Pleistocene Epoch (‘ice-ages’)

(2.6 Ma to 11,700 BP)

 

The Midlandian involved three distinct phases :

local glaciation in Munster

advance of the Midlandian General ice front

local glaciation in Wicklow

Ice eroded whaleback composed of gneiss and surounded by blanket bog at the top of Killerry Mountain, Ox Mountains, Co Sligo

Benwiskin, Dartry Mountains, Co Sligo.  it is believed that the remarkable shape was carved by a glacier during the Midlandian.  This feature could be classed as a truncated spur.

Maps of Ireland showing the ‘Drumlin Belt’

Copied from ‘Geography of Ireland’ website by Wesley Johnston

http://www.wesleyjohnston.com/users/ireland/geography/physical_landscape.html#drumlin_belt

Landscape trends



Diversion of drainage / breaches of watersheds

The River Bonet follows a course that effectively curves through 180 degrees before draining into the sea as the Garavogue River in Sligo town.  It flows south-eastwards out of Glenade Lough, southwards near Manorhamilton, and then swings westwards to pass through Dromahair before passing through Lough Gill and north-westwards into the sea.  This route makes no sense unless the glacial erosion that deepened the valleys of Glenade, Glencar and Lough Gill are taken into account (see diagrammatic map shown below).



Misfit rivers

Glaciers usually follow the course of pre-existing river valleys.  Their passage widens and deepens the valley then at the end of the glaciation the ice wastes away.  A river follows the course again but this time it appears as a small river in a very wide and deep valley.  This is called a misfit river.  The Drumcliff River and Bonet River would be good examples of misfit rivers.



Misfit estuaries

There is no such term as ‘misfit estuaries’ to my knowledge but the exceptionally large size of the three estuaries of Drumcliff Estuary, Sligo Harbour and Ballysadsare Bay have all come about through the over-deepening of relatively small river moths through glacial erosion followed by the drowning of those river mouths when the sea level rose..

Maximum extent of ice sheets over north-west Europe during the peak of the Midlandian Glaciation at about 22,000 BP.  The Sligo-Leitrim area and Donegal Bay are under ice sheets.

(modified from Ulamm, 2013-01-30 under the GNU Free Documentation License)

Granite erratic from Lough Easky Adamelite pluton left sitting on Carboniferous wave-cut platform at Coanmore Bay, Easky, Co. Sligo.

South-east to north-west trend in the landscape at Moy Estuary

South-east to north-west trend in the landscape at Lough Arrow (formed by drumlin alignment) and in the adjacent limestone bedrock of the Bricklieve Mountains.

Glacial geology of North Mayo and West Sligo as mapped and drawn by Charlesworth (1928)

Small arrows on this map show the direction of ice advance.

The grey concentric bands represent glacial moraines that were deposited as ice sheets and glaciers retreated towards the south-east.

Boulder clay (till)

This is unsorted mixtures of clay, silt, sand, gravel, rocks and boulders with no features of stratification.  The seemingly unorganised appearance of this sediment is typical of the eroded material dumped by glaciers in drumlins and moraines.  In contrast, sediment carried by water is sorted and deposited in layers and often has cross-bedding and other fluvial features.  The rocks and stones found in boulder clay are also more angular than the rounded shapes of the material that has been transported by water.

Detail taken from the above map of Charlesworth (1928)

References


Charlesworth, J.K. (1928)  The glacial geology of north Mayo and west Sligo.  Proceedings of the Royal Irish Academy 38B(6):100-115 (with 1 plate).


Coxon, P. (1985)  Quaternary geology and Gleniff - protalus rampart.  Pages 1-12 in: THORN, R.H. (ed.) Sligo and West Leitrim. Irish Association for Quaternary Studies Field Guide No. 8.   IQUA. Dublin.

Coxon, P. (1988)  Remnant periglacial features on the summit of Truskmore, Counties Sligo and Leitrim, Ireland.  Zeitschrift fur Geomorphologie N.F. Suppl.-Bd. 71:81-91.


Coxon, P. (2005)  The late Tertiary landscapes of western Ireland.  Irish Geography 38(2):111-127.


Coxon, P. & P. Browne (1991)  Glacial deposits and landforms of central and western Ireland.  Pages 355-365 with 3 plates in : EHLERS, J.; P.L. GIBBARD & J. ROSE (eds.)  Glacial Deposits in Great Britain and Ireland.  A.A. Balkema, Rotterdam.


Finch, T.F. (1977)  Western Ireland.  International Union for Quaternary Research, Birmingham.  40 pages.


Greenwood, S.L. and C.D. Clark (2009)  Reconstructing the last Irish Ice Sheet 1: changing flow geometries and ice flow dynamics deciphered from the glacial landform record.  Quaternary Science Reviews 28 (27-28):3085-3100.


Lemon, K. (2009) Extreme geology in Co Cavan.  Earth Science Ireland 6:32-33.


Meehan, R.; X. Pellicer and M. Sheehy (2014)  Debris of the ice ages mapped out.   Earth Science Ireland 15:32-35.


Thorn, R. (ed.) (1985)  Sligo and West Leitrim. Irish Association for Quaternary Studies. Field Guide No.8. (84 pages).


Synge, F.M. & N. Stephens (1960)  The Quaternary Period in Ireland - an assessment.  Irish Geography 4(2):121-130.

Roches moutonée known as Tullaghan Hill in the Ox Mountains

viewed from road linking Coolaney with the main Ballysadare-Ballina road

Diagram showing how roche moutonées are formed by the action of ice sheets

(diagram after Brooklyn College web site http://depthome.brooklyn.cuny.edu/geology/core332/central.htm)

Corrie lake of Lough Aghree below Knockachree, Ox Mountains

Landscape features associated with glacial processes


Over 90% of the Irish landscape is overlain by quaternary deposits (Meehan et al. 2014).  The authors describe how data for Mayo, and extending as far as Knocknarea in Sligo, has combined various data sets in a computer generated geomorphology map to identify features created from these deposits.  The result is the Quaternary Geology Map.  They list 23 identified features but not all are represented in Sligo :


Glacial trimlines (76 individual nunataks) - e.g. Truskmore, Sligo/Leitrim

Corries (777 recorded)

Ribbed moranines

Minor ribbed moraines

Drumlins

Non-orientated bedforms

Striations

Crag and tails

Mega-scale glacial lineations

Thrust block moraines

Eskers

Glaciofluvial terraces

Hummocky sands and gravels

Meltwater channels

Moraines

Deltas and fans

Kame terraces

Glaciolacustrine deposits at surface and under peat beds

Ice-damed lakes

Landslides

Limits of erratic dispersal flowlines



The impact of ice sheets and glaciers in the Sligo and Leitrim region can be seen everywhere.  Some people place all of County Leitrim and most of County Sligo into a landscape region they call the ‘Drumlin Belt’ as shown on the following maps.

Glacial features can be divided into those

caused by erosion (by ice and by melt water running from retreating ice fields) and those created by the deposition of sediments.  There are also features characteristic of places at the edges of ice sheets and glaciers that are not glaciated but arise from the impact of severe cold such as ‘frost and thaw’ on the landscape.  Such conditions often arise towards the end of a period of glaciation. These are called periglacial features and include nunataks, solifluction lobes and patterned ground.





Glacial Erosion


Using examples from the Sligo-Leitrim region, features of glacial erosion are briefly described and illustrated in the following account.  All features date from the late Midlandian Glacial period known as the Midlandian Cold Stage.



Corrie

Corries are ice eroded hollows on the sides of mountains with an amphitheatre shape backed by the mountain cliffs.  In this part of the world they usually face north-west or north because this is where conditions were at their coldest.  The word ‘corrie’ comes from the Gaelic for a pot or cauldron, and these are the same feature as a ‘cirque’ in France, Switzerland etc.  Very often the hollow bowl is filled with water to hold a small round ‘corrie lake’.  In the Sligo-Leitrim area there are corries at Lough Aghree and Harte’s Lough on the side of Knockalongy in the Ox Mountains and at the head of the Horseshoe Glen near Ballaghnatrillick in the Dartry Mountains.

U-shaped valleys

Ice typically fills the whole valley and erodes the sides, truncating the spurs of the hills to either side, whilst gouging out the valley floor in a wide sweep.  Rivers tend to cut a well defined channel but glaciers leave a wide deep U-shaped valley behind.  Glencar and Glenade with their long narrow ‘ribbon lakes’ are good examples of U-shaped glacial valleys in this area.

Roche moutonées

Literal meaning ‘a sheep back’.  These are rocky hills that have been shaped by ice passing over them.  The approaching ice erodes a long even slope on the upstream side and as it moves away plucks an irregular vertical face on the downstream side.

Glacial striae

These are scratches on smoothed rock faces that were caused by boulders under the ice sheet being dragged along in the direction of ice flow (no photograph).




Ice polished rock

The rocks that show this feature have survived for so long after the last glacial because they are resistant to chemical weathering.  Limestone is far too easily dissolved in rain water to maintain such a surface.  In the Sligo-Leitrim area it is the metamorphic rocks of the Ox Mountains that occasionally have exposures that fit with this criterion.  The same rocks may also exhibit glacial striae.

Whaleback

is a streamlined outcrop of the bedrock that has been formed by the smoothing action of ice passing over a landscape. Typically they are about 1-2 m high and about 3 m long and are smoothed and rounded on all sides.

Truncated spur

This feature is formed when a glacier in carving out a valley, cuts off or truncates the spurs which were at the lower slopes of the hills to either side.  Benwiskin with a shape like the Matterhorn could be considered to be a truncated spur.

There were eight more Glacial / Interglacial cycles during earlier times in the Pleistocene but evidence of their impact on the landscape is lost because the scouring action of each subsequent glaciation almost entirely removed the evidence of previous glacials and interglacials.  In Ireland, evidence of two distinct glacial stages are well known (Munsterian and Midlandian) with a third one barely detected (Pre-Gortian).


The earliest evidence of the Pleistocene in Ireland is of the Pre-Gortian Glacial from two small sites in south County Galway.  The Munsterian (also the Munster General Glaciation or Older Drift) is quite well represented in southern Ireland but there is no evidence for it from the Sligo-Leitrim area because this has been obliterated by the most recent glaciation.  Evidence for the Midlandian (also called the Midland General Glaciation or Newer Drift) is to be seen everywhere through its impact on the landscape of Sligo and Leitrim.  The map shown below indicates that the northern two-thirds of Ireland were buried under an ice sheet at the height of this glaciation which was at about 22,000 years BP.

Glacial and Interglacial stages of recent Pleistocene (below)

Stadial and Interstadial stages of Midlandian (above)

Synonomy shown of some names for Ireland, Great Britain and mainland Europe.

Chronology and an indication of sea level changes also shown.

NOTES : Dates are approximate and there are different interpretations in the literature

Read this table from bottom to top.

The Quaternary Period


The Quaternary Period is the most recent of the three periods of the Caenozoic, the two previous ones being the Palaeogene and the Neogene.  The start of the Quaternary Period is marked by a global downturn in temperatures and the onset of the ice ages.  It is deemed to have started at 2.6 million years ago and comes right up to the present time.  The Quaternary Period is further sub-divided into two epochs; the Pleistocene Epoch and the Holocene Epoch.  The Pleistocene Epoch is often known as ‘the ice-ages’ and only came to an end 11,700 years ago, therefore it comprises more than 99.5% of the Quaternary Period.  The Holocene Epoch runs from 11,700 BP to the present time and is the remaining less than 0.5% of the Quaternary Period.




Pleistocene Epoch (The Ice-Ages)

The emerging picture of what happened over these two and a half million years is very complex.  All around the world the Pleistocene Epoch is marked by a large number of oscillations in the climate characterised by cold periods (glacials) followed by warm periods (interglacials).  There are known to have been at least 11 glacials during which up to 30% of the Earth’s surface was covered by ice and beyond the ice there was a zone of permafrost.  The ice sheets were between 1.5 and 3 km thick so they had trapped within them a great deal of water which resulted in the sea level dropping up to 120 m.  The interglacials were often quite warm periods with a climate in Europe at times resembling tropical conditions.  With most of the ice having melted, sea levels reached to over 100 m above what they are today.  Features such as drowned coastlines and raised beaches mark times when the sea level was much higher.  Within the glacials there were shorter and less severe warmer times called interstadials; and similarly within the interglacials there were short periods of cold weather called stadials.


Details of the story for the last 800,000 years have been pieced together with year by year precision using measurements from Greenland ice cores.  Studies of thin layers of muds taken from deep sea sediments and of tree growth rings have provided independent verification of ice core results for more recent events.


The bottom lines of the table shown below summarises the last three glacial stages and the last two interglacial stages.  It provides names used for them in Ireland, Great Britain and on the European Continent; their approximate dates; and sea level maxima.  The top lines of the table are expanded details of the stadials and interstadials during the last 19,000 years of the Midlandian Glacial period.

Moraines

Moraines are deposits of boulder clay dropped by the glacier either along the sides (lateral moraines) or at the leading edge when the glacier is melting (terminal moraines).


A fine example of a fan moraine in County Sligo is the Meenamore Fan Moraine situated in the Ox Mountains roughly between Lough Talt and lough Easky at  G4217.  Unfortunately this moraine is gradually being destroyed as it is being mined for aggregates.  Out in Sligo Harbour off Gibraltar there are the remains of glacial moraines poking out above the sand flats that are used by mussels and other forms of marine life for their attachment.



Erratics

These are large rocks and boulders that have been transported and dropped by a glacier.  They have often been carried over many kilometers and as a result they are of a different rock type to the bedrock upon which they are now sitting.  Along the Sligo coast there are erratics at Easky made of granite that are sitting on Carboniferous limestone and on the northern shore of Ballysadare Bay there are erratcs of gneiss dumped on the limestone.  In both instances the erratics have originated in the Ox Mountains and have been carried northwards by the glaciers.  A very well known erratic in Sligo is the ‘split rock’ near Easky.

Drumlins

These were formed at the interface between the ice sheet (or glacier) and the ground.  They are deposits of glacial till (often called boulder clay) that were dragged along by the moving ice and shaped into streamlined hills.  Until recently these features were all called drumlins; a word derived from the Irish word droimnín meaning a small hill.  The term was first used by the Rev Maxwell H. Close in 1867 to describe a glaciogenic hill or ridge composed of glacial till.  These drumlins often occur in ‘swarms’ in what is called the ‘basket of eggs topography’.

Examples are to be found forming the peninsulas and islands in Lough Arrow where they are oriented along a south-east to north-west axis (note that Chapman studied the orientation of drumlins in the Erne lowlands).


Recent observations of the drumlin landscape nearby in County Cavan, by using satellite imagery, has shown that the relationships between the hills is more interesting that previously thought.  What were thought to be drumlins are in fact ribbed moraines, very much like huge ripple marks that one might see sand on a beach (Lemon 2009).  It would be interesting to see if any of the drumlins in north Leitrim might also be ribbed moraines.



Eskers

Eskers are created when a river channel beneath a glacier becomes filled with sediment.  Two main eskers in County Sligo have largely been removed for sand and gravel.



Protalus rampart

A protalus rampart is a line of angular rock debris accumulated below a cliff that is shedding material as it is affected by ‘frost and thaw’ processes.  The debris is organised into low ridges by being transported on snow banks at the cliff base (see diagram below).  This feature has been noted in County Sligo by Pete Coxon (1985).

Ice polished rock at Glen, Ox Mountains near Coolaney, County Sligo

The last glacial stage of the ice ages (called the Midlandian Cold Stage) lasted for 65,000 years, when an ice sheet up to 1 km thick covered three-quarters of Ireland.  All of the northern part of the island was under ice apart from perhaps a few mountain peaks that stood out as nunataks.  The landscape of north-west Ireland would have borne a close resemblance to Greenland as it is today.


Many other parts of the Earth would also have been buried in ice sheets and glaciers during the Midlandian Cold Stage and with so much of the planets’ water taken up as ice and snow it is known that at its height (22,000 years BP), sea level was 120 m lower than it is at present.  If we look at where the -120 m contour is below sea level today this would give us an idea of where the Irish coastline would have been at that time.  In Sligo this would expose a significant amount of sea bed including all of Sligo Bay, Killala Bay etc.


It is probable that few if any plants and animals would have survived in Ireland during this glaciation which came to an end about 11,700 years ago.  Parts of southern Ireland remained ice-free and would have had a flora and fauna.  It has been suggested that Truskmore was a nunatak (Coxon 1988) so it might have supported a limited arctic-alpine flora but this is speculation and remains unproven.

Glacial outwash channel / meltwater channels

The Glen, Knocknarea

Diagram illustrating how a corrie forms through glacial erosion

Glencar valley and lake is typical of a glacial U-shaped valley

Periglacial (edge of ice sheet) features



Nunatak

Named after the Inuit word nunataq this is a hill or mountain that protrudes above the ice sheet and is affected by periglacial processes but is not directly eroded by ice.  The photograph shown below is of Gunnbjørn Fjeld which is a nunatak in Greenland.  Several Irish mountain peaks are believed to have been nunataks during the Midlandian Glacial including Truskmore (647 m) in the Dartry Mountains on the Sligo-Leitrim border.  Evidence for this comes from the presence of a range of periglacial features on the upper slopes.

Glacial Deposition


A major scientific paper published by Charlesworth (1928) includes a map of the pattern of glacial deposits in the North Mayo and West Sligo area.  This work was a major achievement given that modern tools such as aerial and satellite imagery would not have been available to the author at that time.

Drainage basins of north Sligo and parts of north Leitrim

Periglacial features observed by Pete Coxon on Truskmore were most likely formed during repeated freeze and thaw cycles towards the end of the Midlandian Glaciation (Coxon 1988).  The following features were all observed on the south-east facing aspect of the mountain summit.

Gunnbjørn Fjeld, Greenland’s highest mountain is a typical nunatak and may be similar in appearance to Truskmore as it was 22,000 years ago.

Photograph by Mário Gonçalves on his website ‘Ultima Thule’ (http://ultima0thule.blogspot.ie/)

Example of boulder clay (glacial till) in a low sea cliff at Knocklane, Ballineden, County Sligo

More recently, Coxon & Browne (1991 Figure 259) have published a paper showing major glacial depositional features arising from the Midlandian Glacial.  In the Sligo-Leitrim area they depict swarms of drumlins, sand and gravel spreads, an esker on Knocknarea peninsula and cirque moraines on the north side of the Benbulbin uplands.

Rock slope failures / rotational slips

There are several places in Glencar and Glenade where the valley glaciers eroded vertical walls of limestone.  These were left standing unsupported when the ice eventually melted at the end of the Midlandian Glacial.  The jointing in the limestone bedrock eventually failed and great vertical slabs of rock broke away from the high cliffs and slipped away from the main rock face on lubricated beds of clay-rich shale at the bottom of the sedimentary sequence.  The result is an interesting landscape of rotational slips.  The “Swiss Valley” in Glencar is the best known example resulting from this process.





Glacio-fluvial (or Fluvio-glacial) (meltwater) features



Dropstones


Ice flows


Glacial outwash channel

‘The Glen’ on the south-western side of Knocknarea is a gorge-like feature that may have been cut into the rock by meltwater from the ice sheet.  This idea hasn’t been published and I think is still regarded as speculation.


Kame

‘A hill or hummock composed of stratified sand and gravel laid down by glacial meltwater’

The area between Ardnaglass (below Benbulbin), Grange and the coast near Mountedward is a landscape dominated by many humps and bumps that appear to be kames.


Delta kame


Raised beaches

These are benches above high water level, sometimes a long way above high water, that tend to run parallel to the coast and were eroded when sea levels were higher.  There are several places along the Sligo coast where features that appear to be raised beaches can be seen.

Diagram to show how a protalus rampart forms

Trends, also sometimes referred to as the ‘grain of the land’, are shown in the Moy Estuary and the Bricklieves-Lough Arrow area as illustrated by maps shown below.

The ‘Split Rock’ a glacial erratic near Easky, Co. Sligo

Solifluction lobes

Soil creep occurs when the soil on a slope becomes saturated with water, and aided by the processes of ‘frost and thaw’, it very slowly moves downhill.  Solifluction lobes have the physical appearance that the blanket of soil has developed wrinkles or ripples.


Patterned ground

In places where ‘frost and thaw’ processes were active, stones on the ground surface were gradually moved around over many, many years to become arranged in sorted nets (circles and polygons), stone stripes and stone-banked terraces.

Corrie lake of Lough Aghree below Knockachree, Ox Mountains

Lough Dagea corrie lake, Ox Mountains