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GLACIAL CHRONOLOGY OF THE NORTHERN CAIRNGORM MOUNTAINS

Supplement to the QRA/GLWG field excursion to Nairn and the Inverness Firth

26 October 2017

Adrian Hall and Martin Kirkbride

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Northern Cairngorms excursion

The northern Cairngorms is a classic area for the study of glacial landforms and sediments. The ski road gives access within short walking distances to the Cairngorm granite plateau, the northern corries and the flanks of Glen More. For the Quaternary geologist, this provides opportunities to examine corries and glacial valleys and a wide range of periglacial features. Many groups however visit the area to view the superb sequences of ice-marginal landforms and sediments formed during the final stages of the last glaciation. For bibliographies of early work on the northern Cairngorms you are referred to the QRA Cairngorm Field Guide (Glasser and Bennett, 1996).

The aims of this day excursion are (i) to view landforms produced by the Strath Spey ice lobe in a final major advance of the Scottish ice sheet and (ii) to walk into Coire an Lochain to examine the evidence for Loch Lomond Stadial and Late Holocene glaciers. Both sets of features have recently-published 10Be cosmogenic isotope exposure ages.

Deglacial sequences on the northern flanks of the Cairngorms and in Glen More

Figure 1.Glacial limits in Glen More as mapped by the Geological Survey of Scotland (Hinxman and Anderson, 1915).

Towards the end of the last glaciation, ice sourced from the western GrampianMountains of Scotland flowed down Strath Spey to encroach on the northern flanks of the Cairngorm Mountains (Fig. 1). The carry of schist erratics shows that Strath Spey ice encroached to elevations above 800 m on the northern flanks of the Cairngorm Granite massif (Sugden, 1970; Hall and Phillips, 2006). This erratic limit is associated with the highest set of a remarkable, off-lapping assemblage of ice-marginal landforms found on the mountain flanks that includes lateral moraines, kame terraces and meltwater channels (Fig. 1) (Hinxman and

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Anderson, 1915; Gordon, 1993). The ice-marginal forms cover an altitudinal range of ~500 m and can be traced for ~60 km along Strath Spey (Fig. 2). Hence this landform assemblage provides an unusual opportunity to establish the pattern and timing of the final retreat of a major ice lobe within the last Scottish ice sheet.

Figure 2. Elevations of ice-marginal landforms along mid-Strath Spey and their relationship to the M1 and M3 moraine limits. YD Younger Dryas moraines; LE Loch Etteridge; AF Abernethy Forest

Evidence for significant ice advance rather than a standstill during general ice margin retreat includes the widespread presence of glacitectonised glacilacustrine sediments lying beneath till in stream sections along the eastern flank of Abernethy Forest (Bremner, 1934). The maximum of the advance is recorded by a trimline, with glacially-stripped granite surfaces below, and a small lateral meltwater channel at 832 m on Sròn a’ Cha-no (Fig. 3). The sharpness of the morphological contrasts above and below the trimline, the lateral continuity and the relationship to ice flow from the west all point to the limit representing an advance along Strath Spey. In Coire Laogh Mòr and southwards towards Coire na-Ciste, lateral moraine ridges and channels and an upper limit for schist erratics occur at the slightly lower elevation of 810 m. From the col between Sròn a’ Cha-no and Stac na h-Iolaire, sloping, schist-bearing moraines extend into Strath Nethy (Fig. 3). Lateral moraines from both Spey and Nethy ice meet near the base of the western wall of Strath Nethy and demonstrate the contemporaneity of the two ice masses (Fig. 4A). Extensive granite boulder accumulations derived from rock slope failures also occur on the valley floor in upper Strath Nethy (Ballantyne et al., 2009a). The lobate form of ridges on the surface of the boulder accumulations suggests deposition on to glacier ice, followed by glacial transport a short distance down the valley (Sugden and Clapperton, 1975). Residual Nethy ice on the valley floor also may have supported the eastern flanks of large, steeply-sloping fan deltas that extend downslope from the col (Fig. 4A). Horizontal benches cut in bedded gravel and sand on lower valley slopes (Fig. 4A) indicate that Spey ice blocked the outlet of Strath Nethy and so ponded ephemeral lakes between the separating ice margins. The fan deltas were fed by two main sets of meltwater channels in the col, a southern set with an intake at 725 m and a deeper northern set with an intake at 650 m (Fig. 4B). The moraine M1 at 720 m marks the presence of Spey ice at a time when the lower channels continued to pass meltwater through the col. After the ice surface dropped below the level of the Iolaire col, meltwater was

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diverted northward and sequences of lateral channels, some associated with the M2 and M3 moraines at 600 and 540 m respectively (Fig. 4B), mark the down wasting of the ice margin in Glen More. The M3 moraine forms a large ridge seen from the ski road (Fig. 4C).

Figure 3. Geomorphological map of the area between Glen More and Strath Nethy. Modified after Brazier et al. (1996).

1. Moraine ridge. 2. Fan or kame terrace. 3. Boulder accumulations derived from rock slope failures. 4. Cliff. 5. Ice-contact slope. 6. Glacial drainage channel. 7. Tor. 8. Sites with cosmogenic exposure ages.

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Figure 4. Ice-marginal landforms in the northern Cairngorms.A. View west across Strath Nethy.B. View north-east from the Coire na Ciste Car Park.C. View west from below the Coire Cas Car Park.(Hall et al., 2016)

a. Lateral moraine. b. Marginal meltwater channel. c. Kame terrace d. Delta fan. e. Glacial trimline.

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Figure 5.10Be exposure ages for the M1 to M3 moraines (Hall et al., 2016).

Figure 6. Dates constraining final deglaciation of the Cairngorms compared to the GISP2 and GRIP Greenland ice cores and stadial/interstadial events(Hall et al., 2016).

New cosmogenic 10Be exposure ages for boulder samples for moraines at 740-540 m O. D. in the northern Cairngorms indicate deglaciation at 15.1 ± 1.1 ka (Figs. 5 and 6). This timing matches existing age data for deglaciation of other sites along Strath Spey and for the Wester Ross Readvance (Fig. 7). Active ice retreat from the central Grampians was rapid and occurred within the ~1 ka uncertainty of the cosmogenic exposure ages. Comparison with Greenland ice core records (Fig. 6) indicates that ice advance and the onset of final ice retreat occurred at the close of Greenland Stadial 2a (GS-2a) (16.9-14.7 ka).

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Figure 7.The maximum extent of the readvance of the Strath Spey ice lobe is shown. For comparison, the limits are shown of the Loch Lomond Stadial (LLS) ice sheet in the western Grampians and of late stillstands or readvances of the BIIS across northern Scotland. Ice limits are shown at Ballater (B) and the basin of Feugh (F) in the Dee valley (Brown, 1993), at Elgin (E) in lower Strath Spey (Peacock et al., 1968), in Wester Ross (WR) (Ballantyne et al., 2009b) and at Loch Scavaig (Sc) in southern Skye (Ballantyne et al., 2016). C Cairngorm; M Monadhliath. Stars indicate the locations of radiocarbon-dated Lateglacial sites. 1.Abernethy Forest. 2. Loch Etteridge. 3. Morrone.

Figure 8. A schematic summary of known and inferred 10Be cosmogenic isotope exposure ages for boulders in the northern Cairngorms.

A large number of 10Be cosmogenic isotope exposure ages are now available for granite boulders in the northern Cairngorms. This allows discussion during the excursion of key questions related to this method of dating that include:

1. What are the sources of the granite boulders found on moraines? 2. Why do exposure ages vary on individual moraine ridges? 3. What might happen to cosmogenic nuclide inventories if the

current stock of boulders was recycled during a future phase of glaciation?

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Coire an Lochain: glacial chronology and snow avalanche landforms.

This accessible cirque contains a fine assemblage of glacial and nival landforms. Moraines represent two cosmogenically-dated climate stages: a Younger Dryas till sheet is one of the best-dated Younger Dryas deposits in Britain, while the Great Slab moraines in the upper cirque have yielded late Holocene 10Be ages (Kirkbride et al. 2014)

The Great Slab is a stepped 25-30° granite slab which merges upslope with the 100 m-high headwall. The junction of slab and cliff is free of talus. Two prominent debris ridges border the lower slab (Fig. 9). The two ridges do not meet at the base of the Great Slab, but form an opposing pair, and are interpreted as ice-marginal moraines. The Great Slab is a source of full-depth snow avalanches in heavy-snow years, which trim the lower ends and proximal slopes of the moraine ridges to deposit elongate splays of avalanche-reworked debris downslope (Fig. 9). The Great Slab moraines have yielded five 10Be ages ranging from 0.9 to 5.5 kyr BP. The moraines have been interpreted as late Holocene, possibly LIA moraines (Kirkbride et al. 2014), and are the only proposed Holocene glacial deposits in the United Kingdom.

Figure 9. Geomorphological map of Coire an Lochain, showing 10Be ages on granite boulders (Kirkbride et al. 2014)

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A glacial origin of the ridges is interpreted on the basis of their form and location. They contain matrix-supported angular granite clasts in a matrix of coarse sand and grit. Boulders are pink and less edge-rounded than the weathered grey boulders on nearby slopes and till sheets. They are smaller than those in the cirque-floor till sheets, and soil cover is thin with no horizon development. No clast shape analysis has been conducted because of the probable recycling and mixing of debris with different weathering and transport histories.

Suggested evidence against a Late Holocene glacier in the upper cirque

Following the publication of Kirkbride et al. (2014), there was wide interest in the finding that a small glacier could have formed in Scotland in recent centuries. There was (and remains) some scepticism, though no formal challenge has been made in an academic publication. Arguments advanced against a LIA age were varied and imaginative and are summarised below. Discussion of these is reserved for the excursion.

Climatic arguments

The cirque closest to glaciation today is Garbh Choire More (Braeriach) judged by the amount of late snow-lie since the 1930s. This cirque contains a small moraine ridge, but pedological evidence demonstrates that this could not have formed as recently as the LIA. It follows that if there was no LIA glacier there, there cannot have been one in Coire an Lochain, or indeed anywhere else.

Geomorphological arguments

These arguments centre on the interpretation of the landforms as ice-marginal moraines. Other suggested interpretations included protalus/pronival ramparts, avalanche boulder tongues, snow-push ridges, and debris-flow deposits.

Chronological arguments

It follows that if none (or perhaps one) of the five 10Be ages plot within conventional boundaries of the Little Ice Age, there is no chronological evidence to support the LIA age. The alternative interpretation is that the moraines date from the late Younger Dryas, and the dated boulders are either superimposed Holocene avalanche debris, or clasts exhumed by post-depositional moraine degradation. The pattern of ages is therefore significant (Fig. 10).

Glaciological arguments

There is scepticism that climate in the LIA was cold enough for long enough to form a viable and dynamic glacier capable of eroding its bed and constructing moraines. The example was given of a 1,500 year-old snowbed in Japan that has never moved enough to build a moraine. If a thicker glacier did form, it would have soon avalanched off the slab and disappeared.

Historical arguments

Proposed evidence falsifying the LIA hypothesis are varied and interesting, including place names (Coire an-t Sneachda is the “Snowy Corrie”, therefore an LIA glacier would have formed there if it had formed anywhere) and local knowledge (inhabitants of Aviemore would have been able to see the glacier from the valley, and therefore there would have been historical records of its existence).

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Figure 10. Distribution of cosmogenic exposure ages from Coire an Lochain moraines in relation to the main climatic events since the Younger Dryas Stadial. See Fig. 9 for sample locations.

A geomorphological model of post-Younger Dryas evolution of the upper cirque

The validity of the interpretation of 10Be ages is a key part of the LIA hypothesis. The ages could instead be interpreted as later accretion of debris on the moraine (Kirkbride 2016), or exhumation by post-depositional mass wasting (cf. Putkonen & Swanson 2003; Applegate et al. 2010), though Fig. 10 raises questions about the validity of such interpretations of age distribution. Here, a geomorphological history of the upper cirque is proposed which reconciles landform and chronology in a rationale scenario (Fig. 11).

Stage 1: Ice retreat from the Younger Dryas limit

The upper cirque was largely an area of subglacial erosion, and disappearance of the YD glacier would have left behind an ice-scoured cirque floor with sparse boulder cover. The right margin of the receding glacier appears to have formed a lateral moraine to c. 950 m altitude during recession.

Stage 2: Early and mid Holocene weathering phase

Free of ice for millennia and under a climate as least as mild as the present, gentle and moderately-angled surfaces acquired a litter of frost-weathered grit and clasts, soil and vegetation. Such a regolith can be seen beyond the late Holocene moraines at the same altitude and similar slope angle to the Great Slab. A near-complete but thin cover over granite slabs was probably present by the mid Holocene. Snow avalanche activity was less than in the subsequent period.

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Figure 11. Schematic reconstruction of Holocene landform evolution in upper Coire an Lochain.

Stage 3: Late Holocene deterioration and nival processes

Cooler and moister conditions in recent millennia are associated with Neoglaciation in northern mountain ranges. In Scotland, the threshold for glaciations was not crossed until very late. Instead, an increase in snow avalanche activity is possibly recorded in the

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disturbance of 3 out of 12 dated Younger Dryas till boulders on the lower cirque floor. These give ages of 2.7 ± 0.1, 4.4 ± 0.2 and 5.1 ± 0.2 kyr (Fig. 9). All three boulders lie close to each other in the centre of the cirque floor, below the west wall of the cirque.

Stage 4: Late Holocene glacier formation

Eventually the threshold for renewed glaciation was crossed, probably no earlier than 2.8 kyr BP, and possibly in the LIA after AD 1300 (Kirkbride et al. 2014). The equilibrium-line altitude (ELA) of 1050 m lay below the climatic ELA due to enhanced accumulation by blown snow. The 25 m thick glacier generated a basal shear stress of c. 60 kPa.

Debris production, moraine formation, and chronological implications: discussion

A common assumption is that moraine debris is produced during the contemporary glacial period, allowing straightforward interpretations of surface exposure ages. Such an assumption cannot hold when cool periods are short and rates of debris production low. Here, we suggest that a “young” moraine has been constructed from “old” recycled debris.

Order-of-magnitude differences exist cliff retreat rate since the Younger Dryas. If it is accepted that the moraines formed in the LIA, we can estimate how much of their constituent debris would have been contemporary with the cool period, how much reworked from the Holocene slope mantle, and how much originated in the Younger Dryas. We use a present-day rate of 0.01 mm yr-1 for most of the Holocene, 1.0 mm yr-1 for the Younger Dryas (after Ballantyne & Harris 1994), and 0.1 mm yr-1 for cooler phases of the Holocene (these are the 8.2 k event plus a paraglacial period of greater rockfall from the end of the Stadial to the 8.2k Event, and cooler parts of the LIA). Multiplying the duration of each period by the cliff retreat rate and normalising the resulting values to 100%, we estimate that a LIA glacier would have access to the following debris sources. We exclude YDS debris which would have been transported downvalley out of reach of the later glacier.

Total rock wall retreat during the YDS = 1.20 m Total rock wall retreat since 11.7 k yr BP = 0.39 m (100%) Rock wall retreat 11.7 to 0.7 kyr BP = 0.33 m (86.5%) LIA rock wall retreat 0.7 to 0.2 kyr BP = 0.05 m (13.0%) Post-LIA debris produced in the last 200 years = 0.002 m (0.5%)

What can we conclude from this?

1. 87% of the LIA till was reworked from pre-existing Holocene deposits. 2. We cannot expect the moraine to yield clustered contemporary LIA exposure ages,

but can expect a wide range of mid and late Holocene ages. 3. We can expect YDS boulder tills to yield contemporary ages, because debris

production was so much higher during the Stadial.

It is noteworthy that the nine dated YDS till boulders cover a 2.2 kyr range for a cold period lasting 1.2 kyr, but the LIA boulders cover a 4.6 kyr range for a cold interval lasting <0.7 kyr. The debris-production estimates provide a geomorphological rationale for this age pattern.

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Climate and glacier formation in Coire and Lochain

Coire an Lochain represents a topoclimatic niche within the Cairngorm Mountains, which appears to have been the most glacier-friendly locality in the whole massif in the Holocene. The Great Slab receives less short-wave radiation than anywhere at similar altitude (Fig. 12), combined with a relatively gentle surface on which to accumulate a deep snowpack. Comparably shaded localities tend to be much steeper and cannot accumulate snow to the great depths. By implication, there may be no other LIA moraine sites in the Cairngorms.

Figure 12. Modelled annual short-wave radiation receipt under clear-sky conditions for Coire an Lochain (left), Coire an-t Sneachda, and Coire Cas (right). Values <50% of the maximum are masked. Courtesy of Brice Rea (University of Aberdeen).

However a model of glacier cover for a range of climatic scenarios (Harrison et al. 2014) suggests that modest cooling of -1.5°C m M.A.T. combined with a 10% increase in precipitation would be associated with cirque glacier formation in several Cairngorm cirques (Fig. 13). The largest glaciers would be in the cirques on Braeriach (Lochain, Ruadh, Beanaidh and Bhrochain), and the north-east facing cirques on Cairn Toul (Lochan Uaine and Saighdeir). It is interesting that several of these cirques lacked glaciers in the Younger Dryas Stadial, and Bhrochain and Lochain Uaine contain sediments predating the LIA (Sugden 1977; Battarbee et al. 1999). Therefore the model conclusions can only be read as a general indication of the possibility of late Holocene glacier presence, and not as a map of their precise locations or extents. Indeed, visits to all these cirques have failed to identify geomorphological evidence of late Holocene glacial activity (Kirkbride et al. 2014 and subsequent observations).

There are many uncertainties associated with modelling the Cairngorm glaciers, in both the YDS and the LIA. Enhanced accumulation by blown snow is particularly difficult to model, and authors diverge wildly on how they define sources areas. All models are based on a dominant regional wind direction, whereas winter snowfalls often occur on one wind direction and are redistributed on another as pressure systems migrate, with complex local

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wind fields. Harrison et al. (2014) model a constant climate allowing glaciers to grow to equilibrium, though LIA climate was highly variable such that runs of warm years probably prevented glaciers attaining the dimensions that the most glacier-friendly decades would encourage. The model will overestimate former glacier cover, as the field evidence suggests. However, it is open to discussion is whether small LIA glaciers forming on porous boulder beds would leave any geomorphological imprint on the landscape.

Figure 14. Modelled ice cover for -1.5⁰C M.A.T. and 110% precipitation (Harrison et al. 2014).

Snow avalanche landforms in Coire an Lochain

Snow avalanche sources vary according to snowpack distribution, avalanche type and time of year. Late season snowslides from the Great Slab are usually small and in June 2014 modified the eastern Great Slab moraine by creating a small snow-push ridge (Kirkbride 2016). In heavy snow years large full-depth slides can reach the lake, where a small bay represents an impact pit and depositional rampart. The debris apron above shows excellent small-scale landforms including rip-up clasts and sediment tails in the lee of larger boulders. These avalanches also trim the lower ends of the late Holocene moraines.

A high sloping shelf southwest of the main lochan releases avalanches directly into the lake. In January 2015 a series of such avalanches first deposited slush on lake ice, which then froze to the lake ice. A second, large, avalanche displaced lake water and rafts of ice as a slush flow across the cirque floor, displacing boulders and depositing coarse grit and imbricate ice/refrozen snow/.turf stacks. A debris trimline was visible on the opposite side of the cirque floor on a visit in April (Fig. 14), as well as a layer of avalanche-deposited sediment. Anecdotal reports (Bullivant, pers. comm.) of similar events indicate that such avalanche aggradation of the cirque floor may be responsible for the boulder-studded “lawn” of fine sediment between the lochans, superposed on the Younger Dryas boulder till.

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Finally, the grassy western sidewall of the lower cirque collects thick snow cover from the plateau to the west, and mid-winter avalanches runout into the boulder field, depositing small amounts of fresh material and sometimes displacing till boulders (hence three anomalous mid- and late-Holocene 10Be ages from this site). Thus, nival processes in the cirque have been an active landforming process during the Holocene.

Fig. 14. Avalanche debris on lake ice and the grassy cirque floor following the 2014-15 winter, including soil, clasts, turf and vegetation transported by a slush flow of displaced lake water and ice.

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