Scottish erratics and the Irish Sea Ice Stream

 

Map of the northern British Isles, showing the main ice shed (ice divide) area which separated glacier ice flowing NW and W into the Atlantic and ice flowing broadly southwards in the Irish Sea Ice Stream.    Almost all of the reconstructions relate to the Late Devensian Glaciation.......

The traditional map of iceflow directions, based on Wright (1914)

The BRITICE-CHRONO map for 27,000 yrs BP -- ie near the peak of the Late Devensian Glaciation

Now this is interesting.  Geomorphological dilemmas are always interesting -- but they all get sorted out in the end........

It's all got to do with ice sheet summits and the positions of ice sheds.  I say "positions" because they are never static.  Shifts in snow accumulation patterns and drainage route efficiency happen all the time in modern ice sheets, as they did in the past. So during the course of a single glacial episode you may find that an ice shed (between diverging flowlines) may shift by 100 km or more.  And an initial  northward shift (for example) may be replaced later on by a similar or even greater southward shift.

See this article too:  "Flow pattern evolution of the last British Ice Sheet" by Anna L.C. Hughes, Chris D. Clark, Colm J. Jordan

https://nora.nerc.ac.uk/id/eprint/507185/1/Flow%20pattern%20evolution%20QSR.pdf

Hughes, A. L. C., Clark, C. D., & Jordan, C. J. (2014). Flow-pattern evolution of the last British Ice Sheet. Quaternary Science Reviews, 89, 148-168. 

https://doi.org/10.1016/j.quascirev.2014.02.002

The dilemma of current interest relates to the presence of erratics presumed to come from the western isles of Scotland found along the coasts of Devon and Cornwall.  Paul Berry tells is that the famous pink granite erratic at Saunton has probably come from Gruinard Bay in Wester Ross.  That's north of the Isle of Skye and at the same latitude as the Isle of Harris -- on the eastern shore of the Minch, which was (according to current wisdom) one of the great northwards ice discharge routes in the Late Devensian Glaciation.  That's 200 km north of the assumed Late Devensian ice shed position.  I have been reminded of this dilemma by a recent suggestion from Prof Peter Kokelaar that he has seen a number of glacial erratics on the Gower that have come from the Western Isles -- well to the north of the supposed ice divide.

Almost all of the ice shed positions described in the literature show that they lay on a line approximately joining Belfast to Glasgow -- across the North Channel and running NNE --SSW.  Some reconstructions show an ice shed running NE-SW along the Southern Uplands of Scotland, and some show an ice divide approximately on the southern part of the Isle of Arran and running across the tip of Kintyre -- but always the assumption is that the ice from the ice shed flowed across Islay and the Malin Shelf and westwards out into the Atlantic.  I have seen no Devensian reconstructions showing a broadly southward flow of ice through the North Channel.  

Some interesting work on erratic distributions, from Veness et al, shows us that erratic dispersal tracks can be quite different during the advance and retreat stages of the same glacial episode.  This shows up in the maps of Shap erratic distributions and the complex distributions from the Loch Fyne source.  It's clear that the Irish Sea Ice Stream can have carried Shap granite erratics and erratics from the northern end of Loch Fyne hundreds of kilometres to the south.  We should not be surprised by this, in view of the relative abundance of Ailsa Craig erratics in Pembrokeshire and Gower.

See this:
Modelling erratic dispersal accounting for shifting ice flow geometries: A new method and explanations of erratic dispersal of the British–Irish Ice Sheet
R. L. Veness, C. D. Clark, J. C. Ely, J. L. Knight, A. Igneczi, S. L. Bradley,  15 May 2025

https://doi.org/10.1002/jqs.3720

https://onlinelibrary.wiley.com/doi/epdf/10.1002/jqs.3720

See also:  https://brian-mountainman.blogspot.com/2023/01/the-distribution-of-shap-granite.html

Bearing in mind that most far-travelled erratics will have followed zig-zag routes over the course of several glacial episodes, how far to the north might the ice divide have been located during Wolstonian and Anglian glaciations, and possibly even further back in time?

Well, if we look at landscapes rather than glacial deposits, it's clear that southward-flowing ice has played a powerful role in landscape formation in the southern part of the Western Isles.  Loch Fyne itself occupies a long and well defined glacial trough cut by ice flowing broadly southwards.  The Sound of Jura was also probably a glacial transport route.  So too the Firth of Clyde.  So too Loch Awe.  The Firth of Lorn and Loch Linnhe, following roughly the route of the Great Glen Fault and separating the North West Highlands from the Grampian Highlands, are more difficult to interpret because of the powerful structural influences at play; but it does look as if this elongated deep trough once carried ice flowing south-westwards.

So maybe, in trying to explain how erratics from the Western Isles found their way into the path of the Irish Sea Ice Stream -- and were then carried away far to the south -- we have to look at a very long history of glacial episodes, with some entrainment and transport related to local glaciations of the Scottish Highlands (involving local valley glaciers and outlet glaciers from the Scottish Ice cap, and other episodes of erratic transport related to glacial maxima and the work of the British and Irish Ice Sheet.  Sequence of events still to be worked out.......

There are two further matters to be considered:  isostatic and eustatic interactions, and changes in oceanic circulation which can be related to glacier expansion and contraction. These are important if we are to get to the bottom of the "ice rafting" issue.

The ice shed area shown on the map at the top of this post was (at least in the Late Devensian) the area of thickest ice and therefore the area of greatest post-glacial isostatic recovery.  Raised beach ridges occur in many locations (for example near Loch Tarbert of the Isle of Jura) up to altitudes of c 40m.  The raised beaches on Rum and Muck occur up to c 30m, and the highest beach is found at Arisaig in Lochaber, at 45m asl. 

Raised beach ridges at Loch Tarbert on Jura

Another raised beach sequence on Jura

Isolated raised beach remnants resting on an old rock platform near Glenbatrick on the south shore of Loch Tarbert, Jura.  (Oblique satellite image from Google Earth)

The most prominent raised beach ridges may coincide with short-lived RSL stillstands and minor advances of glaciers feeding from the Scottish Ice Cap -- but this sequence of events has still to be worked out.  It's believed that someof the glacier advances involved floating glacier snouts and short-lived calving episodes -- so there might have been some "ice rafting" of erratics as envisaged by some geomorphologists.  Sea level at the time was approxinately at -60m m -- so any erratics transported southwards cannot have been dropped on modern shorelines far to the south.

In any case, extensive southwards iceberg drift seems very unlikely, since the prevailing oceanic  current direction on the western coasts of Britain -- driven by the Gulf Stream and the North Atlantic Drift -- was broady northwards, not southwards.  Even if there was a weakening of the North Atlantic Drift and a reduction in the force of the westerly winds at the time of the Loch Lomond Stadial, it is most likely that any icebergs carrying erratics into Scottish tidal waters will have melted and dropped their loads quite close to their places of origin, as happens today in the Greenland fjords.

We now know quite a bit about the Younger Dryas (Zone III) or Loch Lomond readvance, and we can assume that similar conditions prevailed at times in the Early and/or Middle Devensian prior to the massive expansion of the BIIS and the development of the Irish Sea Ice Stream. 

The blue area on this map indicates the extent of the Loch Lomond Stadial in Scotland

Bickerdike, H.L., Evans, D.J.A., Stokes, C.R. and Ó Cofaigh, C. (2018), The glacial geomorphology of the Loch Lomond (Younger Dryas) Stadial in Britain: a review. J. Quaternary Sci., 33: 1-54.

https://doi.org/10.1002/jqs.3010

This brings us back to the matter of ice rafting and the Devon and Cornwall coastal erratics.  Could ice floes have carried some of them all the way from the Western Isles, through the North Channel and across Cardigan Bay at a time when relative sea level was more or less as it is today?  As indicated above, I can see no set of circumstances in which that was possible.  There are abundant dropstones and sand and gravel layers in the glacio-marine sediments on the bed of the Celtic Sea, but all are connected to the the advance of the Irish Sea Ice Stream ice front prior to 26,000 years ago and to its retreat between 26,000 and 20,000 years ago -- at a time when relative sea level was probaly around -130m.  Towards its outer edge the ice front was afloat, but that was at least 200km away from the tip of Cornwall.

The ice rafting hypothesis really is a non-starter.  So the "western Scottish" erratics must have been derived from very ancient glacial deposits, and they must all have had very complex histories of glacial entrainment and transport.