Collins New Naturalist Library. David Cabot

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      Relief of Ireland. From F.H.A. Aalen, K. Whelan & M. Stout (eds) (1997) Atlas of the Irish Rural Landscape. Cork University Press.

      But what about the physical framework of Irish mountains? What about their environmental conditions – the soils and climate? And finally, what kind of mountain flora and fauna characterises Irish eminences and where are the best places to encounter it?

      Physical frameworks

      Most Irish mountains and uplands are formed of the older and harder rocks, the most ancient dating from the Precambrian period some 600 million years ago. These are the schists and gneisses (formed mainly of quartz, feldspar and mica, differing from granite in the size, colour and configuration of the crystals) that were originally laid down as sediments in the seas prior to severe alteration by pressure and heat. The effects of these processes were to change radically the character of the minerals and particles that made up the sedimentary rocks. Also included amongst the oldest rocks are the granites, originating in the molten material spewed out from deeper sources some 400 million years ago and injected into the surface layers. Mountains built of these earliest rocks are found mainly in Donegal, west Mayo, west Galway and in the Leinster region – especially the Wicklow uplands. The granites of the Mourne Mountains, Co. Down, were formed 350 million years later. During the Cambrian period, slates and quartzites were born out of sedimentary marine muds and sands. These rocks are found mainly in north Wicklow, Wexford and at Howth in Dublin. More recently still, in the Ordovician period, shaly rocks with some sandstones and limestones, including some molten rocks that have flowed into them, formed in the sea as sedimentary depositions. These amalgams of rocks are found mainly in the southeast of Ireland.

      The Devonian grits and sandstones, often called ‘Old Red Sandstones’, and principally made up of fresh or marine water deposits, form the bulk of the Cork and Kerry highlands. Closer to us still, Carboniferous limestones, the result of deposition of millions of tiny calcareous shells and marine creatures in warm tropical seas around 300 million years ago, line the floor of the Central Plain, while in the Burren, Co. Clare, these limestones thrust up to produce grey rounded hills. In Co. Sligo the Benbulbin mountain range has been carved from great thicknesses of such limestones. As to the most recent Irish mountains, the extensive upland plateau in Co. Antrim and eastern Derry, they are the result of outpourings of lava belched up from underground sources some 65 million years ago.

      The largest continuous upland area in Ireland is in Co. Wicklow where the granite hills higher than 300 m range over 520 km² and peak at 925 m on Lugnaquillia Mountain, which, unlike most of the other parts of the Wicklow uplands, has retained its Old Red Sandstone capping. The original body of the Wicklow Hills consisted of sandstones, grits and conglomerates that were laid down in an ancient sea during the Ordovician period. About 400 million years ago a large mass of hot molten granite was extruded from the earth’s belly. This heaving mass pushed the overlaying rocks upwards and humped them into a southwesterly aligned dome. Later on the sandstones and other slatey rocks were eroded away, a few lingering as marginal flanks to the hills, exposing the granite core that now forms the greatest area of granite in Ireland or Britain.

      Further south in Cork and Kerry it is the hard Old Red Sandstone rocks that have endured. Their limestone covering was stripped off after all the rock layers were thrust up by a gigantic lateral earth movement some 300 million years ago, and folded in a series of ridges, aligned west-east. The intervening valleys, however, retain some of the surviving limestone. Towards the western side of this mountain mass are the Macgillycuddy’s Reeks, Co. Kerry. These host amongst a cluster of tall conical peaks, Ireland’s highest mountain, Carrauntoohil, rising to 1,039 m, to the east of which are the famous lakes and mountains of the Killarney National Park.

      The structural geology of Ireland. From J.B. Whittow (1974) Geology and Scenery of Ireland. Penguin Books, London.

      Table 3.1. Sequence of events and dates for the geological history of Ireland. From J.B. Whittow (1974) Geology and Scenery of Ireland. Penguin Books, London.

      The Galway and Mayo highlands are also the result of convulsions that took place during the Caledonian mountain-building period some 400 million years ago. Hard granites in the south of Connemara and quartzites, gneiss, Silurian slates and shales in the north were pushed upwards to create hills and mountains. The Twelve Bens are made up of quartzite surrounded by Silurian schists and separated by valleys covered with blanket bog. Further north, over the Killary fjord, Co. Mayo, lies Mweelrea, the highest mountain in the west of Ireland (814 m), north of Brandon, Co. Kerry. The coastal cliffs at Slieve League, Co. Donegal, the highest sea cliffs within Ireland and Europe, plunge into the Atlantic from a height of 595 m. On the steep northeastern and landward face, one of the richest assemblages of alpine flora in Ireland looks out over blanket bogland and the cold, desolate Lough Agh below.

      Rainfall, soil and environmental conditions

      Levels of rainfall and humidity are much elevated on Irish mountains. The prevailing westerly winds are moisture-laden as they hit the west coast after travelling across several thousand kilometres of Atlantic Ocean, hence the often substantial and persistent falls of rain. Published figures from the highest recording station, set at 308 m at Ballaghbeama, Co. Kerry, show 396.5 cm of rain for the year 1960. At another station in Co. Kerry, at the Cummeragh River, 540 m above sea level, a total of 68.6 cm of rain was logged for just the month of December 1959, more than the average annual fall on the east coast. Further up the west coast at Kylemore, Co. Galway, close to sea level, the average over a 16 year period was 207.7 cm per year. At nearby Delphi, Co. Mayo, on the lee side of Mweelrea, rainfall of 254 cm per annum is not unusual, while in the wettest spots of Kerry and Galway precipitation can be as high as 250 cm per year.³ Such high rainfall encourages the development of boggy wet acidic soil and induces the leaching of nutrients. Still more important is the frequency of precipitation. The mountains of Donegal, Mayo–Galway and Kerry–Cork experience over 220 ‘wet days’ each year (a wet day is a period of 24 hours with precipitation of at least 1 mm).

      Unfortunately no temperature readings are available from Irish mountains. However, for every 150 m rise in altitude the temperature decreases by approximately 1°C, so temperatures at any altitude may be estimated from isotherm maps corrected to sea level. For instance, at a height of 1,000 m the air will be at least 6.7°C colder than at sea level. The increased wind speed at the top of mountains will drop the temperature even further – a phenomenon known as the wind chill factor. Below freezing temperatures are encountered in winter as a thin white mantle covers the summits. On the country’s highest mountain, Carrauntoohil, snow can fall and lie for six months of the year, from November to early May⁴, while on Mweelrea there may be snow around the summit for at least 20 days each winter.

      Despite some extremes, the Irish СКАЧАТЬ