The five glaciations that are now known to have influenced southern County Dublin and northern County Wicklow, Eire, were identified and placed in order by Reference Farrington,Farrington (1944); he summarized his findings in the Journal of Glaciology (Reference Farrington,Farrington, 1949). This sequence has recently been confirmed (Reference Hoare,Hoare, 1975 Reference Hoare.unpublished) but the status of the intervals between events remains to be firmly established.
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1. Slievethoul (Enniskerry) ice cap.
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2. Irish Sea (Eastern General) ice sheet.
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3. Brittas (Brittas) ice cap.
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4. Dublin (Midland General) ice sheet.
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5. Glenasmole (Athdown) ice cap.
(Names in parentheses are those given by Farrington.)
Much of the evidence for the stratigraphy comes from the Brittas area of south-west County Dublin; a single exposure in Ballinascorney townland (061212)Footnote * illustrates the relationship between the first four of these events. The site is a trial gravel pit at approximately 270 m o.D. near the eastern margin of the Ballinascorney–Aghfarrell delta of Dublin glaciation age. There can be few such diminutive sections which have yielded so much detail concerning former glacial episodes.
The Slievethoul and Irish Sea glaciations. A brown (7.5YR 4/4 on the standard soil colour chart of the Fujihira Industry Company, Tokyo, Japan) stony till laid down by the Irish Sea ice sheet occupies the lowest part of the excavation (bed A, Fig. 1 ; Table I). It is at least 1.4 m thick and, although the base is not seen, Lower Palaeozoic bedrock crops out immediately to the south suggesting that the total depth does not greatly exceed this figure. Till samples which appear to be fresh do not react with dilute hydrochloric acid and no shell fragments occur. (A calcareous shelly till forms the lateral equivalent of this deposit in northern and central County Dublin.) The material nevertheless possesses a colour and fracture characteristic of Irish Sea glaciation till.
Fig. 1. The section in Ballinascorney townland, southern County Dublin, Eire.
Table I. Percentage Lithological Composition of the Beds
The green shale, siltstone, greywacke, and grit fragments (Table I) were derived from the local bedrock. The Leinster granite erratics were incorporated from drift carried northward into the area by the Slievethoul ice cap. The nearest in situ granite to the Ballinascorney exposure occurs 2.8 km to the south-east and there is no other route by which these particles may have been moved into the area. Granite erratics form the only evidence of" this early local glaciation in southern County Dublin; till and outwash gravel dating from this event may be seen in coastal sections in County Wicklow (Reference Farrington,Farrington, 1944; Reference Synge,Synge, 1964). The absence of Carboniferous erratics from the till of Irish Sea Basin provenance demonstrates how effectively they were "diluted" by locally derived material. Fabric analyses, striae and erratic carriages throughout County Dublin indicate that the ice sheet advanced from the north-east (Reference Hoare,Hoare, 1975 Reference Hoare. 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 determine lithological make-up is almost completely absent but the composition of gravel from a nearby exposure (064215) is shown in Table I. The chert and black mudstone erratics were derived from Irish Sea glaciation till in the area (although bed A at the Ballinascorney site would not have been able to provide them).
Bed B is equivalent to sand and gravel mounds in the Brittas area which mark the maximum extent of the Brittas ice cap. The distribution of granite erratics related to those found in bed A shows that the earlier Slievethoul glacier advanced beyond this limit.
The Dublin glaciation. The granite sand is sealed beneath approximately 4.2 m of predominantly fine-medium calibre gravel (bed C) deposited as a delta in Glacial Lake Blessington at the height of the Dublin glaciation (Reference Farrington,Farrington, 1942 Reference Farrington,1957; Reference Hoare,Hoare, 1975 Reference Hoare. unpublished). Beds B and C are separated by a clearly defined junction. The gravel is generally moderately well graded but is locally poorly sorted. The maximum diameter of fragments is about 18 cm; layers of washed sand and gravel also occur. A lobe of the Dublin ice sheet pushed south-westward through the Ballinascorney col (0722) from Glena-smole {0823) and delivered the gravel into the pro-glacial lake at about 280 m.o.D. Morainic mounds north of Ballinascorney House (068220) record the farthest extent of this diffluent ice; they are banked against low andesite knolls which control much of the local relief. The moraines are equivalent to bed G in the Ballinascorney section. Directional indicators associated with the Dublin ice sheet show that it advanced from the north-west.
Evidence for the Glenasmole glaciation is very limited in County Dublin and does not occur in the Brittas area.
The sequence of glaciations at the northern end of the Leinster mountains was originally unravelled as the result of the extensive and meticulous work of (Reference Farrington, Farrington 1934 Reference Farrington,1942 Reference Farrington,1944 Reference Farrington,1949 Reference Farrington,1957). The somewhat fortuitous exposure of three horizons in a single small pit within the area, which was so important in Farrington's pioneering studies, has allowed the first four of these events to be confirmed.
