Members Reports and Photos


SATURDAY 21st FEBRUARY 2015                    GLENDALOUGH, CO. WICKLOW 


At the end of the Cambrian period, c. 510 million years ago (Ma), Laurentia (w/W+NW Ireland) and Gondwana (w/SE Ireland) started to come together with the closing of the Iapetus Ocean. The plates came together in the Late Silurian, c. 400 Ma, with the final closing of the Iapetus Ocean when Avalonia detached from Gonwanda and fused the two parts of Ireland. The resulting fault line runs from Clogher Head, Co. Louth to Loop Head, Co. Clare.
The collision caused the Caledonian Orogeny to occur. In a period of relaxation after the collision the Leinster Granite was emplaced, c. 405 +/- 2 Ma. Then uplifting occurred with a north-east to south-west orientation. The base of the crust was pushed down under the weight of the mountains, and the heat at depth caused it to partially melt producing a large volume of granitic magma. This rose up and was intruded as the Leinster Granite Batholith in five discreet bodies or plutons. In many cases the country rock was uplifted and is found on some mountain peaks such as Lugnaquilla and Conavalla.
During the Ordovician-Silurian Period, 510-410 Ma, there was continuing deposition of marine sandstones and mudstones, similar to what occurred in Cambrian times at Bray Head, and which became more continental as the plates came together. In addition, there was considerable volcanic activity, because the oceanic crusts were being subducted under the lighter continental crusts resulting in submarine volcanic arcs, similar to those in modern Japan, i.e. a northerly arc from Balbriggan to Kildare, and a southerly one from Wicklow to Waterford.
Walking from the car park at the Upper Lake westwards towards the granite outcrop the Lower Ordovician rocks of the Maulin Formation can be seen. These are dark to mid gray siltstones and shales, which have been metamorphosed by the heat of the granite intrusion. As the granite is approached, evidence of metamorphism is apparent. The latter is only low grade, seldom reaching the mica-schist phase. The thermal aureole is c.2 km and starts near the car park. At the latter site, furthest from the contact, we have dark grey phyllites; pale to dark grey/green chlorite; and muscovite which has a grey-green silvery sheen. Sedimentary features such as folding of the rocks, and even bedding, are also commonly seen. Close to the contact area clay minerals convert to micas and form mica-schists (50% platy minerals), which include the high temperature mineral Andalusite (Aluminium Silicate: pink/red, lead-pencil crystals), and Quartizites.
Leinster Granite is the largest body of granite in the British Isles. It covers an area of c. 1,500acres and consists of five plutons, all with different mineralogies: The Northern, (Killiney Hill, which is seen in Dalkey Quarry), The Upper Liffey, The Luggnaquilla, here in Glendalough, The Tullow, and The Blackstairs. They stretch from Blackrock to New Ross and from Glendalough to Ballymore Eustace.
The main constituents of granite are quartz (SiO2), feldspars (complex Na-K-Ca aluminium silicates), and micas (Fe+Mg sheet silicates: biotite & muscovite being the commonest). Sometime Amphiboles (hydroxyl molecules with needle like crystals) occur, these are coarse grained due to rapid cooling, the commonest being the dark coloured Hornblende. Leinster Granite has complex mineralogical assemblages, which differ in each pluton The Lugnaquilla pluton has predominantly Adamellite (Equal volumes of Ca+K feldspar), which is what is seen here, and Ganodiorite (Na/Ca more than K feldspars), which occurs to the south.
The primary economic ores are Galena (PbS) Lead (Pb) and Sphalerite Zinc (ZnFeS).  These are found along the granite contact stretching from the Dalkey coastline southwards. In addition, native silver is found in association with lead. Mining started at Glendalough between 1812-1817 and gunpowder, packed into holes drilled using a hammer and long chisel, was the explosive used; adits ran right through mountain. The broken rock was loaded by hand into carts, which ran on rails. From 1928 onwards, the ore was sent to Ballycorus. Lead was used for lead sheeting and  piping, whereas lead oxide (red lead) was used for paints. Mining continued until 1965, and over the years some 46,000 tons of lead and 330,000 ounces of silver were extracted. At the height of the industry, over 2,000 people were employed in Glendalough and Glendasan. Neither Copper, nor granite, nor slates were mined in Glenadalough. However, copper was mined at Avoca,  granite at Dalkey, and slates elsewhere.
No Mesozoic or Tertiary rocks cover bedrock in much of Leinster. During the Pleistocene, 2.6 million to 11,000 years ago, there were a number of ice ages. The most important one affecting the landscape (because each glaciation removes evidence of previous ones) was the last one, the Midlandian Glaciation, 120,000-11,600 years ago; it was an intensely cold period with tundra conditions prevailing. The maximum ice advance occurred 24,000-15,000 years ago, when ice covered most of Ireland. Ice moves as snow accumulates in mountain corries, and pressure converts snow to ice. As the ice gets thicker it deforms and begins to move in the form of glaciers that carry debris. Scoured out U-shaped valleys, as typified at Glendalough, result when the debris in the ice flow gouges out the sides of a valley. The ice flow here was from north-east to south-west. Over deepened sections, lakes in less resistant rocks (schist and slates), allow water into cracks which refreezes. The rock material eroded by the ice was carried until the ice melted and was then deposited. Large granite erratics may be found all over the valley floor, dumped there by the mass of moving ice. A medial moraine is a ridge of moraine that runs down the centre of a valley floor. It forms when two glaciers meet and the debris on the edges of the adjacent valley sides join and is carried on top of the enlarged glacier. Two glaciers coalesced to form a medial moraine at the site of Glendalough Monastery. The flat-topped area of sand and gravel provided a dry, flat and defensible position.
Pegmatite at Waterfall
A pegmatite is an holocrystalline, intrusive igneous rock composed of interlocking crystals usually larger than 2.5 cm in size; such rocks are referred to as pegmatitic. Crystals grow perpendicular to the direction of intrusion.  Most pegmatites are composed of quartz, feldspar and mica that have a similar basic composition as granite. In most cases, pegmatites tend to occur in the aureoles of granites and are usually granitic in character, often closely matching the compositions of nearby granites. Pegmatites thus represent exsolved granitic material, which crystallises in the main earlier than emplaced granite.


Glenealo Valley Reserve                             Guide to Minerals                                  Field Club            


 Pegmatite in Granite                         Pegmatite                                                  Granite              


Mine Workings                                                                Mine Workings        


Quartzite                                               Spoilheap                                                         Quartz


             Mica-shist                                             Metamorphosed Sedimentary


    Muscovite                                              Schist (R) & Granite (L)


St Kevin's Bed                                      Granite (R) & Schist (L)

Photographs © P. Lenihan

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