"Killiney beach "

Killiney station car park, Killiney, Co. Dublin

Leader : Sarah Gatley (GSI)

Start from Killiney Station Car Park or from grass area near Shanganagh River bridge,
accessed via Bayview Estate (Bayview Lawn and walk under the DART railway bridge, keeping to right-hand tarmac path and up steps).

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This leisurely 3.5 km walk (plus an optional drive to Whiterock) along the sweep of Killiney Bay, will enable us:

- to discuss the different rocks forming the surrounding landscape,
- to examine the pebbles underfoot and the glacial deposits of the Shanganagh Cliffs above,
- and finally to see close up, the dramatic contact between two contrasting rock types at Whiterock.

These few hours will encompass over 500 million years of geological time, from some of the older rocks in Ireland, of Cambrian age, to the relatively recent Quaternary, or Ice Age deposits of the last one and a half million years. Although we can see only a small sample of the varied rocks of Ireland from this viewing point, we will be able to demonstrate most of the geological processes at work and talk about Ireland's place in the geological column and its plate tectonic setting.

For a shorter walk, look at the pebbles on the beach just south of Killiney Station (Stop 2a) and at the cliffs extending southwards from behind the lifeguard shelter (Stop 3a).

The Shanganagh glacial cliffs,
looking northwards towards Killiney Hill and Whiterock.

Stop 1. The landscape around us

Our starting point (a 20 minute walk southwards from Killiney Station car park) will be at the southern end of the beach, on the grass area just above the Shanganagh River Bridge. From here we can see the relatively low-lying land of the bay area itself and farther inland, and the enclosing rim of hills and mountains. The low land is underlain by mainly shales and sandstones, that were laid down as mud and sand sediments in a deep ancient ocean called Iapetus (see Stop 4) during the Ordovician period of geological time (about 475 million years ago). As comparatively soft rocks, they have proved less resistant to erosion during the millions of years since their formation. In contrast, the surrounding higher ground of Killiney Hill and most of the Wicklow Mountains are composed of a harder, igneous rock called granite, that cooled from a molten magma forced up into the surrounding shales during the Devonian (around 400 million years ago). Where the granite heated the adjacent shales, a new type of (metamorphic) rock called schist was formed (see Stop 4). The schist also forms some of the east Wicklow peaks, such as Lugnaquillia, as both granite and schist have proved more resistant to erosional forces over time. Bray Head, the Little and Large Sugar Loaf and the small hump of Carrickgollagan also stand relatively high because they are made up of quartzite, a tough rock that has formed from the hardening of sands deposited on the continental shelf in Cambrian times (about 500 million years ago). Quartzite generally forms sharp, conical peaks and granite, the more rounded hills.

Stop 2. Pebbles on the beach

Most of the pebbles have been derived from the glacial cliffs behind the beach, having been plucked as fragments from various rock formations nearby and farther afield, and carried by ice age glaciers as they scoured the landscape (see Stop 3). Further rounding and smoothing has been effected by the action of the sea. They represent a stage in the ongoing tectonic rock cycle of igneous (molten rock) intrusion, mountain building, erosion, sedimentation of the detritus, compaction, deformation and metamorphism. Looking at these pebbles is a good way of seeing a variety of rocks, minerals and fossils that make up the three rock groups known as sedimentary, igneous and metamorphic, and of talking about the environments in which they formed.

Pebbles on the beach: spot the three rock groups.

Amongst the sedimentary rocks, you will be able to identify examples of grey Carboniferous limestone containing fossils of the sea life in existence at the time (around 330 million years ago), such as corals, sea shells and sea lilies; and red conglomerates formed in the desert landscape of Devonian times. Igneous pebbles include spotted Ordovician volcanic rock called porphyry, from Lambay Island; and granite, including a distinctive microgranite speckled with small bluish minerals, that was transported all the way from Ailsa Craig island off the Ayrshire coast of Scotland by the ice. The metamorphic types are well represented by smoothed pieces of mica-schist, mostly derived locally from Whiterock and of quartzite from Bray Head.

Stop 3. Glacial cliffs

Much of the solid rock of the low-lying areas is covered by relatively recent superficial (subsoil) deposits, mostly of sands, gravels and boulder clay (till) brought southwards across Ireland by ice sheets during two glacial periods, most recently around 18,000 years ago. These glacial deposits also form the line of cliffs at Shanganagh and the higher cliffs (reaching 17 m) that are being "propped up" near Killiney Station. This instability is due to rapid coastal erosion which appears to be at the rate of about one metre per year. These cliffs, which extend for about 4 km to just south of Bray, have provoked much interest and argument since the mid-nineteenth century, concerning the provenance of the ice sheets that deposited them. The presence of lower tills and gravels containing shells dredged from the Irish Sea floor point to input from the Irish Sea Ice Sheet advancing from the northeast. Later deposition by the Midlands Ice Sheet is supported by the northwest-southeast trending glacial scratches, or striae, seen on the smoothed granite surfaces of Killiney Hill.

Fluvial-glacial channel cut into sands and gravels showing cross- bedding.

Another theory suggests that due to downwarping of the Irish Sea Basin because of the ice load, a rising sea level broke up the Irish Sea Basin ice, exposing its subglacial tunnels. These were then filled with a poorly-sorted mixture of glacial-marine debris pumped in through a northwesterly channel from the Ballybrack direction. Whatever their exact origin, we will be able to see clearly gravel-filled channels cut into the Shanganagh cliff section and other features of glacial meltwater deposition, such as cross-bedding. The gravels and boulders in the cliff mirror the variety of pebbles on the beach, as the latter have merely been prised from the boulder clay by erosion.

In the cliffs near to Killiney Station (Stop 3a), though much overgrown, it is possible to see sorted pebbles and sands dipping quite steeply towards the south, representing the deposits of the glacier meltwaters. Higher in the cliff section, a distinct line of truncation (picked out by vegetation) marks the base of the later Midlandian boulder clay.

Stop 4. Whiterock granite contact

The recent boulder defences erected at the northern end of Killiney beach, mean that access to Whiterock is only safely achieved by driving north from the station, parking halfway along the Vico Road and taking the footpath over the railway line and steep steps down into Whiterock bay.

At Whiterock you can see close-up, an outcrop of rocks showing the contact between the granite of Killiney Hill and the adjacent Ordovician mica-schists. Unlike the pebbles and the glacial deposits, these rocks are in situ, which means that we are looking at the original relationship between them from the time of the granite intrusion.

The granite (on right) and mica-schist contact at Whiterock.

The so-called Leinster granite that forms the Wicklow Mountains was intruded during a mountain building episode (the Caledonian Orogeny) in the early Devonian Period around 400 million years ago. This orogeny was the culmination of the collision of the two originally separate northwestern and southeastern halves of Ireland, along the line of the closing Iapetus Ocean that had been gradually shrinking (by the mechanism of plate tectonics) throughout Ordovician times. The granite pluton rose upward as molten rock into the root of the Caledonian Mountain chain, and started to cool 5-10 km below the surface. The present-day 'unroofing' of the granite is the result of millions of years of erosion.

When viewed closely, the granite can be seen to be composed of three main minerals which occur as grains a few millimetres across; glassy grey quartz, white feldspar and shiny plate-like micas, which impart the glint to the rock. Within the granite, finer-grained veins are called aplites and coarser-grained veins, pegmatites: both formed from left-overs in the granite melt, and lack the darker type of mica. The granite was worked from a quarry on the Dalkey side of the hill for the construction of the Dun Laoghaire harbour piers between 1817 and 1859. Six million tonnes of granite were transported along a straight narrow-gauge tramway, the line of which still exists in places as the 'Metals' pathway.

The rocks against which the granite abuts are called mica-schists, characterised by thin, parallel bedding planes along which the rock tends to split. Extreme heat from the granite intrusion metamorphosed what were originally compacted sandstones and mudstones from the Iapetus sea floor, into schists. Shiny plate-like crystals of mica are orientated along the bedding planes as a result of pressure from the granite. New minerals such as garnet and andalusite also formed in the zone of metamorphism. The andalusite can be picked out as a dark criss-cross pattern of crystals covering the bedding planes of the mica-schists.

Andalusite crystals on a mica-schist bedding plane (pen for scale).

Stop 5. Mine shaft

The contact between intruded granites and their surrounding 'country' rocks is significant in that this is also the site of economically important mineralisation. A short distance northwards from the granite-schist contact, there is an old mine entrance in the cliff (it is unsafe to enter), where lead and copper was worked sporadically during the 1750's and 1820's. From the southern end of the beach we were able to see the smelting chimney at Ballycorus, where veins of lead ore were mined at the granite's edge in the first half of the nineteenth century. These were relatively insignificant operations; greater production continued in the Glendalough area of County Wicklow, from where the lead was also sent for smelting at Ballycorus.

Old mine entrance in the cliff : it is unsafe to enter.