62

THE GEOLOGY OF THE MALLARANNY AND SLIGO DISTRICTS. With notes on the country between Dublin and Sligo.*

By GRENVILLE A. J. COLE, M.R.I.A., F.G.S. ReadJune 14th, 1912. DUBLIN TO MALLARANNY. HE Excursion to the west of Ireland in 1912 was T planned on somewhat broad lines, in consideration of the distances to be traversed. The journeys to and from Dublin were arranged so as to illustrate the general geology and physical geography of the interior, and the local excursions dealt in large part with the origin of scenic features. The Department of Agriculture and Technical Instruction for Ireland authorised the co-operation of the officers of the Geological Survey in guiding the Association in the field, and the office of the Survey at 14, Hume Street, Dublin, was available as a bureau of geological information for those who intended making further excursions in the country. The railway from Dublin to Westport furnishes an impressive picture of the central plain of Ireland. The traveller has entered the country by the broad gateway at the north end of the Leinster Chain, where the Carboniferous Limestone comes out to the sea. To the north, Silurian rocks, the continuation of the Southern Uplands of Scotland, are exposed in the low hummocky hills that extend as far west as Longford. To the south, the Leinster granite appears in the headland of Killiney, and folded Ordovician and Cambrian strata form picturesque foothills on either side of the domed summits on the igneous core. The Cambrian quartzites are responsible for the rocky bosses on Howth and Bray Head, and for the fine cone of the Great Sugarloaf, 1,659 ft. above the sea. The Great Carboniferous Limestone plain has been worn down nearly to sea-level by prolonged sub-aerial wasting. In Cretaceous times it was probably far less extensive, and uplifted Triassic and Coal Measure strata still covered much of the interior. On this irregular surface the rivers ran with a general southerly trend, which is still traceable in the courses of the Shannon, the Suir, the Nore, the Barrow, and the Slaney. These rivers cut the overlying strata into outliers, and widened their valleys as they reached the Carboniferous Limestone, until a general coalescence of the mature valleys reduced a great area of the interior to a plain. • This Paper was included in the Pamphlet issued for the use of the Memhers attending the Excursion of July 31st-Aug. 9th, '9'2. P ROC. G EOL. Assoc., VOL XX IV . P L AT E r 3

A.- GLACIATED S UR FACE OF Gl': E JSS WITH !:;' CLUSroNS M ANORH A MILT ON.

[P;'oto by G.A. t . Cole. B .-THE GLACIATED GNEISSIC R ID GE FRO ~l NEAR S LIGO.

To face page 62. THE GEOLOGY OF THE MALLARANNY AND SLIGO DISTRICTS. 63

Meanwhile, the more resisting rocks, Old Red Sandstone and Silurian slates, folded into ridges of the Caledonian and Hercynian series, appeared as the covering beds were worn away. The ancient ridge of Leinster, though trenched by the Slaney and the Barrow, asserted itself as a moorland chain, owing to the removal of 2,000 ft. of rock on either side. The Silurian axis of Newry was laid bare, and the Slieve Bloom Mountains (1,677 ft.) stood out at the northern end of a Hercynian mass parallel with the Caledonian chain of Wicklow. During the Glacial epoch the plain was occupied by ice that flowed down from the country near Lower Lough Erne, while its eastern side was for a time invaded by the Irish Sea ice, which had its source in Scotland. The shrinkage and final disappearance of these glaciers left boulder-clay, and gravels washed from the boulder-clay, as widely spread deposits on the limestone. The detritus that was banked up in the water­ courses under the stagnating ice remained as long sinuous eskers, which are often the only conspicuous surface-features." Lakes gathered in the hollows between drumlins, which occur mostly in the north, and in broader depressions in the central plain. Peat in time grew across these lakes and. choked them. Peat­ bogs become conspicuous some twenty miles west of Dublin, and cover much of the country south of Athlone. The rivers of the plain are still seeking their former valleys through the accumulations of glacial drift. A large driftless area, however, is traversed after the Shannon has been crossed at Athlone, and the grey limestone often appears in terraces on the surface. Here and there an esker rises, with angular blocks dropped on it from the upper layers of the ice. The Suck, the great parallel tributary of the Shannon, is crossed at Castlerea, and a small patch of Old Red Sandstone is exposed between this town and Ballyhaunis. At Castlebar a band of intrusive dolerite forms a ridge parallel with the Ox Mountains, which rise beyond. The line then reaches the Atlantic at Westport, where the Carboniferous Limestone, in a synclinal between the older rocks of northern and southern Mayo, forms a western gateway to the plain, corresponding with that left behind at Dublin. From Westport to Newport the railway runs among drumlins of boulder-clay, and the large stones, carried from the east, are conspicuous in the numerous cuttings. The drift-covered islets in Clew Bay provide fine sections where they are cut away by the waves upon their western sides, and are clearly the result of the submergence of a hummocky country, such as that which con­ tinues the hollow landwards. At Mallaranny, on the narrow neck joining Curraun Achill with the main mass of Mayo, the traveller reaches the Dalradian quartzites of the west.

• See especially W. J. Soli as, .. A Map to show the Distribution of Eskersin Ireland." Sci. Trans. R. Dublin Soc" vol, v (1896), p. 785. GRENVILLE A. j. COLE ON

MALLARANNY AND (See Plate IS). The Upper Old Red Sandstone conglomerates that underlie the Carboniferous Limestone in this area crop out along the lower slopes at Mallaranny, dipping seaward at about 10°. They rise on the hills as high as the crest of Curraun Achill (1,784 ft.), where they rest with clear unconformity on the Dalradian series. The main features of the country, however, are formed by the Dalradian quartzites, intercalated with quartzose mica-schists. Although the schists attain considerable heights in the north­ west of Achill Island, they may have been preserved for a time by overlying quartzite. This is suggested by the quartzite patch remaining on the summit of Slieve More, 2,204 ft. above the sea. On the west of the island the Atlantic has eroded the bold cliff of , 1,950 ft. in height, the crest of the hill (2,192 ft.) rising a very short way inland from the edge. The Minaun Cliffs farther south-east are 800 ft. high, and their structure can be well studied on the shore at the Cathedral Rocks. Here sea­ caves have been cut out of horizontally bedded flaggy quartzite, and pillars have been left between them as the hollows have united beneath the hill. A broad stretch of sand forms a sea­ front for the low schistose ground, covered by bog, between the Cathedral Rocks and the next rising slope at Keel. The whole of this remote and interesting island is clearly a mass detached by subsidence from western Mayo. At low water Achill Sound is in part occupied by a mere stream. The sinuous inlet ending near Mallaranny in Bellacragher Bay almost severs Curraun Achill from the mainland. Submerged peat occurs in Achill Sound, as in so many places around Ireland, and attests the recency of a downward movement of the coast. In the Memoir of the Geological Survey on Clare Island, now in the press, Mr. T. Hallissy shows how Clew Bay was at one time choked by drift from side to side, Clare Island being thus con­ nected with the land near Louisburgh and also with Mallaranny. The latest subsidence has no doubt assisted the sea in its destructive work. The glacial features near Mallaranny still require to be worked out in detail. Ice from the north-east no doubt moved along the low ground traversed by the railway to Achill Sound, and numerous bands of detritus occur here, resembling lateral moraines. The great bank of stones that runs as a kind of terrace along the north side of the hills probably represents an accumulation due to snow-slides, held up by the ice that stagnated during its later stages in the lowland. Messrs. G. H. Kinahan and M. H. Close" have shown how at one epoch glacier-ice, generated in southern Mayo, crossed the Clew Bay area, abutted on the hills near Mallaranny, and oozed northward through the • .. The General Glaciation of Iar-Ccnnaught " (1872), p. 12. P ROC. G EOr.. Assoc., VOL. XXI V. PLAT E 14.

[Phot o by R . Wel clt, Copyr igld . S CAR P OF U PPE R CARBON IF E ROUS LI MESTONE. GLE NCAR, Co. L EIT RHI . T o fa ce pa ge G4. THE GEOLOGY OF THE MALLARANNY AND SLIGO DISTRICTS. 6S pass. The main ice-sheet from the east and north-east, which preceded this more local flow, was similarly influenced by the gap, and streamed across the lowlands of Achill Island.

MALLARANNY TO SLIGO. From Claremorris, the junction for Sligo from the west, the railway returns towards the Ox Mountains, a range of schists, gneisses, and granites, with a typical Caledonian north-east trend. The line keeps to the lowland of limestone and bog-covered boulder-clays; but the quartzite dome of is usually visible above the southern portion of the chain. At Tobercurry the hills are neared, and they are crossed in a picturesque stream-gap at Collooney. The oscillations of the Irish coast-line are here indicated by the fall on the River, which at an early date carved out the gap, but which has been rejuvenated, like the Erne, by recent uplift.

THE GENERAL STRUCTURE OF THE SLIGO DISTRICT (See the map facing page 68).

The variety of scenery near Sligo is due to the exposure of the Ox Mountain ridge through the prevalent terraced Carboni­ ferous rocks. The Ox Mountains, at one time described as Laurentian, are no doubt in part pre-Cambrian, being formed of Dalradian schists, quartzites, and basic igneous masses, invaded by granite and converted for the most part into a composite gneiss. There is no evidence that Silurian rocks have been worked up into the ridge, though the folding that gave it its present trend is no doubt of early Devonian (Caledonian) age. It is unfortunately not clear if the granite that forms so much of the mass was intruded prior to the folding, or as an accompani­ ment of it. In the latter case, it would be of the same age as that of Leinster. The evidence in the field goes to show that what have often been styled" shearing planes" in the composite gneiss are structures due to igneous flow. This conclusion proves to be in complete agreement with what has been observed in Canada and elsewhere." The inclusions of garnetiferous amphibolite that are common near Ballysadare, Drumahaire, and Manorhamilton, have been described as boulders in a meta­ morphosed conglomerate; while the more highly siliceous portions of the granite have been mapped as quartzite. The former masses are in reality residues from the Dalradian envelope, completely re-crystallised in the bath of invading aplite. * See especially F. D. Adams and A. E. Barlow, .. Geology of the Haliburton and Bancroft Areas. Ontario." Geol. Suru, Canada, Mem. 6 (1910), p. 120, etc. 66 GRENVILLE A. j, COLE ON

The aplite, by retention of the garnets from these masses, and by carrying them along in its flow-planes, becomes locally a garnetiferous gneiss (Fig. 9). In other places, the horn blendic blocks become streaked out into sheets, hornblende becomes converted into dark mica, and a gneiss arises, with strongly

FIG. 9.-THIN SECTION OF GARNET-GNEISS (COMPOSITE GNEISS), WITH FLUIDAL STRUCTURE, CASTLEORE, NORTH OF BALLINTOGHER.

FIG. IO.-BLOCK 80 CM. WIDE, CASTLEORE, NORTH OF BALLINTOGHER, SHOWING TROUGH-LIKE INTRUSIONS AND DYKE OF FLUIDAL GNEISS IN AMPHIBOLITE. marked banding, in complete contrast with the pale granular aplite that is continuous with it at other places (Plate 13A.). It is interesting to consider if the bands mapped as quartzite, running north and south across the strike of the chain, may not be ghosts, as it were, of Dalradian quartzite, only partially absorbed by the invader. The glaciated surfaces of the Ox Mountain ridge afford ample PROC. GEOL Assoc., VOL. XXIV. Pun: 15

w-f--f--+

C1 s W BA Y INDEX z LOWI:II Limestone. . - . Quartzite. ceroonireroos .000 '. d' LowerCerboniterous Sendstone andSh41e Boulder-deposit, limestone, Graphitic & Mica-Schist. Basal beds ofcerbomreroos Sandstone cd ijMica-Schist, with beds orrine Conglomerate. .J • ~ (or Old Sandstone). IJpper Red "~f' Granite, f'oliateulll Hornblende -rock iliSjj Lower Old RedSandstone. Granite, non-Toliatted. and EpldlorJte. SCi/Ie _ ce --''-- ~ -L.,9 _<:.::....12 =_,IS _',1a MILES .

GEOLOGICAL MAP OF N .-W. MAYO.-By :/. R. Kitro., A.R.C.Sc.I. ReproducedIrom Proc, Royal Irish Acad.my. Vol. 26B, by Permis#on 01 the Council 01 the A cade/llY.

To lace pag e 66. THE GEOLOGY OF THE MALLARAN""Y AND SLIGO DISTRICTS. 6] opportunities for the study of its structure (Plate 13B.). Its highest point in this area is south-west of Manorhamilton (1365 ft.) ; but the Bonet River trenches the range down to within 20 ft. of sea-level at Drumahaire, only five miles away. Another trench, parallel with and between those of the Bonet and the Ballysadare Rivers, has been eroded from Castleore down to Lough Gill along the weak zone caused hy a dyke of serpentine. This mass, a thousand feet across and exposed down the valley for a mile and three-quarters, results from the alteration of an olivine rock containing about one per cent. of nickel oxide. A number of faults run parallel with it, and these lines of fracture may have determined its course, and, in far later days, that of the old consequent streams across the chain. The serpentine is post-Silurian but pre-Carboniferous, and may be interestingly compared with those associated with Silurian rocks on and Clare Island. A second small intrusive mass of serpentine occurs in the Bonet valley north-west of Drumahaire. The floor of schist and gneiss is exposed again in the pro­ montory of Rosses Point, north of Sligo, as an inlier in Carboniferous Limestone. The Carboniferous strata were deposited against and across the Caledonian axis, and have been faulted down over its north­ west side (Fig. 11). The local series, up to Lower Coal Measures, is complete between Drumahaire and Lough Allen, and the scarp that is here so characteristic of the Upper Limestone is con­ spicuous above the Bonet Valley near Larkfield (Fig. 12). But the most striking features occur on the seaward side of the chain. Tables of limestone stand out on the upland near the pass of Benbo, and culminate in the huge flat-topped mass of Ben Bulben (1]22 ft.). The highest points of this plateau (200] and 2113 ft.) retain outliers of " Yoredale " Sandstone, and the local presence of this type of rock has no doubt led to the preservation of much of the Upper Limestone in the past. The slopes of Glencar and Glenade, with steep walls of Upper Limestone and gentler slopes of more shaly rock below, afford fine examples of weathering on a large scale (Plate 14), and the circular outlier of indicates how much Carboniferous material has been removed. The Upper Limestone scarps are marked by fairly regular vertical grooves, due to the temporary rills following on rains, and the absorptive power of the rock allows it to develop surface­ features characteristic of far more arid lands. Landslips of the Upper Limestone over the more yielding "calp" strata arc conspicuous in Glencar. The Glacial epoch has left abundant boulder-clay over the lower ground, as is well seen on the Bundoran coast. Extensive peat-bogs spread between Bundoran and Sligo, and mountain­ peat remains over a large part of the Ben Bulben tableland. 0­ 00 Lough Gill. Dr umah nire,

->;.::>~~ ~ c

Slali~Jl(J)'OjJia.) Q :<: t'l Z <: r:: r­ .,: -0 t'l 0" _0 u ~== ?' l\'lorerah. Benbo. ';: 5 '""" .....c" 3 ':-' o o r­ t'l o Z

FIG. I2 .-SE CTION E AST AND WE ST, ACRO SS TH E B ONET VALLEY, SO UTH OF M ANORIIAMlLTO:-l.

m rission (R eproduced/ rom lite Geological S urv ey Mem oir 10 Slu e/ 55. by ,~e if the Controller 0/ H .lI! . Stationery OjJice.) FI Gs. 3 and 4. - (a) Metamorphi c axis. (b) Lower Carboniferous Limestone and Sha le. (c) Middle Carboniferous Limestone and Shale . (d) Upper Carboniferous Limestone. ( e)" Yoredale " Series. (j') Mill stone Grit . I I DO NEGAL lJ A IIY GEOLOGICAL MAP OFTHE SLIGO DISTR ICT

Ih-,~ llq>r odlw ed /rom- th/"o{}rrhum cb (f,nd, (/~()u~git:(,l.\·71rf'';.¥.I/r'P~·HJilk .wz" "tif7 fl" MillstolU- Grit, orIh~·'f..

J ·/.... 'd ,," .\ ~) "Yore~o Series" (~&Sanilstonet)

~ f:,~:"~~/III'''~I' ' Ja·i,,,,,.'~".I··'~ )7.':$.)::;;',"'. Upper Carbonireroics L i7ne6tone series. D Mid cflq· &Low er C.L. S er Ws. .::.::::·······:::::: EIJ.,. " . Sandstone mMiilill/! / C.L .Series ,

SL .J a 0 /f A S 'andetone at bas e of' C.L .Series . 8 ~ o J}o~om..:i. D Schist & Gnei.ss .

~.Do"Le,ri~ . .Basalt,•, ~ SerpenOUlA?- o THE GEOLOGY OF THE MALLARANNY AND SLIGO DISTRICTS. 69

Glenade and Glencar Loughs, as well as Lough Melvin and Lough Gill itself, offer attractions for students of the origin of surface-features. The uncertainty of river-courses on the Irish lowland of Carboniferous rocks is shown by the fact that Belhavel Lough, south-east of Drumahaire, lying only 213 ft. above the sea, drains westward by a short course through Lough Gill to Sligo Bay and also eastward into the Shannon Basin through Lough Allen, and so down into the central plain.