Curlew Mountains

(419 to 359 Ma)


The Curlew Mountains Inlier is a long (52.5 km), narrow (average 3 km and maximum 4 km) outcrop of red sandstones and conglomerates.  They run in a west-south-west direction from just east of Drumshanbo, Co. Leitrim through parts of Roscommon and Sligo and into Co. Mayo just west of Ballaghaderreen, Co Roscommon.  In their eastern half the topography is low and undulating but at the western end of the outcrop they form a ridge of low hills.

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Conglomerate boulder of Old Red Sandstone (Devonian Period)

on Mullaghthee near Lower Lough Gara

Typical rock sample of Old Red Sandstone (Devonian Period) in the Curlew Mountains, Cornameelta near Boyle, Co. Roscommon

These rocks were deposited in the Devonian Period (419-359 Ma ago) and are often referred to as Old Red Sandstone (ORS) because of the red colour that is caused by iron oxide that coats the sand and pebbles.  This is very typical of sediments laid down in hot desert conditions.  Similar ORS of the same period is to be found in the south of Ireland and in south-west England indicating that desert conditions were quite widespread in this whole region at that time.  In some parts of south-west England marine deposits from the same period are of limestone with a rich fossil fauna indicative of tropical coral reefs.  Geologists have determined that this part of the world was very close to the earth’s equator during the Devonian Period.

Cliff of Old Red Sandstone in the Curlew Mountains Cornameelta near Boyle, Co. Roscommon

Closer examination of rock outcrops in the Curlew Mountains shows some grading of the sediments so that over a short distance one layer of rock may change from conglomerate with large quartz pebbles to small pebbles to sand and muddy sand.  There is no doubt that such sorting and deposition patterns are caused by fast flowing water.  It is concluded that this desert was occasionally subjected to torrential rainfall events resulting in flash floods and the erosion and deposition of sediments.  Examination of the pebbles in the conglomerates shows them to be predominantly composed of quartzite with some red sandstone pebbles and even some jasper.  Typical of deserts there was little plant or animal life and so very few fossils have been found in these sedimentary rocks.

The rocks show quite a high degree of homogeneity so it is difficult to sub-divide them into different beds.  Simon (1984) distinguished between ‘quartz-rich sandstones and thin mudstones’ that he named the Keadew Formation, and ‘conglomerates and pebbly sandstone and mudstone’ he called the Moygara Formation.  However other geologists found that in some exposures this distinction was not always upheld.

After these rocks had been deposited the whole area was subjected to the effects of a mountain building event at the end of the Devonian named the Acadian Orogeny (375-325 Ma ago).  This orogeny was caused when a portion of a sea called the Rheic Ocean closed up and the continents of Avalonia and Laurentia collided.  Its impact on the ORS of the Curlew Mountains was to fold and tilt the sediments so that today the rocks in some places are vertical rather than horizontal.  Some volcanic activity also took place and a lava flow on the Boyle side of Ballaghaderreen is one piece of evidence for this.  There are also andesitic pyroclastics that would have been ejected into the air during violent volcanic eruptions.  These are shown on the map as the Sheegorey Member within the Keadew Formation.

The long, narrow outcrop of these hills that is aligned from north-east to south-west is in-keeping with the landscape trend created by the Caledonian Orogeny (mountain building period), a trend also seen in the Ox Mountains.

The Curlew Mountains have been described as an ‘inlier’ in this description.  An ‘inlier’ is akin to an island of older rocks completely surrounded by a sea younger rock.  This is a very appropriate description because during the Carboniferous Period that followed the Devonian, the Old Red Sandstone of the Curlew Mountains was an island in a Carboniferous sea and deposits of limestone were laid down all around the ORS and probably even buried them in marine limestones.


Charlesworth, H.A.K. (1960)  The Lower Palaeozoic inlier of the Curlew Mountains anticline.  Proceedings of the Royal Irish Academy 61B(2):37-50.

Charlesworth, H.A.K. (1960)  The Old Red Sandstone of the Curlew Mountains inlier.  Proceedings of the Royal Irish Academy 61B(3):51-58.

Jukes, J.B. & F.J. Foot (1866)  On the occurrence of flestone traps and ashes on the Curlew hills north of Boyle.  Journal of the Royal Geological Society of Ireland 1:249-250.

Kinahan, G.H. (1881)  Report on the rocks of the Fintona and Curlew Mountains district.  (With palaeontological remarks by W.H. Baily).  Proceedings of the Royal Irish Academy Series 2 Volume 3:475-500 with 2 plates.

MacDermot, C.V.; C.B. Long & S.J. Harney (1996)  A geological description of Sligo, Leitrim, and adjoining parts of Cavan, Fermanagh, Mayo and Roscommon, to accompany the Bedrock Geology 1:100,000 Scale Map Series, sheet 7, Sligo-Leitrim, with contributions by K. Claringbold, G. Stanley, D. Daly and R. Meehan.  Geological Survey of Ireland, Dublin.  (Pages 57, 59-60).

Simon, J.B. (1984)  Sedimentation and tectonic setting of the Lower Old Red Sandstone of the Fintona and Curlew Mountains Districts.  Irish Journal of Earth Science 6:213-228.

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