Glacial Trimlines and Nunataks in Assynt Region of Scotland

Title

To what extent are glacial trimlines and nunataks present in the Assynt region of North West Scotland and how does this affect features above and below the boundary?

or

To what extent is a glacial trimline present in the Assynt region of North West Scotland and how does it affect features above and below the proposed boundary?

Abstract

The Assynt region of North West Scotland, north of the town of Ullapool, is located on The Moine Thrust belt, which stretches from Lock Eriboll on the north coast to the Isle of Skye, approximately 120 miles south. This marks the point where the old Moine schist rock, around 1,000Ma, thrust over younger rocks, creating an unconformity between the Moine schist and the Durness limestone, which was metamorphosed and altered below the thrust, from 500Ma. The area is rich in Quaternary geology, providing evidence of direct ice action and periglacial features not directly linked to ice flow. These Quaternary features are split by a theoretical thermal boundary called a glacial trimline, supposedly representing the highest vertical extent of the glacier, with periglacial features lying above the boundary and ice flow erosional features below. These features will be studied in order to provide evidence for the trimline, with the measurement of rock hardness around the area providing the best information.

Background Geology

The oldest rocks present, gneisses of the Lewisian complex, of Archaean age, have undergone three major periods of deformation, the first of these being the Badcallian event, where dominant foliation was produced, followed by the second period of deformation called the Inverian event. The Scourie dykes, a suite of dykes, intruded the Lewisian complex before being deformed during the third period of deformation, named the Laxfordian event, dated around 1.7Ga. The Lewisian complex can be divided into the Rhiconich, Assynt, Gruinard and Southern Terranes. The boundary between the Assynt and Gruinard terranes lies along the Canisp Shear Zone. Both hold different tectonic histories, but were combined by the Palaeoproterozoic, around 2.4Ga, evident from the intrusion of the Scourie Dykes. (Trewin, N.H, 2003)

The Archaean Lewisian rocks are then unconformably overlain by the Torridon group of red sandstones and conglomerates, deposited in fluiviatile and lacustrine environments, dated approximately between 1.2Ga to 1Ga in the Proterzoic. These red sandstones were introduced by rivers and buried under old hills and mountains. The Torridon sandstones, tilted, eroded and overlaid the previous Stoer group around 1Ga. (http://www.earth.ox.ac.uk/~oesis/nws/nws-geolhist.html)

After a subsequent period of uplift and non-deposition, the region was transgressed and marine Cambrian quartz arenites unconfomormably overlaid the Torridon and Lewisian groups. These quartz arenites differ from the Torridon sandstones, particularly in their white colour and via the presence of vertical burrows from ichnogenera Skolithos and Monocraterion, highlighting the early Cambrian as the upper age bound. (K.M. Goodenough et al, 2009)

The Fucoid Member, a thin detailed unit of brown weathered siltstones, overlies the quartzites. (Trewin, N.H, 2003) This in turn is followed by the Salterella Grit member, a very thin layer of quartzites, which overlies the fucoid member, all of which are dated as early Cambrian in age. Comformably overlying the clastic unites is the Durness group of carbonates, the youngest sediments in the region, which range in age from early Cambrian to early Ordovician, around 542-475 Ma. (Trewin, N.H, 2003)

Abundant thrusting is present throughout the Assynt region from late Ordoivician to early Silurian times with widespread deformation having occurred. Four thurst sheets are present, the lowest of which, the Moine sheet, containing units allocated to the Moine Supergroup, settled upon the Lewisian complex, deposited around 900Ma. (Krabbendam, M. and Leslie, A. G, 2010) The sediments within the Moine supergroup are predominanty shallow marine arkosic sandstones. Major movement along the Moine thurst occurred around 440-430 Ma, recorded via Rb-Sr dating of mylonites, also showing the fine grained platy rock mylonite formation along the thrust. (Freeman, S,R et al, 1998)

Quaternary Geology of the area

Over the last 2 million years, the landscape of North West Scotland has been dramatically altered by climate change, believed to fit the ‘Milankovitch’ timescale. Fluctuations of temperature, from periods of warmth to periods of cold and ice have specifically transformed upland areas. The weight of the ice caused the country to be lowered, coupled with lower sea levels due to the capture of water in the ice, before abrupt climate change forced melting of the glaciers and ice sheets. This triggered the release of vast amounts of water, depositing sands and gravels offshore and in river valleys. (Lowe, J. J. and Walker, M. J. C, 1997) Alongside this, sea levels dramatically rose due to the massive increase in water, forming beaches above the sea level, left today as raised beaches. (http://www.snh.org.uk/publications/on-line/geology/scotland/ice.asp) The movement of the ice due to gravity under its own weight and its fluid nature caused destruction in its path. Due to its destructive nature, evidence is removed, making timing, extent and individual impacts difficult to record.

During the past 30,000 years, there have been three major periods of glaciation, with interglacial periods interspersed, where there was no ice during summer months except in some areas of high latitude and altitude. Shorter periods between glacials are termed interstadials, when warm temperatures present and stadials, where temperatures are relatively cool. Two of these glacial periods had a profound effect on the Assynt region. The first and oldest of these glacial periods, named the Late Glacial Maximum, is dated approximately between 29,000-22,000 years ago. An ice sheet of over 800m in thickness was present, leaving only a small number of nunataks visible. Nunataks are exposed peaks or ridges above the ice sheet, often rocky in nature.

The youngest glacial event is the Loch Lomond Stadial, around 13,000 to 11,000 years ago, named after the Loch itself, which formed as a result of glacial movement due to the removal of rocks, dug out by the ice. (http://www.scottishgeology.com/geo/regional-geology/midland-valley/south-end-of-loch-lomond). It left moraines, nunataks and outwash terraces in many valleys and some small moraines in corries, with the moraines helping to chart the ice margin retreat. The period was ended due to a rapid increase in climatic temperature, subsequently starting the Holocene.

Glacial features are present around the Assynt region which help to chart ice flow direction. These include striations, grooves, crescent gauges and friction cracks to name a few, each of these mostly on a small scale and therefore easily recordable. They are found most commonly on the Cambrian quartzite and the pipe rock; however, small numbers have been mapped on Torridon sandstone. Striations are formed by abrasion of loose rocks and pebbles at the base of a glacier, forming scratches in the rock, the direction of the scratch indicating the directional flow of the ice. At times however, they can be confusing due to different glaciers at later dates cross cutting the previously formed striations from a different direction. The striations have to be subsequently studied in detail to determine which period of ice movement came first. Gauges, in the form of crescent moon shapes, form when boulders within an ice sheet or glacier are pressed against the bedrock. These boulders rotate slightly as the ice sheet or glacier moves, dragging them simultaneously with the rotation, causing crescent shaped indents in the bedrock. Gauges are useful for determining flow direction, as the flow of ice often points in the same direction as the gauge. Gauges can often be easily confused with friction cracks if they have been altered by weathering; however, gauges are normally greater in size. Friction cracks are formed due to an increase in friction between the ice sheet and bedrock below it, with boulders and pebbles bouncing off the bedrock, meaning pressure is not continuous. In terms of ice flow direction, they point in the opposite way to the gauges.

Moraines, another feature of glacial movement, are accumulations of deposited till. Different moraines are formed in different areas passed by the glacier. Terminal moraines form at the terminus, or end of the furthest point reached by the ice, whereas lateral moraines form at each side of the glacier and medial moraines are formed at the intersection between two glaciers. The deposition of the till can happen in three different areas of the glacier, with subglacial at the bottom of the glacier, marginal deposition on the margins of the ice, and supraglacial sitting on the surface of the ice sheet. Fluvial action can subsequently rework the deposited till and moraines, mutating their characteristics and morphology. Till fabrics can also be studied in order to provide evidence of glaciation. Tills are deposited at different areas of the ice flow, with the position of these and the orientation of the clasts helping to map the direction of ice flow in the area.

Periglacial landforms are also present in the region, categorized as areas that form adjacent to glacial terrain or in areas of close similarity and that hypothetically form above the proposed trimline, where freeze thaw weathering often occurs. Patterned ground features are some of the most common structures found, including stripes, nets, circles, polygons and steps, each formed either by sorting or non-sorting of sediment. Nets and stripes are the two most common of these features found in the Assynt region. Nets are found between polygons and circles, with small scale earth hummocks with a core of mineral soil being a common unsorted net. Stripes form on steep slopes, with sorted stripes comprising of alternate stripes of fine and coarse material and are particularly prominent under conditions of permafrost. (Washburn, A.L, 1979) It is believed that both are formed by repeated freeze thaw weathering on sloped ground. Blockfields are one of these features, predominantly found on mountain plateaus in unglaciated areas, helping to provide evidence of the trimline. They form as a result of freeze thaw weathering, where rocks are shattered in situ and jointed, both vertically and horizontally. They are often made up of shattered quartzite. Solifluction is another feature of periglacial weathering, involving the mass wasting from freeze thaw cycles. Silty and sandy soils are common in solifluction, with the process forming lobes, terraces, stripes and hummocks.

Aim – Trimlines

The aim of the project is to discover the existence of a glacial trimline, which marks the highest point of the most recent glacier or ice sheet. However, it is apparent that in some areas, unmodified periglacial terrain survived glacial maxima under cold based ice and in these scenarios, the trimline represents a thermal boundary between cold based ice and warm based ice. (Elias, S.A, 2006). Other hypothesis include a timeline cut by glacial readvance during ice-sheet downwastage, or the trimline forming during initial ice-sheet downwastage under periglacial conditions. ((Goudie, A.S, 2003) The sharpness of this boundary relied upon the effectiveness and intensity of glacial erosion, the degree of frost weathering after its formation and the downslope mass movement during and after deglaciation. (Goudie, A.S, 2003) Schmidt hammer measurements, detailing hardness, the roughness of the rocks present around the proposed boundary and measurements of differential relief are amongst some of the ways in which these hypotheses have been tested. Studies in other areas, such as the Gap of Dunloe, Ireland, using these measuring techniques, have shown that periglacial trimlines mark the upper limit of a body of ice. (Rae, A.C, Harrison, S et al, 2004). Similar results are expected to be seen in the Assynt region.

What we need

For the project to be successful and for our research to be undertaken, a number of items will be necessary. Field maps will be vital in order to navigate to proposed sites, whilst also allowing outcrops and features to be marked. These maps will range in scale from large maps of the whole area, at a 1:10000 scale to small more precise maps for more detailed study and navigation. To study our hypothesis of glacial trimlines, Schmidt hammers will be needed in order to measure the hardness of the rocks, where the rocks should be softer above the boundary. A GPS system will also be necessary, equipped with an altitude reader, allowing site positioning to be recorded precisely, for revisits for further study. The size of certain facies and outcrops will need to be measured accurately, so a long tape measure will be needed. A compass clinometer will be necessary for measuring strike and dip of glacial features such as striations and to ascertain the direction that certain features face, allowing ice flow direction to be understood. A geological hammer would also be a useful addition to the study, allowing segments of rocks unaltered by moss and weather conditions to be studied. Coupled with this will be a hand lens and grainsize charts, allowing the rocks to be studied in precise detail. Due to the nature of our study, in regards to finding the thermal trimline boundary, a large number of mountain peaks will have to be scaled, so warm and weatherproof clothes will be needed according to weather conditions. The Schmidt hammer, GPS, compass clinometer and tape measure will be borrowed from the university geology department, where the maps needed will also be highlighted and printed.

Methodologies

To test the hypothesis of the existence of a glacial trimline, Schmidt hammer measurements will have to be taken around the peaks of mountains. The Schmidt hammer is a portable instrument, which measures the distance of rebound when pressed against the outcrop using a spring. This measures the hardness of the rocks, allowing a difference to be seen in the rocks above and below the boundary. The rocks above at or above the boundary should be softer as they have been affected by periglacial weathering. (Rae, A.C, Harrison, S et al, 2004) A number of readings, between 20 and 30, will be taken over a transect of an outcrop, allowing an average to be recorded. This method will be repeated at a number of different outcrops on a number of different mountain peaks, eventually showing the parameters of the trimline. The Schmidt hammer data will later be recorded in graphs and tables, noting where the hardness of the rocks changed dramatically.

Ice flow features will be present in large quantities below the trimline. These include striations, grooves, crescent gauges and friction cracks. A range of these measurements, approximately 20-30 will be taken of each feature over a number of outcrops in order to gain an average and to ascertain from the results an ice flow direction. These will be measured using rulers to ascertain the size of the feature, whilst a compass clinometer will be used to measure their strike and dip and the overall distance it faces. These features can be drawn onto rose diagrams, clearly and concisely showing the flow direction of the ice.

Till fabric analysis, in the form of a sedimentary sequence and log, will be performed in a systematic fashion, rather than being determined by natural geology and morphology like the methods highlighted above. This will be done over a chosen exposure, where it will be carefully logged by choosing clasts one by one on a transect across the exposure, measuring their dip direction and roundness, before noting their rock type. This will be repeated at a number of different heights, before converting the figures recorded during the day into a sedimentary sequence and stereonet diagrams.

References

Andrews, J.T. Techniques of Till Fabric Analysis. Technical Bulleting No. 6, British Geomorphological Research Group, pp 43, 1971

Ballantyne, C.K & Harris, C, The Periglaciation of Great Britain, Cambridge University press, 1995

Bradwell, T & Krabbendam, K, Lateral plucking as a mechanism for elongate erosional glacial bedforms: explaining megagrooves in Britain and Canada, British Geologic society, 2011

Elias, S.A, Encyclopedia of Quaternary Science, Elsevier Science Ltd, 2006

Fabel, D, Ballantyne, C.K & Xu, S, Trimlines, blockfields, mountain-top erratics and the vertical dimensions of the last British Ice sheet in NW Scotland, Quaternary Science reviews, Vol 55, pp 91-102, 2012

FREEMAN, S. R., BUTLER, R. W. H., CLIFF, R. A. and REX, D. C. ‘Direct dating of mylonite evolution: a multi-disciplinary geochronological study from the Moine Thrust Zone, NW Scotland’,Journal of the Geological Society, 155(5), pp. 745–758, (1998)

Goudie, A, The encyclopaedia of geomorphology, Routledge, 2003

Harris jr, S.E, Friction cracks and the direction of glacial movement, The Journal of Geology, vol 51, no. 4, 1943

Krabbendam, M. and Leslie, A. G. ‘Lateral variations and linkages in thrust geometry: the Traligill Transverse Zone, Assynt Culmination, Moine Thrust Belt, NW Scotland’,Geological Society, London, Special Publications, 335(1), pp. 335–357, 2010

K.M. Goodenough et al, Digital surface models and the landscape: Interaction between Bedrock and Glacial geology in the Ullapool area, British Geological Society, 2009

Lawson, T.J, Former ice movement in Assynt, Sutherland, as shown by the distribution of glacial erratics, Scottish Journal of Geology 26, 1990

Lawson, T.J, Glacial striae and former ice movement: the evidence from Assynt, Sutherland, Scottish Journal of Geology 32, 1996

Lowe, J. J. and Walker, M. J. C.Reconstructing Quaternary environments. 2nd edn. United Kingdom: Prentice-Hall, 1997

Mendum, J.R et al, Lewisian, Torridonian and Moine Rocks of Scotland, GCR Volume No. 34, 2009

McCarroll, D., Ballantyne, C. K., Nesie, A. & Dahl, S.-O. 1995. Nunataks of the last ice sheet in northwest Scotland. Boreas, 24:305–323.

Stoker, M. & Bradwell, T. 2005 The Minch palaeo-ice stream, NW sector of the British-Irish Ice Sheet. Journal of the Geological Society, 162 (3). 425-428.

Trewin, N. H.The Geology of Scotland. 4th edn. United Kingdom: Geological Society Publishing House. 2003

University of Birmingham field guide – Assynt field course

Washburn, A.L, Geocryology, Edward Arnold, London, pp 122-156, 1979

http://www.physicalgeography.net/fundamentals/10ag.html

http://www.earth.ox.ac.uk/~oesis/nws/nws-geolhist.html

http://www.discoverassynt.co.uk/landscape-geology.php

http://www.see.leeds.ac.uk/structure/assyntgeology/extra_info/about_us/project_details.htm

http://www.scottishgeology.com/geo/regional-geology/midland-valley/south-end-of-loch-lomond/

http://www.snh.org.uk/publications/on-line/geology/scotland/ice.asp

http://www.snh.org.uk/pdfs/publications/geology/scotland.pdf