Journal of Glaciology, Vol.
20,
No.
82,
1977
THE GLACIAL RECORD IN SOUTHERN COUNTY DUBLIN,
EIRE
By PETER HOARE
(D e partment of Geography, Cambridgeshire College of Arts and Technology, Cambridge CBI 2AJ,
England)
ABSTRACT. Early work by Farrington a t the northern end of the Leinster mounta ins es ta blished a fram ework for glacial episodes throughout Irela nd . The recent excava tion of a small pit in commercial gravel
workings in southern County Dublin has confirmed the rel ationship between four of the fi ve events described
by Farrington.
R EsuME. Les souvenirs glaciaires dalls le sud de COUllty Dublill, Eire. Un precedent travail dfl a Farrington a
l'ex tremite Nord des montagnes de Leinster a etabli les grandes ligncs des episodes glaciaires a travers
l' Irla nde. Le forage recent d'un petit puits da ns une carriere commerciale de gravier, dans le sud de COUllty
Dublin, a confirme les rapports en tre quatre des cinq episodes decrits par Farrington.
ZUSAMMENFASSUNG. Der AblmU der Eiszeit im slidlichen County Dublill, Eire. Farringtons fruhere Untersuchungen am Nordende der Leinster Mountains schufen ein Gerust fur eiszeitliche Geschehnisse in ganz Irland.
Die jungst erfolgte Grabung eines kl einen Schachtes in einer Kiesgrube des sudlichen County Dublin ha t di e
Beziehungen zwisch en 4 der 5 von Farrington beschriebenen Ere ignisse bes tatigt.
THE five glaciations tha t are now known to have influenced southern County Dublin and northern
County Wicklow, Eire, were identified and placed in order b y Farrington ( 1944) ; h e summarized his
findings in the J ournal of Glaciology (Fa rrin g ton , (949). This seq uence has recently been confirmed
(Hoare, 1975, unpublished) but the status o f the intervals b etween even ts r emains to be firmly es tablished.
5.
4.
3.
2.
I .
Glenasmole (Athdown) ice cap .
Dublin (Midland General) ice sh eet.
Brittas (Brittas) ice cap.
Irish Sea (Eastern Gen eral ) ice sheet .
Slievethoul (Enniskerry) ice cap.
(Names in parentheses are those give n b y Farrington. )
Much of the eviden ce for the stratigraphy comes from the Brittas area of south-west County Dublin ;
a sing le exposure in Ballinascorney townland (06 12 12)* illustrates the relation ship between the first
four of these events. The site is a tri a l gravel pit at a pproximately 270 m 0 .0. n eal· the eastern margin
of the Ballinascorney- Aghfa rrell delta of Dublin glaciation age. There can be few such diminutive
sec tions which have yielded so much d etail concerning form er g lacial episod es.
The Slievethoul and Irish Sea glaciations. A brown (7.5YR 4 /4 on the standa rd soil colour chart of the
Fujihira Industry Company, Tokyo, J apan ) stony till laid d ow n by the Irish Sea ice sheet occupies
the lowest p art of the excavation (bed A, Fig. I ; Table I ) . It is at least 1.4 m thic k and, althoug h the
base is not seen, Lower Palaeozoic bedrock crops o ut immedi ately to the south su ggesting tha t the total
d ep th does not greatly exceed this figure . Till samples which app ear to be fresh do not react with dilute
hydrochloric acid and no shell fragments occur. (A calcareous shelly till forms the lateral equival en t of
this d e posit in northern and central County Dublin .) The m ater ia l nevertheless possesses a colour and
fracture ch a racteristic of Irish Sea glaciation till.
The g reen shale, siltstone, greywacke, and g rit fragm ents (Table I ) were d e ri ved from the local
bedrock. The Leinste r granite erratics were inco rporated from drift carried northward into the area by
the S lieve thoul ice cap . The nearest in situ granite to th e Ballinascorney exposure occurs 2.8 km to the
south-eas t and there is no other route by which these particles may have been moved into the area.
G ranite erratics form the only evidence of this earl y local g laciat ion in southern County Dublin ; till
a nd outwash g ravel dating from this event m ay be see n in coastal sections in County Wicklow
(Farr ing ton, 1944 ; Synge, 1964). The absence of Carboniferous erratics from the till ofIrish Sea Basin
provenance d emonstrates how effectively they were " diluted" by locally d erived material. Fabric
• Grid references lie within sub-zone 0 of the Irish National Grid.
223
JOURNAL OF
224
Fig.
I.
GLACIOLOGY
The section in Ballinascornry townland, southern County Dublin, Eire.
TABLE
I.
PERCENTAGE LITHOLOGICAL COMPOSITIO N OF THE BEDS
Phyllite
Local shale, siltstone, greywacke and grit
Leinster granite
Carboniferous limestone and chert
Carboniferous black mudstone
* Exclusively
Bed A
66·9
12·3
16.0
Nil
N il
Bed B
10.0
5. 6
76 .4
2. I
0·5
*
BedC
13·4
24·7
19·5
23·4
Nil
chert.
analyses, striae and erratic carriages throughout County Dublin indicate that the ice sheet advanced
from the north-east (Hoare, '975, unpublished).
The Brittas glaciation. The till is overlain by up to 2.1 m of granite-rich fluvioglacial sand (bed B)
associated with the northerly expansion of the Brittas ice cap; the contact between beds A and B is sharp
but slightly irregular. There is no weathering profile in the upper part of the till, although a thin
(8- 10 cm) layer of pebbles represents the re-worked top of bed A. The sand is current bedded throughout
but with highly variable foreset directions. Material coarse enough to d etermine lithological make-up
SHORT
NOTES
is almost comple tely absent but the composition of g ravel from a nearby exposure (064215 ) is shown
in Table I. The ch ert and black mudstone erratics w e re d erived from Irish Sea glaciation till in the
area (a lthough b ed A at the Ballinascorney site would not have been able to provide them) .
Bed B is equivalent to sand an d gravel mounds in th e Brittas area w hi c h mark the maximum extent
of the Brittas ice cap. The distribution of g ranite erratics related to those found in bed A shows that the
earli er Slievethoul g lacier advanced b eyond this limit.
The Dublill g laciation . T he g ranite sand is sealed b e neath approximate ly 4.2 m of predominantly
fin e-medium calibre g ravel (bed C ) d eposited as a delta in G lacial Lake Blessington at th e h eight of the
Dublin g lacia tion (Farrington , '942 , ' 95 7 ; Hoa re, ' 9 75, unpublish ed ) . Beds Band C are separated by a
clearl y d e fin ed junction. The g ravel is generally mod erately well g ra d ed but is locally poorly sorted.
The maximum diameter of frag m e nts is a bout , 8 c m ; layers of wash ed sand and gravel also occur. A
lobe o f the Dublin ice shee t push ed south-westward t hro ug h the Ballinascorney col (0 722 ) from Glenasmol e (0823 ) and d eli vered the gravel into the pro-g lacia l lake at about 280 m 0.0. Morainic mounds
north of Ballinascorn ey House (068220 ) record th e farth est extent of th is diffluent ice ; they are banked
aga inst low andesite knolls wh ich co ntrol much of th e local reli ef. The morain es are equiva le nt to bed C
in th e Ballinascorn ey section . Direc tional indi cators associated with the Dublin ice sh eet show that it
advan ced from the north-west.
Evidence for th e G lenasmole g laciation is very limite d in County Dublin and does not occur in the
Bri ttas a rea.
Th e sequen ce of glacia tions at the northern end of th e Leinster mountains was origin a ll y unravelled
as the result of the extensive a nd meticu lous work of Farrington ( 1934 , '942, 1944, '949, ' 957 ) . T he
somewhat fortuitous exposure of t hree horizons in a single small pit w ithin the area, which was so
important in Farring ton's pionee l'ing studies, has allowed the first four of th ese events to b e confirmed .
MS. received 4 March 1977
REFERENCES
Farrington, A. 1934. The glacia ti on of the Wi cklow mountains. Proceedings of the Royal Irish Academy, VO!. 42B,
No. 7, p. 173- 209.
Fa rrington, A. 1942. The granite drift near Brittas, on the bord er between County Dublin and County Wi cklow.
Proceedings of the R oyal Irish Acadenry, VO!. 47B, No. 12, p. 279- 9 1.
Farrington, A. 1944. The glacial drifts of the district around Enniskerry, County Wicklow. Proceedillgs of the
Royal Irish Academy, VO!. 50B, No. 6, p. 133- 57.
Fa rrington, A. 1949. The glacial drifts of the Lei nster mounta ins. J oumal of Glaciology, Vo!. I , No. 5, p. 220- 25.
Fa rrington, A. 1957. Glacial La ke Blessington. Irish Geography, Vo!. 3, No. 4, p. 2 16-22.
Hoare, P. G. 1975. The pattern of gla cia tion of County Dublin. Proceedings of the Royal Irish Academy, Vo!. 75B,
No. 9, p. 207- 2 4.
H oare, P. G. Unpublished. T he glacia l stratigraphy of County Dublin . [Ph.D. thesis, University of Dublin,
197 2 .]
Synge, F. M . 1964. Some problems co ncern ed with the glacial succession in south-east Ireland. Irish Geography,
Vo!. 5, No. I , p. 73- 82.