Cairngorm Workshop

The history of the Cairngorms: granite, landscape and processes

An informal workshop at the British Geological Survey, Murchison House Edinburgh.

Organiser: Dr Chris Thomas

Monday 17th November, 2003.

Abstracts


Cairngorm Deglaciation: A cosmogenic 10Be dated stillstand

Dr. J. Everest

jdev@bgs.ac.uk

British Geological Survey, Murchison House, Edinburgh

Past discussion has centred on the interaction of the SIS in the Spey, and Cairngorm ice. Debate has focussed on the extent of glaciation in the Cairngorms during the Loch Lomond Stadial. The significance of Cairngorm glaciation must be seen in the light of the spatial relationship of the massif to ice sheets further west, and the combined response of Scottish glacial systems to hemispheric climate change.

New cosmogenic 10Be data is presented here confirming a long-postulated stillstand in both the Cairngorms and the Scottish Ice Sheet during deglaciation. Ages derived from three sets of moraines argue for an event synchronous in both ice masses. The event is constrained by geomorphology, radiocarbon and tephrochronological data.

The implications of the study are far reaching. The ages confirm that there was no extensive valley glacier system in the Cairngorms during the Loch Lomond Stadial. The synchrony of the results argues for a large scale, climate-driven event, rather than a local alteration in ice dynamics, the most likely trigger being Heinrich Event 1.

 

Preliminary findings from Ryvoan Pass kettlehole

 

Dr. Mike Kaplan

mkaplan@geo.ed.ac.uk

Institute of Geography, School of GeoSciences, University of Edinburgh,

Drummond Street, Edinburgh EH8 9XP

Abstract

A core taken from a well developed kettle hole feature in the Ryvoan Pass in 2002, has yielded a wealth of new data regarding climatic and environmental change in the region. The basal section of the core (at 7.4 to 8.1m) is made up of finely laminated minerogenic clays, overlain by peat. The very base of the core at 8.1 m is composed of an organic clay layer.

Physical examination of the core has been carried out using magnetic susceptibility, grain size fractionation, percentage mass loss on ignition, and x-ray greyscale variation. To provide a temporal framework downcore, a series of samples have been dated using 14C .

The results reveal the signature of a highly variable climate over a short timescale, and combined with the 14C data provide information about the environment around Ryvoan before, during and after the Loch Lomond Stadial period.

Future work, in collaboration with the University of Plymouth and BGS is planned to carry out palynological, diatom and radiometric dating on cores to be extracted from the basin in 2004.


 

Discriminating Younger Dryas moraines from earlier deposits using boulder edge-roundness

Dr. Martin P. Kirkbride

m.p.kirkbride@dundee.ac.uk

Department of Geography, University of Dundee, Dundee DD1 4HN.

 

A method is presented for quantifying the degree of edge roundness of large crystalline boulders using a simple instrument. The method assumes that the edges of supraglacially-transported boulders were initially sharp and have subsequently become rounded by granular disintegration, so that the degree of rounding is some function of time. Edge roundness can be precisely measured in the field if strict morphological criteria are adhered to when selecting boulders to measure. Roundness can be expressed as a length measurement between fixed points on adjacent facets, as a normalised index, or as a radius of curvature. Field measurements from Cairngorm corries reveal two groups of deposits differentiated by the degree of weathering. On this basis, Younger Dryas glaciers appear to have been fewer and less extensive than previously mapped, and an earlier Lateglacial advance is recognised.

 

New perspectives on a classic landscape of selective linear glacial erosion

 

Dr. Adrian Hall

am.hall@landforms.eu

Fettes College, Edinburgh

The Cairngorms is the type landscape for selective linear glacial erosion (Sugden, 1968). Terrain configuration has determined basal ice conditions which have led, in turn, to sharp contrasts in depths of glacial erosion. Recent observations show that glacial erosion on the Cairngorm plateau has varied in both space and time. Data from morphology, relative age criteria such as weathering pit depths and cosmogenic exposure ages indicate significant glacial modification of parts of the plateau during at least two phases of ice cover. The oldest rock surfaces on the plateau may predate the first Cairngorm ice caps but most surfaces have continued to evolve over the last 106 yr.


 

Geological constraints on landscape development in the Cairngorms

Dr. Martin Gillespie

mrg@bgs.ac.uk

British Geological Survey, Murchison House, Edinburgh

The Cairngorm massif is renowned for its distinctive landscape character: it is an erosionally-incised dome characterised by extensive upland plateaux, deep linear troughs, spectacular corries and tors. The massif is underlain by a single geological entity, the Cairngorm Granite, pointing to a close genetic relationship between the landforms and solid geology. Detailed examination of the granite and the landscape features formed upon it has shown that the incised character of the Cairngorm massif is the result of focussed erosion along linear zones of rock that was altered and weakened by the passage of hydrothermal fluid shortly after the granite was emplaced. This suggests that many or most of the valleys we see today probably began to develop as soon as the granite was unroofed, around 400 million years ago. The basic architecture of the Cairngorm landscape may therefore have changed little since the Devonian Period. 

 

Luminescence-age constraints on the stability of the Strath Spey ice stream - new data for the timing of Cairngorm deglaciation

Nick Golledge

nrg@bgs.ac.uk

British Geological Survey, Murchison House, Edinburgh

The application of blue-light stimulated single aliquot luminescence techniques to glaciolacustrine sequences of laminated sand and silt in two valleys of the northern Cairngorms has yielded age determinations that indicate stability of the southern margin of the Strath Spey ice stream for > 5 ka. The sample sites in Gleann Einich and the Lairig Ghru, formerly described by Brazier et al., (1998), reflect ice-dammed lake formation during Late Devensian deglaciation, resulting from separation of the Strath Spey ice stream from outlet lobes of Cairngorm plateau icefields. Whilst the existence of these glaciolacustrine sequences has been known for over a century, the OSL data described here are the first to provide an absolute constraint on the time of their formation. Significantly, lake sedimentation appears to have taken place sporadically over several thousand years, with the main phase of ponding occurring at around 17 – 16 ka BP (calendar). The implications of the range of dates are, 1) that ice separation in these valleys occurred earlier than previously thought, perhaps soon after the Late Devensian Glacial Maximum, and 2) that although the Strath Spey ice stream oscillated in response to Heinrich Event 1, the precipitation-starved Cairngorm ice did not.

 

Modelling the sub-Devonian land surface in NE Scotland: implications for long-term denudation of Highland Scotland

Prof. David Macdonald

d.macdonald@abdn.ac.uk

Department of Geology & Petroleum Geology, University of Aberdeen, Aberdeen AB24 3UE

As part of a NERC-funded project on evolution of continental margin drainage, a team from Aberdeen and Glasgow has developed a GIS-based erosion model for Highland Scotland and carried out work modelling past erosion surfaces, handling these in software packages used to model seismic reflection surveys.  We can use isochores between dated erosion surfaces as an accurate guide to volume and nature of denuded material.  Work to date has concentrated on the sub-Devonian erosion surface, represented by numerous outliers of Old Red Sandstone across the eastern and northern Highlands.

Our results indicate that the modelled Devonian surface is close to the present-day surface.  Less than 3,000 km3 of material has been derived from below the post-Caledonian (Devonian) erosion surface (for comparison, there are c.5,000 km3 of Paleocene deposits in Quad 15 alone).  This means that much of the Tertiary denuded product was Devonian or younger sediment, rather than primary erosion of Caledonian basement.  This finding is consistent with unpublished data of Dr N.J. White (U. Cambridge) suggesting only 1-2 km of Highland erosion since the Cretaceous and modelling work by Dr R.W. England which suggests rapid slowing of denudation through the Tertiary.

We believe that Highland Scotland, including the Cairngorms, is close to an exhumed land surface which has been affected by relatively modest rates of Neogene denudation.

 

The pattern of deglaciation on the Gaick Plateau

Jon Merritt

jwm@bgs.ac.uk

British Geological Survey, Murchison House, Edinburgh

The pattern of glacial retreat on the Gaick Plateau has been established from mapping the distribution of ice marginal glacial drainage channels and morainic ridges. At first Strathspey ice began to retreat from the NW margins of the plateau whilst Perthshire ice retreated from the southern margins. Ice sourced in the Cairngorms also began to retreat from the valleys of the Upper Feshie and Geldie Burn, to the NW of the plateau. Ice sourced on the plateau subsequently advanced into vacated valleys, particularly on its southern side. The advances were probably short-lived as no well-developed terminal moraines were formed; they were probably diachronous. An independent ice dome centred over the eastern parts of the Gaick shrank concentrically towards the lowest ground. Evidence within, and to the west of the central breach indicates that ice retreated westwards towards the Pass of Drumochter.

The pattern of retreat that emerges is not too dissimilar to that deduced by Charlesworth, who recognised two independent ice dispersion centres on the Gaick Plateau, but it is difficult to reconcile with Sissons’ model of a single ice cap that built-up entirely during the Loch Lomond Stadial.

 

Cosmogenic nuclides applied to Cairngorm geomorphological problems

Dr. Bill Phillips

wmp@geo.ed.ac.uk

Institute of Geography, School of GeoSciences, University of Edinburgh,

Drummond Street, Edinburgh EH8 9XP

The cosmogenic nuclides 10Be and 26Al are widely used to construct glacial chronologies by surface exposure dating of erratics and glacially eroded bedrock. Accuracy of cosmogenic dates may be sensitive to local geomorphic factors including 1) post-depositional erosion; 2) transient shielding of sampled surfaces by soil or snow; 3) prior exposure of surfaces (nuclide inheritance). All three problems potentially complicate the use of cosmogenic techniques in the Cairngorms. Grain-by-grain erosion at rates similar to those in other arctic/alpine environments (1-3 mm/ka) do not greatly influence exposure ages. Recent spalling of blocks from the edges of erratics significantly reduces exposure ages. Snow or ice shielding is important for corries and on plateau sites where tors have undergone multiple episodes of glaciation. Shielding of bedrock surfaces by peat and exhumation of small erratics by soil erosion from moraines is likely at lower elevation sites. Nuclide inheritance dominates at higher elevations where feeble erosion by cold-based ice has failed to reset the cosmogenic nuclide clock.