GLWG 18 – Inverness, Moray Firth and Nairn, September 2017

I’m rather excited to be attending the 19th GLWG trip in September this year because I get to visit Iceland. I can barely wait, and in the excitement, it had me looking back over the report I wrote for last year’s GLWG trip to Inverness, the Moray Firth and Nairnshire, to look at the excellent glacial geomorphology preserved since deglaciation of the Celtic ice sheet. I thought I’d share this report with you via my blog, but it was also published first in the Quaternary Newsletter (QN 144, Feb 2018) of the QRA. So here it is:

JOINT QRA/GLACIAL LANDSYSTEMS WORKING GROUP FIELD MEETING 18

Nairn and the Inverness Firth – 26th-29th October 2017

Introduction

This year’s annual Glacial Landsystems Working Group trip, now in its 18th year, was to the area around Nairn and the Inverness Firth. The trip ran from Friday 27th October until Sunday 29th October with an optional extra day prior to the trip on the 26th October. The trip was well attended, with 37 attendees at all stages of careers from PhD, through professorship all the way to retirement, as well as attendees from industry and heritage and those just attending for personal interest.

Fig_1
Figure 1.
(Most of) the field meeting attendees at the Daless Viewpoint. Photo: Dave Evans

Optional day – Thursday 26th October – Cairngorm

The optional day was a visit to the slopes of Cairngorm to give an overview to the Devensian and Holocene history of the mountainous region. After an introduction to the area from Martin Kirkbride and Adrian Hall, we were led into the hills, past the Younger Dryas ice limit and into Coire an Lochain, up to the Great Slab. Here evidence was presented for and against the presence of a Late Holocene glacier. The Great Slab is a clean, low angle (25-30°) face of granite which has on either side two ridges which converge in a downhill direction, but do not meet underneath the slab itself. These ridges are interpreted to be ice-marginal moraines and were considered not to meet due to later avalanching processes removing their downhill extent. Martin provided a good deal of evidence to support the presence of a small glaciation but welcomed ideas which would explain the geomorphological evidence. A lively discussion ensued, despite the rapid deterioration in weather conditions, with members of the party arguing for avalanching processes building up the ridges. Further reading on the evidence for, and modelling in support of, a Little Ice Age glacier in Coire an Lochain can be found in Kirkbride et al. (2014) and Harrison et al. (2014).

After a rapid fish supper, introductory talks were given that evening by Jon Merritt, Clive Auton, Emrys Phillips, and Callum Firth, at the quirky Pagoda events venue in Grantown. During these talks we were introduced to the late Quaternary history of the area and shown many fantastic images of the field areas we were to visit over the following few days. Inevitable discussions broke out about the longer-term evolution of the area, as well as the short-term post-glacial sea level history, but as they were threatening to reduce pub time before last orders, they were remembered and saved for the specific field localities we would be visiting.

Day 1 – Friday 27th October – Sites close to the Moray Firth coast

Thankfully, the Friday morning dawned bright and cheerful for a classic Scottish autumn day, with the sun’s first rays irradiating the autumnal arboreal hues. The convoy set off (note- try not to get stuck behind an old BGS Hilux that produces more black smoke than a hydrothermal vent) through the melt-water channel of the Slochd, driving past Inverness and reaching our first stop for the day.

Site 1 – Alturlie Point

The first stop was to visit the active Bothyhill gravel pit to see the deltaic sequences of the Bothyhill Gravels Member of the Alturlie Gravels Formation. Unfortunately, quarrying activity has reduced the impact of the site, and the kettle-hole infills which were once visible have been removed. Kettle holes to the south of the gravel pit lie topographically lower than the top of the delta/subaqueous fan complex, indicating the presence of ice whilst the sand and gravel was being deposited.

Site 2 – Ardersier Peninsula

Further to the east, the coastline becomes delineated by a large push moraine, running east-west, before arcing to north-south at Ardersier. The push moraine itself is evidence of the Ardersier Oscillation, where ice from the Great Glen and Inverness Firth readvanced and deformed glaciomarine sands and silts of the Ardersier Silts Formation. At the gully section close to the car park, we were treated to spectacularly-deformed rhythmically-bedded silts, with Emrys Phillips excitedly explaining to us the deformation processes involved in push moraine formation.

Site 3 – Kingsteps

After a picnic at Ardersier and a detour through a petrol station forecourt, we arrived at Kingsteps to study the raised shorelines exposed as a set of terraces in fields below Lochloy House. Two or three terraces could be seen, which are cut by, and therefore predate the so-called ‘Main Postglacial Cliffline’, which formed during the Younger Dryas. Whilst farming activity has degraded the terraces, they are still visible, but are better preserved in the Christmas Tree plantations to the east. The raised shorelines and shingle ridges provide evidence which constrains the postglacial isostatic rebound of the area, however, some members of the trip felt that this, combined with the poorly-constrained nature of the work presented by Lambeck (1995), was too far outside of a quantifiable mean sea-level range to provide adequate evidence of punctuated RSL change, as many different permutations of a sea-level curve could be viable.

Site 4 – Easterton

A quick stop was taken at some farm buildings on top of a transverse ridge. These transverse ridges have been interpreted as crevasse-squeeze ridges. Little evidence of the typical sedimentology associated with subaqueous landforms could be seen within the Easterton section. This, combined with their altitude and their interrelationship on DEMs, led most members to agree that the landforms were more consistent with a corridor of crevasse-squeeze ridges, similar to those observed in Saskatchewan, Canada (Evans et al., 2016). Other transverse ridges occurring towards Elgin and on Tarbat Ness, to the north of the Inner Moray Firth, have been described as De Geer moraines but are in some places at 60 m above OD. These are unlikely to be glaciomarine De Geer moraines, if they are at all De Geer moraines, due to their age and elevation, unless there was a relative sea-level fall of 30m in less than 500 years (~60 mm/yr), against a background Global Mean Sea-Level rise of approximately 5 mm/year (Bassett et al., 2005).

Site 5 – Cloddymoss

Another quick stop was taken at Cloddymoss to view the distal Ardersier Silts contacting the underlying subglacial Easterton Member of Finglack Till Formation. These are the formations into which the Main Postglacial Cliffline is eroded.

Site 6 – Grange Hill

The final stop for the day was the large, flat-topped ridge surrounded by Late Devensian and Holocene marine deposits. These have previously been interpreted as chasm fill in a large crevasse (Peacock et al., 1968). However, the arcuate nature of this and the neighbouring ridges (Figure 2) are reminiscent of the Ardersier push moraine and, similar to that seen in Jamieson’s pit earlier in the day at Ardersier, is also capped by a thin orange-brown diamict. Another possible interpretation is therefore that the ridges are push moraines formed during what has been termed the Grange Hill Oscillation, formed by readvance of the Moray Firth Glacier, temporally between the Elgin Oscillation and the Ardersier Oscillation seen earlier in the day.

Fig_2
Figure 2. View from the flat-topped Grange Hill, over Holocene marine deposits, to the adjacent, arcuate flat-topped ridge.

Day 2 – Saturday 28th October – Sites within the Lower Findhorn Valley, Strathnairn and Strathdearn

Site 1 – Meads of St John

Similarly to the previous day, we were fortunate to experience more glorious weather the next morning. Even more glorious were the subglacial hydrofractures through the Devonian bedrock (Figure 3). Healthy discussion was had, led by Emrys Phillips, about how such features form. Discussion of the geomechanical regime necessary to form such features was had, including the source of the overpressure required to form a hydraulic fracture, as well as the role of principle stresses and stress transference from subglacial movement causing sub-horizontal fracturing.

Fig_3
Figure 3.
Complex hydraulic fracturing in Devonian Old Red Sandstone bedrock at Meads of St John.

Site 2 – Dalcharn

Driving down the unmetalled road towards Dalcharn took us from the Last Glacial Maximum all the way back into the Last Interglacial. At Dalcharn West, a series of subglacial tills underlie and overlie a palaeosol. Although the pollen record suggests a Hoxnian age (Walker et al., 1992), an infinite radiocarbon date of 41,300 BP together with maximal luminescence ages of 68 and 50 ka suggest that the palaeosol formed during the Ipswichian. Hard graft put in by a few intrepid team members removed fallen trees to uncover what remains of the palaeosol. This revealed an inverted sequence to what was originally observed, with the peaty Dalcharn Biogenic Member underlying the siltier Dalcharn Cryoturbate Member. Whether or not the formation can be considered as autochthonous and deformed or rafted in later is still uncertain. Clearly, the site has great potential to yield vast information about the longer-term palaeoclimatic and glacial history of Scotland if more work is put into unravelling its mysteries.

Site 3 – Clava

Clava is a historical locality, having been visited by such notable figures as Horne, Jamieson, Agassiz and even Charles Darwin. Their interest, and those of many authors since, had been drawn by a shelly till at 150 m above OD. The origin of these marine deposits has been debated frequently, being 110 m higher than any other known marine deposits. Most now agree that they have been ice-rafted, but explaining the mechanism by which a raft can remain relatively undeformed after being rafted up to 50 km laterally and over an hundred metres vertically is difficult. No evidence of freezing is seen in thin section, therefore Phillips and Merritt (2008) and later, yet to be published, revised models propose a mechanism of elevated fluid pressures subglacially squeezing the imbricated stack of glacitectonically rafted blocks along the base of a glacier. These duplexes were then accreted onto the side of a more resistant bedrock high as the Great Glen Glacier was forced upwards and over metamorphic bedrock. This conceptual model still has a few issues and trip members were quick to point these out, such as how little internal deformation there is without having been frozen to the bottom of the glacier, although were unable to offer further explanation without further thought. The actual presence of the shelly till was hard to determine though, as the field meeting party only found one 2 mm bivalve shell throughout the entire outcrop!

Site 4 – Flemington Eskers

With darkness encroaching, a quick final stop was made to visit the eskers and flat topped ridge at Meikle Kildrummie. The Flemington Eskers are a series of beautifully-preserved braided eskers that become a flat-topped ridge towards the east. The flat-topped ridge is proposed to be a glaciomarine delta formed subaerially at the mouth of the braided glaciofluvial esker system, and the presence of kettle holes on the ridge support this. The position of the eskers is at the approximate confluence of the Monadhliath and Great Glen ice streams, therefore these eskers were considered to have formed under the shear margin where the two ice streams met. Such an interpretation may lead to the reinterpretation of the flat-topped ridge to be an ice-walled chasm, similar to that proposed at Grange Hill, which lies further east, but on the same trend as the Flemington Eskers.

Day 3 – Sunday 29th October

Site 1 – Daless Viewpoint

The final day began with a short, steep stroll up to a fantastic viewpoint (Figure 4) over the Middle Findhorn Valley where Adrian Hall and David Jarman explained the significance of the valley in terms of the long-term denudation history of Scotland. The lack of geomorphological evidence of extensive valley glaciation and the smooth “summit surface” topography implies that cumulative glacial erosion here was not extensive, and that the Nairnshire mountain tops are close to their original height. This interpretation is backed up by the extent of weathering of the Caledonian granite, which has been severely degraded to loose sands up to a depth of 8-10 m.

Fig_4
Figure 4.
David Jarman describes how big the one that got away was at Daless Viewpoint.

The glacial geomorphology of the valley is mainly glaciofluvial and glaciolacustrine in origin. Up to 14 terrace levels have been identified, mainly consisting of kame terraces but also fans and deltas. The promontory which provides the viewpoint would have provided a dam for proglacial lakes in this area. The rest of the dam was proposed by some members of the party to be consistent with observations of more dynamic ice (compared to that of the relatively stagnant, and therefore causing little erosion, ice in situ) flowing back up the valley from the south, back-filling the valley and providing further damming for proglacial lakes.

Site 2 – Banchor

The final stop for most of the party was to the banks of the River Findhorn at Banchor, where erosion has revealed a fantastic sequence of glaciofluvial and glacilacustrine sediments. The bottom of the sequence exposed beautiful climbing ripples (Figure 5) from a glaciofluvial sequence that transitions through delta bottomset gravels before muddy and sandy rhythmites are exposed from the proglacial Lake Findhorn. Occasional debris flows containing angular blocks can be seen sporadically throughout the sequence; these are thought to have been rock fall from the slopes of the surrounding hills flowing laterally into the lake. After this, we said our long goodbyes and faced our journeys home (mine was to Keyworth, 7 hours’ drive away, but others had to drive to London!)

Fig_5
Figure 5.
Climbing ripples in glaciofluvial sediments at Banchor.

Acknowledgements

I would like to take this opportunity to express my thanks on behalf of all the field meeting attendees to the organisers, Jon Merritt, Clive Auton, Emrys Phillips, Callum Firth, Adrian Hall and Martin Kirkbride, as well as the valuable discussions and contributions of all others. The trip was an undeniable success, the weather was almost perfect, the logistics ran smoothly and most of all, everybody was able to visit some fantastic glacial geomorphology and learn, discuss and debate (albeit in a friendly manner) the fascinating, sometimes enigmatic features visited. The field guide which accompanied the trip, The Quaternary Around Nairn and The Inverness Firth, is available now from the QRA field guides page on the website. I encourage you all to buy this impressive book, in which you will find far more detailed information about the field sites, as well as other references. Jon Merritt and Emrys Phillips are thanked for their comments on an earlier version of this report.

References

Bassett, S.E., Milne, G.A., Mitrovica, J.X. and Clark, P.U. 2005. Ice Sheet and Solid Earth Influences on Far-Field Sea-Level Histories. Science. 309(5736),pp.925–928.

Evans, D.J.A., Storrar, R.D. and Rea, B.R. 2016. Crevasse-squeeze ridge corridors: Diagnostic features of late-stage palaeo-ice stream activity. Geomorphology. 258,pp.40–50.

Harrison, S., Rowan, A. V, Glasser, N.F., Knight, J., Plummer, M.A. and Mills, S.C. 2014. Little Ice Age glaciers in Britain: Glacier–climate modelling in the Cairngorm Mountains. The Holocene. 24(2),pp.135–140.

Kirkbride, M., Everest, J., Benn, D., Gheorghiu, D. and Dawson, A. 2014. Late-Holocene and Younger Dryas glaciers in the northern Cairngorm Mountains, Scotland. The Holocene. 24(2),pp.141–148.

Lambeck, K. 1995. Late Devensian and Holocene shorelines of the British Isles and North Sea from models of glacio-hydro-isostatic rebound. Journal of the Geological Society. 152(3),pp.437–448.

Peacock, J.D., Berridge, N.G., Harris, A.L. and May, F. 1968. The Geology of the Elgin District, Memoir of the Geological Survey, Scotland, Sheet 95.

Phillips, E. and Merritt, J. 2008. Evidence for multiphase water-escape during rafting of shelly marine sediments at Clava, Inverness-shire, NE Scotland. Quaternary Science Reviews. 27(9–10),pp.988–1011.

Walker MJC, Merritt JW, Auton CA, Coope GR, Field MH, Heijnis H, Taylor BJ. 1992. Allt Odhar and Dalcharn: two pre-Late Devensian (Late Weichselian) sites in northern Scotland. Journal of Quaternary Science 7: 69-86.

 

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