of ‘glacitectonite’ (sensu Benn and Evans, 1998). Excellent sections in deformation till and glacitectonite may be found in the catchment of the Allt Cuaich (Merritt, 2004f) and in valleys south of the Gaick plateau (Merritt, 2004d).

2.6.2 Morainic deposits

Morainic deposits are typically hummocky and form several distinctive suites of sediment- landform assemblages. They are typically very poorly sorted and consolidated accumulations of boulders, gravel, sand and sandy diamicton, forming mounds up to 20 m high. Some particularly gravelly or bouldery deposits are prefixed SG or B respectively on the maps.

The deposits are highly variable lithologically and include complex interdigitations of matrix- and clast-supported diamicton, stratified and unstratified silty boulder gravel, and beds of sand, silt and clay. Most of the deposits in the district are ‘constructional’ moraines that formed at ice margins during ‘active’ glacial retreat (Plate 30). They were deposited by several processes, including dumping of debris from the glacier surface, ‘bulldozing’ of loose debris at the ice front during forward movement of the ice, squeezing of soft till from under the glacier margin and by the proglacial thrusting and stacking of slabs of frozen proglacial outwash sediment, debris-rich ice and weak rock at the ice front (cf. Benn and Evans, 1998; Bennett et al., 1998).

The ice marginal moraines are generally intimately associated with meltwater channels that were eroded parallel to the ice margin, either subaerially at the margin or a short distance beneath the glacier. Taken together, the features provide a record of glacial recession across the district. They are generally most pronounced on ground that sloped towards the receding ice margin, notably along the eastern slopes of the valley of the River Feshie downstream of Glenfeshie Lodge (Werritty and McEwen, 1993) (Figure 17), the eastern slopes of Gleann Chomhraig and the valley of the Allt an Dubh-chadha (Golledge, 2000), to the west of the lodge, and on the north- western flanks of the Gaick plateau (Merritt, 2004f).

In some areas, notably within the Pass of Drumochter, Cairngorms and the Gaick, the moraines are apparently not so clearly related to the former positions of ice margins and have a more jumbled and chaotic distribution. Such ‘hummocky moraines’ were thought to result from the ‘downthrusting’ of stagnant ice, particularly at the end of the Loch Lomond Stadial (Sissons, 1967), but most of them are now believed to be closely-spaced push and dump moraines, deposited during repeated minor, perhaps annual, readvances by dynamically active glaciers (Lukas, 2003, 2005). Although appearing to be chaotic on the ground, linear patterns are generally apparent from air photographs and Digital Elevation Models and the mounds commonly form chains arranged en echelon across valley sides. Excellent examples of such moraines have been described in Glen Geusachan (Bennett and Glasser, 1991), Glen Eidart (Golledge, 2000), Glen Luibeg (Golledge, 2003) and the valley of the Edendon Water (Merritt, 2004c).

Good examples of morainic deposits formed mainly of sand and gravel may be examined in road cuttings along the A9 road within the valley of the River Truim and in the lower reaches of the Cuaich catchment (Merritt, 2004f). Here the moraine ridges range up to 20 m in height and their cross-sectional asymmetry, orientation and disposition clearly indicate that they formed periodically at the margin of an ice sheet that retreated towards the west-south-west (Plate 31).

They are typically composed of pale yellowish-brown, poorly compacted, coarse, extremely poorly sorted, yet relatively well-rounded, gravel with a ‘floury’ silty, sandy matrix. The deposits commonly pass laterally and vertically into heterogeneous stratified diamictons containing many boulders and blocks. Here, as elsewhere, it is commonly difficult to distinguish morainic and moundy glaciofluvial deposits, as both contain many rounded clasts.

A distinct suite of morainic deposits occurs on the Gaick plateau, particularly within the eastern Gaick. Assigned to the Gaick plateau Moraine Formation, these deposits are typically composed of yellowish brown, poorly consolidated, mainly clast-supported, gravelly, sandy, silty diamicton, and matrix-rich gravel (Merritt, 2004e). The gravel fraction is dominated by fragments of weathered psammite and porphyry. The suite takes the form of low, arcuate, recessional moraine ridges and parallel, shallow glacial drainage channels that record the decay of the ice concentrically toward the upper reaches of the valleys of the Tarf Water and Bruar Water (Plate 24).

A distinctive set of sharp-crested moraine ridges occur in Glen Loch, in the south-eastern corner of the district, where some of them allow passage across Loch Loch at low water levels (Plate 32).

These features are probably De Geer moraines that formed at the margin of ice that retreated northwards towards an ice divide situated across the eastern Gaick. Features such as these formed at ice fronts that terminated in water (Golledge and Phillips, 2008).

2.6.3 Glaciofluvial deposits

Where possible, moundy glaciofluvial ‘ice-contact’ deposits have been distinguished from terraced glaciofluvial sheet deposits and assigned to one or other of the glacigenic subgroups represented within the district. The ‘ice-contact’ deposits are generally a little older than ‘sheet’ deposits occurring at a particular locality, the former having been largely destroyed and redeposited by outwash streams to form terraced spreads (sandar). However, moundy and terraced spreads commonly merge together in which case boundaries drawn between them are somewhat arbitrary. Glaciofluvial terraces are generally distinguished from alluvial terraces occurring at lower levels in the valleys by the presence of ‘kettleholes’ that were formed by the melting of blocks of ice trapped within the sediment. However, many alluvial terraces were probably created only a short time after the glaciofluvial ones. Some large outwash fans have been distinguished separately as glaciofluvial fan deposits.

Ice-contact deposits mostly comprise sand and gravel, but include subsidiary beds of diamicton, silt and clay. The deposits were laid down on, under, or against glacier ice. Hummocky topography is characteristic and includes steep-sided ridges of gravel (eskers), flat-topped kame plateaux (often former deltas) and rounded hillocks of sand and gravel (kames), which were often deposited within ice-walled, water-filled chasms and enclaves. All ice-contact deposits are typically studded with kettleholes, and steep former ice-contact slopes are commonly recognisable. The kames and kame-plateaux are typically composed of coarsening-upward sandy deposits that formed as fan-deltas in ephemeral ice-marginal lakes. Most glaciofluvial deposits were laid down in an ice-marginal setting, but some esker deposits may have accumulated in tunnels beneath the ice some distance from the contemporary ice margin.

Glaciofluvial sheet deposits include sands and gravels underlying terraces and flat valley-floor spreads, together with some outwash fans and the more extensive kame terraces. The deposits were laid down by outwash streams proglacially and they generally become better-sorted and sandier away from the former ice front. The ‘sheet’ deposits are typically more gravelly and densely-packed than ice-contact deposits, also more uniform in composition and thickness (Plate 33). However, some spreads contain so many kettleholes locally that they are hummocky in appearance.

Glaciofluvial fan deposits typically fine upwards and consist of very dense, very poorly sorted, silty, matrix-rich cobble-gravel, with subordinate lenses of sand and sandy gravelly diamicton. Branching distributary channels are commonly preserved on the surface of the features.

2.6.3.1 The Spey Valley and Glen Truim

The most widespread glaciofluvial deposits occur within the Spey Valley, where some deposits may be several tens of metres thick. Deposits lying on the north-western side of the valley around Newtonmore and Kingussie, and to the west of Loch Gynack, form kame terraces and benches that merge into moundy deposits. Deep kettleholes abound locally, as within Newtonmore. An esker extends south-westwards from Newtonmore, beside the A 86 road, towards thick, moundy deposits in the vicinity of Creagdhubh Lodge [NN 673 954]. Thick deposits occur on the southern side of the valley too, as around Nuide [NN 730 987] and Ralia Lodge [NN 713 976].

The valley of the River Truim was also a major route taken by glacial meltwater, as evidenced from a fragmentary esker on its western flank. The valley also contains fragmentary terraces that broaden out towards the confluence of the Truim and Spey. It is clear that meltwaters were at first diverted north-eastwards in the vicinity of the Falls of Truim, possibly when a large outlet glacier still occupied the upper Spey valley terminating in the vicinity of Ralia. These diverted meltwaters laid down a complex of eskers, mounds and terraces within the misfit valley that stretches from Etteridge towards Inverton [NN 745 993]. These moderately well-sorted deposits of glaciofluvial sand and gravel fan out and thicken towards the Spey Valley, between Milton of Nuide [NN 735 981] and the Ruthven Barracks, near Kingussie. The deposits are flanked by a spread of more poorly sorted, ice-marginal gravels that stretches to the north-east of Lùibleathann [NN 737 971]. Core taken through lacustrine deposits and kettlehole infillings in the vicinity of Loch Etteridge have provided an important record of late-glacial to Holocene vegetational, environmental and climatic change (Walker, 1993).

At an earlier stage in the deglaciation, meltwaters flowed through the valley occupied by Loch Cuaich [NN 695 878], where eskers are preserved to the north-east of the loch. Deposits lying within the Cuaich catchment have been described by Merritt (2004f).

2.6.3.2 Glen Tromie

Meltwaters also became concentrated within the valley of the River Tromie during ice-sheet deglaciation, laying down terraced spreads of cobble gravel, particularly in the vicinity of Bhran Cottage [NN 753 913]. Meltwater flowed from the south-west for a time, via cols to the north and south of Meall Chuaich, whilst western-sourced ice still occupied the Cuaich catchment. However, ice capping the Gaick plateau to the south was a more enduring source of meltwater. A very large outwash fan indeed was laid down at the mouth of the central breach, mostly on the eastern flank of Glen Tromie. The feature stands over 100 m above the valley floor and is apparently formed mainly of poorly-sorted, matrix-rich cobble gravel capped locally by boulders, blocks and slabs of micaceous psammite up to 0.4 m across. The gravel contains a relatively large proportion of pink granite, porphyry and felsite in addition to the ubiquitous micaceous psammite. The matrix of the deposit is yellowish brown, silty fine-grained sand with poorly developed lamination. The surface of the feature reaches about 545 m OD in the vicinity of Bruach Dhubh [NN 7675 8825], from which the fan has been named, and has been dissected by several distributary channels than fan out northwards from the apex of the feature at Glac nam Meirleach [NN 7650 8765]. The origin of the fan is probably quite complex, having been built up against the margin of Strathspey ice as it retreated westwards and parted from ice occupying the central Gaick (Merritt, 2004b).

A much smaller, lower-elevation outwash fan formed mostly of cobble gravel lies adjacent to the northern shore of Loch an t-Seilich. It is likely that the fan formed whilst a glacier of Loch Lomond Stadial age occupied the loch basin and that the present cliffline is approximately situated at the former ice contact (Merritt, 2004b).

A large glaciofluvial outwash fan with branching distributary channels on its surface lies at the mouth of Glen Tromie. It apparently merges northwards into a moundy spread of glaciofluvial deposits on the southern flank of the Spey Valley, suggesting that they formed roughly contemporaneously when ice still occupied the latter valley. The large alluvial fan at the mouth of the Glen Tromie probably began to accumulate soon after ice retreated from the Spey Valley and meltwaters had dissected the higher, older fan.

2.6.3.3 Glen Feshie

Glaciofluvial terraces abound within the lower reaches of the valley of the River Feshie and Allt Fearnasdail, where they are associated with alluvial terraces at lower elevations, alluvial fans and debris cones. The valley is one of the most important sites in Britain for the study, interpretation and dating of such geomorphological features and of the processes of valley-floor development (see Werritty and McEwen, 1993 for summary). At least five periods of incision have occurred since ice-sheet deglaciation. Many of the terraces preserve distributary palaeochannels.

2.6.3.4 White Bridge and southern Gaick

A large glaciofluvial fan lies at Chest of Dee [NO 015 884]. It was mostly laid down by meltwater emanating from Glen Dee, to the north-west, and partly from the valley of the Geldie Burn, to the west. Like most fans the feature is composed mainly of very dense, very poorly sorted, silty, sandy boulder gravel. Terraced spreads of glaciofluvial gravel of cobble to boulder grade occur upstream within Glen Dee, and downstream of White Bridge, commonly concealed beneath peat. Peat-covered terraces also floor the misfit valley of the Allt an-t Seilich, to the south.

A deposit of dense cobble-gravel containing pebbles of pink granite and psammite crops out from beneath till in the southern bank of the River Dee, 270 m downstream of White Bridge. This particular deposit has not been named, but elsewhere more-extensive units of sand and gravel underlying till to the south of the Gaick plateau have been assigned to the Ceardaich Sand and Gravel Formation (see lithostratigraphy above).

2.6.4 Glacioclacustrine deposits

Fine-grained sand, silt and clay laid down in standing water form part of many coarsening upward, glaciofluvial (deltaic) sequences in the district, but it is generally impractical to map out these fine-grained deposits unless they are particularly widespread or form discrete units within glacigenic sequences. They may also occur as lenses within morainic deposits and till, for example, in upper Glen Feshie (Golledge, 2004, fig. 52). Mapped units are typically composed of stiff, pale yellowish brown clay interlaminated with silt and very fine-grained sand. Many deposits are varved (with annually-formed laminae) and contain dropstones.

Most deposits were laid down in ice-marginal, proglacial lakes that formed in the upper reaches of valleys whilst the lower reaches remained blocked by ice during ice-sheet deglaciation. For example, in the catchment of the Allt Cuaich there is a flat-lying area [NN 6767 8665], presumably a former lake basin, that is underlain by over 1.2 m of thinly interlaminated, silty fine-grained sand, silt and clay resting on stony diamicton. The basin is bounded by former shorelines to the east (Merritt, 2004f). Evidence of a temporary ice-dammed lake dammed by an ice lobe during deglaciation in Glen Luibeg, between the Lairig Ghru and Glen Derry, has been described by Golledge (2003). North-trending reverse faults and other glacitectonic structures evident in the laminated silt and sand accumulation reflect deformation resulting from oscillations of an ice margin to the south of the lake (Phillips et al., 2007). This glacier also appears to have supplied the majority of sediment that formed the northward-sloping deltaic feature in upper Glen Luibeg, (Golledge 2003).

An excellent section revealing >3.1 m of rhythmically laminated, fine-grained deposits was discovered beneath till during the recent survey in Coire Mhic-sith [NN 658 745] (Lukas and Merritt, 2004; Phillips et al., 2007). The glaciolacustrine unit (Plate 34) becomes increasingly compact, sheared and disturbed upwards, displaying overturned folds, some detached along thrusts towards the east (Plate 35). The sense of thrusting is towards the east, indicating that the ice that dammed the lake advanced from the Loch Garry basin to the west, probably during the Loch Lomond Readvance (Benn and Ballantyne, 2004).

The deposit in Coire Mhic-sith has not been named formally, but elsewhere more extensive, laminated, fine-grained units underlying till to the south of the Gaick plateau have been assigned to the Linn of Pattack Silt Formation (see lithostratigraphy above).

2.6.5 Alluvial and organic deposits

The alluvial deposits in the district are of five types; (1) fluvial deposits underlying the floodplains and low-lying terraces of rivers, (2) lacustrine alluvium, (3) alluvial fans and (4) river terrace deposits. Organic deposits include peat and organic mud.

2.6.5.1 Alluvium

Gravelly alluvium underlies the floodplains and low-lying terraces of most rivers in the district, which are commonly bordered by steep, unstable bluffs up to 25 m high. Most streams are fast flowing and braided, with beds of cobble and boulder gravel, bifurcating channels and shifting linear bars of shingle. The river gravel is generally less than about 5 m thick, water-saturated, with subangular to well rounded, variably sorted clasts. Thicker deposits probably underlie stretches of some of the larger rivers, such as the Truim, Feshie and Dee. Tabular boulders exhibit a pronounced upstream-dipping imbrication. Unless streams have become engorged into bedrock they are prone to alter course periodically following flood events. Abandoned braid-bars soon become rough, boulder-strewn scrubland whilst abandoned channels become filled with sand and silt. Slower flowing stretches are associated with more sandy beds, wider floodplains and meander belts. Here riverbank sections commonly reveal clast-supported gravel (shingle) capped by ‘overbank’ deposits that accumulated during the waning stages of periodic flood events. The latter deposits consist of one or two metres of laminated, humic, micaceous, silty sand (loam), locally intercalated with peat.

The River Feshie is generally taken to be a textbook example of an upland stream and is one of the most studied in Scotland (see Werritty and McEwen, 1993 for summary of research).

The most extensive deposits of alluvium in the district are associated with the River Spey, the floodplain of which is about 1 km wide in the vicinity of Kingussie. The river bed is formed mainly of sand or subrounded to rounded gravel, but the floodplain is generally underlain by compressible, fine-grained ‘overbank deposits’ that are between 0.5 and 2.5 m thick. These deposits consist of yellowish brown, coarsely laminated, humic, micaceous, silty, sand (loam), locally intercalated and/or capped with organic mud and peat. Former meander channels and ox- bow lakes are filled with organic mud and peat, although the more modern features may have provided repositories for farm waste and other materials locally. Artificial levées have been constructed to help constrain flooding, which occurs regularly. The overbank deposits of the Spey are underlain by many metres of water-saturated sand and gravel commonly grading down into fine-grained sand and silt (see transect on Sheet 64W Superficial and Simplied Bedrock).

2.6.5.2 Lacustrine alluvium

Flat spreads of interbedded humic sand, silt and clay have been mapped adjacently to some bodies of water, such as Loch Gynack, but it has generally been impractical to distinguish between lacustrine alluvium and the alluvial ‘overbank’ deposits of streams flowing into lochs, such as on the southern side of Loch an t-Seilich, in the Gaick. Some lacustrine alluvium began to accumulate during late-glacial times, as for example, that associated with Loch Etteridge, where organic sediments preserve records of vegetational and climatic change that has occurred in the district since deglaciation (Walker, 1993). Gravelly lacustrine beach deposits have been mapped around Loch Einich.

2.6.5.3 River terrace deposits

Dissected remnants of former floodplains flank the alluvium of many of rivers in the district. These form terraces that slope gently down-valley and are typically underlain by several metres of poorly stratified, clast-supported cobble gravel; many are capped by peat. Terrace aggradation occurred mainly during late-glacial and early Holocene times, associated with braided streams. The gravel is locally overlain by spreads of sand and silt up to about 2 m thick, laid down as ‘overbank’ deposits during the waning stages of periodic flood events. These fine-grained deposits are most widespread on the broad terraces of the major rivers, including the Spey, Truim and Feshie, where they produce well-drained, light sandy soils. The terraces of the River Feshie have been described by Werritty and McEwen (1993).

Most river terraces in the district are judged to have formed during ice-sheet deglaciation and consequently have been mapped as ‘glaciofluvial sheet deposits’. These terraces contain kettleholes and commonly merge into spreads of moundy, ice-contact glaciofluvial deposits.

2.6.5.4 Alluvial fan deposits

Fans composed of very poorly sorted, matrix-rich gravel, silty sand and gravelly diamicton are common throughout the district where tributary streams with relatively steep gradients debouch onto flatter ground. The deposits were laid down by braided distributaries that migrated slowly across the fan surface, and during sheet flood events. Many of the features are now relatively inactive and have been partially dissected by drainage. General speaking, the rate of fan accumulation was greatest immediately following deglaciation from when it diminished exponentially for several thousand years ‘paraglacially’ (Ballantyne, 2002).

Many excellent examples of high-angle alluvial fans occur within the central breach of the Gaick (Merritt, 2004b) in and around the Cairngorms and in Glen Loch. There are few natural sections allowing fan deposits to be studied in detail, but exceptionally one occurs in the valley of the Edendon Water [NN 715 776], 1.3 km south of Stronphadruig Lodge, where a tributary stream has formed a small fan that has been dissected by the main river (Ballantyne and Whittington, 1999; Ballantyne, 2004a). Together with studies of several large fans and debris cones on the eastern flanks of the Feshie Valley (Werrity and McEwen, 1993), and within the Pass of Drumochter (Ballantyne, 2004b), immediately to the east of the district, radiocarbon dating of organic layers indicates that the accumulation reflects a small number of extreme rainstorm- generated flood events rather than anthropogenic influences or long-term climate change.

Several large, relatively low-angle alluvial fans occur within the Spey Valley, including those at the mouths of the Truim and Tromie, and at Newtonmore and Kingussie. Another occurs at Cuaich, where the Allt Cuaich joins the valley of the River Truim. These features are likely to be formed of less coarse, better sorted sand and gravel than the steeper fans formed on mountainsides.

2.6.5.5 Peat

Most deposits of peat in the district comprise blanket upland accumulations of wet, acidic, partially decomposed vegetation, generally less than 2 m thick, but locally over 5 m. They are most widespread within topographical depressions, over the more poorly drained, gently undulating parts of the Gaick plateau, in the upper catchment of the River Feshie, and on the more extensive spreads of till. The hill peat is locally undergoing severe erosion, especially on the Gaick plateau to the west of the central breach (Morrocco, 2004).

2.6.6 Periglacial deposits

A wide range of periglacial features and deposits occur across the district, but it is not possible to depict many of them on the maps. For example, ‘stone lobes’ are a characteristic feature of many slopes above about 750 m OD (Plate 36). These are lobe-shaped accumulations of boulders are typically up to a few tens of metres wide and up to 5 m or so high. Piedmont lobes commonly occur where several lobes have coalesced. The lobes are the washed-out, wind-deflated remnants of large gelifluction lobes that formed in the harsh periglacial conditions that prevailed in the district during ice-sheet deglaciation and during the Loch Lomond Stadial (Ballantyne and Harris, 1994; Ballantyne et al., 2009). The boulders were transported to the front of the lobes where they are now piled up following the subsequent removal of the original finer-grained matrix. The Cairngorms and the Gaick plateau are internationally important areas for the study of such phenomena, which include a gamut of ‘fossil’ late-glacial features together with smaller scale, active periglacial forms (Ballantyne, 1984, 1987). The particularly good range of phenomena occur within the Lairig Ghru, including fossil rock glaciers and protalus ramparts (see Gordon, 1993 for summary of research; Glasser and Bennett, 1996; Brazier et al., 1996)

2.6.6.1 Head

Head deposits are poorly sorted and poorly stratified sediments that have mainly formed as a result of the slow, viscous, downslope flow of waterlogged soils (solifluction and gelifluction), soil creep and hillwash. Solifluction was most active whilst periglacial conditions pertained in the district during the latter part of the main Late Devensian glaciation and the Loch Lomond Stadial. It occurred during the summer months when the uppermost 0.5 to 1 m of the soil, the so- called ‘active layer’, thawed, whilst the ground below remained permanently frozen. The thickness and potential mobility of active layers depends very much upon the cohesiveness of the sediments affected, hence the thickest head deposits tend to occur where thoroughly decomposed rocks and clayey tills have been remobilised.

Most mapped deposits in the district are accumulations of angular, clast- or matrix-supported fragments of weathered rock with a matrix of silty sand. Much of the material is derived from frost-shattered rocks and mountaintop regolith. Most deposits are thinner than they appear to be, but thicken substantially on lower slopes. Deposits of head of this type are particularly widespread on the steep slopes of Glen Tilt, the Lairig Ghru and around the Gaick plateau, where they have been substantially reworked by debris flow processes. The widespread head deposits mapped in the Cairngorms are mostly composed of granite boulders and ‘corestones’ in a matrix of gritty, quartzo-feldspathic sand.

Some particular bouldery deposits (blockslopes) have been identified, for example, around Meall Chuaich and Beinn Dearg.

2.6.6.2 Blanket head (regolith)

The category has been applied to sheets of in-situ or nearly in-situ weathering products lying on mountaintops, including disintegrated rock, rock fragments and mineral grains. It is commonly a compact, yellowish brown to orange, matrix- to clast-supported diamicton, formed of angular fragments of weathered rock in a silty sand matrix. Granular, non-cohesive, non-frost-susceptible deposits tend to accumulate over coarse-grained igneous rocks such as granite and psammite whereas mica-rich, clayey, cohesive, frost-susceptible deposits form on schistose rocks. The more massive psammites, quartzites and granites commonly break down into blockfields (see below). The granular regoliths are generally well drained and are typically covered by smooth, mossy turf, such as on the Gaick plateau (Morrocco, 2004).

A feature of regoliths is that gravel-sized clasts and larger fragments generally form a wind- deflated pavement at the surface, below which the material is finer grained. This vertical sorting is the result of frost churning, a process that is particularly effective in the more clayey regoliths (Ballantyne and Harris, 1994). Some frost churning takes place on mountaintops at the present day, but it was far more rigorous in past periglacial climates. Horizontal sorting has also occurred in favourable circumstances to produce ‘patterned ground’. This generally takes the form of polygonal networks of erect stones with turf developed on finer materials at the centre of each polygon. The polygons commonly become elongated downslope and merge into ‘stone stripes’.

Granular regolith derived from psammite and felsite is widespread across the Gaick plateau (Merritt, 2004f), where it forms a blanket 1 to 5 m thick. This gravelly deposit is relatively well drained and produces the grassy turf that provides important summer grazing on the plateau for red deer (Plate 22). More rubbly deposits (gelifractates) mantle some summits where the underlying rock is less decomposed. In general, thin deposits of till on the lower flanks of the plateau give way upslope to rubbly, frost-shattered rock at about the 575 m contour, where the mountainside steepens. The slopes become steeper about 100 m higher, where they are underlain by thin deposits of head on deeply weathered and broken psammite. Blanket head has been mapped above about 800 m where low solifluction lobes and turf-backed terracettes are commonly developed.

A unit of very compact, ferruginous, clast-supported diamicton resembling regolith crops out from beneath till within the deeply incised valley of the Fèith Gorm Ailleag [NN 8029 7984], to the south of the Gaick plateau. This unit (Ailleag Diamicton Member of the Pattack Till Formation) is probably a head deposit that predates the last regional glaciation of the area. The till, about 2 m thick, is overlain by orange-brown sandy gravel that contains sparse cobbles of grey granodiorite in addition to angular to subrounded clasts of local psammite and pink granitic vein rock. The gravel is about 20 m thick locally and quite widespread over the plateau surfaces of the eastern Gaick, where it thins away from the valleys towards the surrounding interfluves. It was probably formed as a result of sheet wash and debris flow processes redistributing weathered and decomposed psammitic bedrock, but is not delineated from blanket head.

2.6.6.3 Blockfields and blockslopes

Blockfields are in-situ or nearly in-situ mountaintop accumulations of blocks produced by the frost shattering of underlying rocks. They are generally clast-supported, open-work and are rarely covered by soil. Blockfields have developed on many mountaintops in the Cairngorms, notably on Ben Macdui and Derry Cairngorm (Plate 13), in the eastern Gaick, such as on An Scarsoch, on Meall Chuaich (Merritt, 2004f) and on Carn an Righ (Figure 12).

The summit of Beinn Dearg is capped by a particularly extensive boulder-field (Merritt, 2004e) (Plate 37). It mainly occurs above about 900 m OD and rises to the main summit at 1008 m by way of several high, concentric ramparts. The rounded boulders constituting the feature are up to 3 m across and composed entirely of local pink, biotite-granite. The blockfield is divided into three segments by two narrow, shallow, WSW-ENE-trending depressions that may have been created originally as subglacial gouges. If so, the blockfield largely predates the last regional glaciation and invites comparison with the partially eroded blockfields and tors that occur on the Cairngorms (Hall and Glasser, 2003).

2.6.6.4 Scree

Scree, or talus, is typically a clast-supported accumulation of angular rock fragments derived from cliffs or steep rock slopes above by mechanical weathering. Scree deposits may be referred to as gelifractates, because most of the fragments have been dislodged by freeze-thaw processes. Actively accumulating screes are not common within the district, but there are many that are being reworked by debris flow processes during heavy rainfall and by snow avalanches, notably within the Lairig Ghru and Glen Tilt. The rate of scree accumulation was greatest immediately following deglaciation and during the Loch Lomond Stadial (Ballantyne, 2002). Particularly coarse screes occur on both sides of the valley of the River Tromie upstream of Glentromie Lodge, and overlooking Loch Cuaich. Other notable examples occur within the central breach of the Gaick plateau overlooking Loch an t-Seillich, Loch Bhrodain and Loch an Dùin (Merritt, 2004b).

2.6.6.5 Talus cone deposits

These deposits consist of steep, matrix-rich, cone-shaped accumulations of rock fragments at the foot of gullies or chutes that are steeper than those identified as alluvial fans. They are commonly associated with the reworking of screes and head deposits on very steep slopes across the district, but only the larger ones have been delineated. Debris cones are mainly formed catastrophically during heavy rainfall as a result of landslides that develop quickly downslope into debris flows; many cones are bounded by levées up to 2.5 m high (Ballantyne and Harris, 1994). Talus cones are particularly common on the steep slopes of Gleann Einich, the Lairig Ghru (Plate 38) and in the central breach of the Gaick plateau. Some cones in the Lairig Ghru are demonstrably the result of snow avalanches (Ballantyne, 1996).

2.6.6.6 Rock glacier and protalus rampart deposits

These enigmatic deposits are very uncommon, but both are reported to occur within the district. Rock glacier deposits are thick, lobate or bench-like masses of interlocking, predominantly angular rock fragments on valley floors at the foot of, but separated from precipitous slopes by up to several hundred metres. They typically have steep bouldery margins with transverse ridges and deep, enclosed depressions akin to kettleholes. The origin of rock glaciers is controversial; some may result from the downslope creep and deformation of ice-rich talus in a periglacial environment, whereas others may result from rockfall onto glacier ice (Ballantyne and Harris, 1994). The former type of rock-glacier deposit has been identified in Coire Beanaidh [NH 956 007] (Ballantyne, 1996), in the Cairngorms, and possibly in Coire Cas-eagallach [NN 976 737] during the recent survey. Ballantyne et al. (2009), however, cast doubt on the identification of all rock glaciers in the region, concluding that most, if not all of them result from catastrophic rock falls that occurred shortly after deglaciation locally.

Protalus ramparts are ridges or ramps of predominantly coarse debris that accumulated at the foot of steep, talus covered slopes covered by perennial firn snow in a periglacial environment. They are commonly up to about 12 m high and 15 m broad, lying parallel to, but separated from the slope by about 12 m, and with accumulations of boulders on their proximal (upslope) sides (Ballantyne and Harris, 1994). Protalus ramparts have been identified at 650 m OD on Devil’s Point [NN 978 947] and 940 m OD on Sròn na Lairige [NH 970 010] (Ballantyne, 1996), in the Cairngorms, and at 450–480 m OD on the slopes below Sron na h-lolaire (Golledge, 2004).

2.6.7 Landslide deposits

Landslides are common within the central breach of the Gaick plateau and in the southern corner of the district around the Beinn a’ Ghlo massif and adjoining mountains. As elsewhere in the Highlands, rock slope failure is principally a paraglacial response to glaciation and deglaciation involving stress relief and re-equilibration (Ballantyne, 2002; Jarman, 2007). Landslides are probably under-represented on the maps, because many are poorly developed and difficult to identify unambiguously. For example, antiscarps can be misidentified as ice-marginal glacial drainage channels. Furthermore, some landslides undoubtedly failed before the last glaciation and have been subsequently modified and masked by glacial deposits. Landslides have influenced the long-term landscape evolution of the region and they tend to cluster within relatively recently formed glacial breaches (Hall and Jarman, 2004; Jarman, 2007).

The landslides that occur within the Gaick have been described by Jarman (2004). A cluster of them occur around the remarkable hill of An Dún, west of Loch an Dúin. Other notable examples occur on buttresses to the west of Gaick Lodge and within the valley of the Allt Gharbh Ghaig, east of the lodge, where at [NN 776 820] there is a jumbled array of steep-sided, boulder-strewn, conical mounds (Merritt, 2004b). Tension cracks have been identified from air photographs on the flanks of Srón Bhuirich [NN 751 827] and A’ Chaoirnich, to the south-west. These features rise and branch diagonally up the mountainside and are associated locally with 4 to 7 m wide benches and antiscarps (Plate 39), but with no evidence of sliding or bulging (Jarman, 2004). They are indicative of weak slope deformation and compression. Similar tension cracks have been identified to the east [NN 773 804], near Carn na Moine.

Another cluster of landslides is associated with the Beinn a’ Ghlo massif, especially within the steep sided valley of the Allt Fheannach; some may be more extensive than mapped by BGS (D. Jarman, written communication). A relatively recently formed tension crack [NN 9575 7450] was observed behind the main backscarp of one of these landslides. Yet another cluster occurs at the head of the valley of the Allt Fearnach, to the east of Glen Loch. A potentially major landslide was identified on air photos encompassing Dubh Chlais [NN 975 755], on the western flank of Glen Loch, but insufficient corroborating evidence has been found. The feature may instead represent a remnant of a knickpoint in a Cainozoic erosion surface that can be traced around the northern and eastern flanks of Beinn a’ Ghlo at this general elevation (D. Jarman, written communication; Hall, 2004). It possibly represents an incipient corrie that was occupied by a niche glacier like several others that have been identified in the Gaick (Merritt, 2004a).

2.6.8 Features of glacial erosion

Evidence of glacial erosion on a large scale is ubiquitous in the district (Figure 8 and 9), including huge corries in the Cairngorms epitomised by Choire Etchachan (Gordon, 1993, 2001), deep glacial breaches such as the Lairig Ghru, the Gaick Pass and one occupied by Loch Cuaich, and enormous ‘U-shaped’ valleys, including Glen Dee, in the Cairngorms, and Cama’ Choire, to the west of the Gaick Pass. Massive glaciated rock walls are a feature of Gleann Einich, Stob Coire Etchachan and The Devil’s Point, providing some of the toughest rock climbing in Scotland. Other large-scale evidence includes severely ice smoothed, plucked and mamillated rocks on the western side of the Spey Valley overlooking Newtonmore and Kingussie, and the mega- roche moutonnée features of Creagan a’ Choin (Plate 23) and Ordan Shios, south of Newtonmore, which have tails of drift stretching in the former up-ice direction. Evidence on a small scale takes the form of ice-scratched rock surfaces (glacial striae) and glacially-smoothed, whaleback-shaped crags, some with ice-plucked ends facing in the former direction of ice movement (roches moutonées). The most commonly observed indicator erratics are large boulders of white porphyritic granodiorite from the Strath Ossian Granodiorite Pluton lying to the west of the district.

2.6.9 Features of glaciofluvial erosion

2.6.9.1 Glacial meltwater channels

Channels cut by glacial meltwaters are very common throughout the district. Three broad genetic types of channel have been recognised: subglacial channels, ice-marginal channels and proglacial spillways. Although the best examples of each type are distinctive, most channel systems formed time-transgressively, causing distinctions between them to become blurred. Furthermore, many of the larger ones probably have a long and complicated history spanning more than one glaciation.

Subglacial, ice-directed channels

Channels of this type formed whilst most of the district remained buried beneath ice. Subglacial meltwaters were constrained by the regional hydraulic gradient to flow parallel to the regional direction of ice movement, irrespective of the local subglacial topography. Meltwaters flowed uphill in places (as in a syphon) within subglacial channels, giving them their characteristic ‘up and down’ long profiles. They are most commonly preserved in cols cutting across topographic barriers lying at an oblique angle to the former direction of ice movement. The channels commonly begin at the crests of such barriers and spurs where the hydraulic gradient and discharge was greatest. Many ice-directed channels are to be found on the north-eastern sides of major protuberances in the north-western of the district.

Ice-marginal channel systems

Complex networks of shallow, 1–5 m deep, arcuate channels are encountered across most of the district. They are interpreted here to have been eroded by meltwater at, or closely within the margins of receding, typically cold-based, subpolar ice sheets or outlet glaciers (Benn and Evans, 1998). They are characteristically curved or crescentic in plan view, asymmetric in cross profile, and commonly occur in anastomosing flights across hillsides, where higher channels truncate, or feed into, lower ones, indicating that they formed progressively as the ice margin retreated. Some shallow gradient channels pass into steeper submarginal ‘chutes’ and vice versa. Many occur as benches on steeper slopes (one-sided channels) or as isolated flights of short channels that loop into the hillside (‘in-out-channels’).

The channels are commonly intimately associated with ice-marginal moraine ridges. Taken together, these arcuate features provide a record of glacial recession across the district. They are generally most pronounced on ground that sloped towards the receding ice margin, notably along the eastern slopes of the valley of the River Feshie downstream of Glenfeshie Lodge (Werritty and McEwen, 1993), the eastern slopes of Gleann Chomhraig and the valley of the Allt an Dubh- chadha (Golledge, 2000) (Figure 17), to the west of Glenfeshie Lodge, and on the north-western flanks of the Gaick plateau (Merritt, 2004f). An excellent suite of meltwater channels incised into bedrock occurs on the southern slopes of Glen Luibeg, most probably marking a stillstand position of the Luibeg glacier following its separation from the Derry Glacier during ice-sheet deglaciation.

Ice-marginal channels are intimately associated with the distinct suite of morainic deposits that are assigned to the Gaick plateau Moraine Formation. These occur on the Gaick plateau, particularly within the eastern Gaick, where they record the decay of the ice concentrically toward the upper reaches of the valleys of the Tarf Water and Bruar Water (Merritt, 2004e) (Plate 24).

Proglacial spillways and glacial lake shorelines

In contrast to the subglacial and ice-marginal drainage channels described above, proglacial spillways generally have regular longitudinal profiles. When cut into bedrock, they also typically have a pronounced V-shaped cross-section, and interlocking spurs. Many formed as ‘overflow’ channels when major drainage routes became blocked by ice and the resulting ice-dammed lakes drained across cols. This situation occurred in Coire Mhic-sith, in the south-west corner of the district, where a set of such features (Glas Mheall Spillways) cut across the col [NN 676 764] at 787 m OD lying between Coire Mhic-sith and the valley to the south-east (Lukas and Merritt, 2004). The spillways connect with a well-defined lake shoreline. Other possible shorelines occur in flights on the south-eastern side of Coire Mhic-sith [NN 669 760], below Glas Mheall Beag, and on the opposite spur of Meall Uaine [NN 664 763]. The benches are between 1 and 3 m wide and cut mostly into bedrock. The near-horizontal bedding would have favoured the formation of lake shorelines, particularly by the process of ice riving (cf. Dawson, 1980), and it has probably contributed to their conservation. Bench features that may also represent former shorelines occur on the eastern side of the valley of the Allt Dearg [NN 758 775], south of the Gaick plateau, but no associated spillway was identified.

Concealed channels

Concealed channels are likely to be common in the district, but they are generally only discovered from site investigations, as for example, in upper Glen Tromie. Tunnelling operations revealed concealed channels on either side of the present river at its outlet from Loch an t- Seilich. The base of the channel to the west lies beneath 76 m of glacigenic sediment, the channel to the east beneath 46 m. The tunnelling encountered cavities within the glacigenic deposits up to 27 m from the surface that possibly formed after the slow melting of blocks of ice originally buried within the sediment (Anderson, 1951).

References