Killarney National Park is underlain by two distinct and contrasting rock formations. A major geological boundary runs across the National Park, generally from north-west to south-east, mostly beneath the waters of Lough Leane and Muckross Lake. It separates a mountainous area of Devonian rocks to south and west from a plain underlain by Carboniferous rocks to north and east.
The Devonian rocks, generally referred to as Old Red Sandstone, consist of numerous layers of sandstone interbedded with grits, slates and conglomerates. The oldest Devonian rock formations, exposed in the valley of the Upper Lake, are predominantly green in colour, while the succeeding formations are generally red, purple or grey. The proportion of fine-grained rocks among the hard erosion-resistant sandstones also tends to increase from older to younger.
The Devonian bedrock outcrops very extensively at the surface, and is only very locally covered by deep glacial deposits, soils or peats.
The Carboniferous rocks are predominantly limestone, including both massive unbedded reef limestone formations and bedded calcarenites. There are also relatively thin but significant chert (i.e. siliceous) formations, comprising layered and nodular black cherts among finely stratified limestones and calcareous mudstones. The Carboniferous formations outcrop extensively along and near the Muckross Lake and Lough Leane shores, the outcrops including expanses of irregular limestone pavement with numerous vertical fissures.
Away from the lakes the Carboniferous rocks are generally covered by thick glacial deposits. One of the chert formations has been partially altered to form the hard white and pink Killarney marble, which outcrops on the Muckross Peninsula and elsewhere, and was occasionally quarried in the past for local building use. A considerable amount of mineralisation also occurred in some Carboniferous strata, with the formation of copper minerals which were mined in prehistoric times and again at Ross Island and at Muckross in the 18th century.
For the most part the Devonian/Carboniferous boundary near Killarney is marked by the Millstreet – Muckross Fault Line, an enormous thrust fault which raised the lower older Old Red Sandstone beds high above the younger Carboniferous limestone. However, transitional formations from Devonian to Carboniferous do outcrop on the Muckross Peninsula. These consist of grey sandstones and laminated black siltstones and claystones.
Glacial Features and Other Landforms
The contrasting influence of the two main rock formations on the landforms of the National Park has already been referred to. The Old Red Sandstone is relatively resistant to weathering, and forms rugged mountainous country. Various forms of erosion have worn down the Carboniferous rocks to form a low-lying plain.
Apart from the nature of the rock formations, the strongest influence on the landforms of the area was Quaternary ice movements. While the smooth summit ridges of Mangerton, Purple and Tomies Mountains may represent pre-glacial surfaces that stood above the ice as nunataks, the predominant landforms of the mountain area are those of glacial erosion. At high levels, erosion by frost and ice carved out horseshoe-shaped corries, some now filled by lakes, such as the Devil’s Punchbowl.
At lower levels pre-glacial valleys, including the major valley now containing the Upper Lake and Long Range, were deepened by glaciers, most recently those that advanced north-eastwards during the last glaciation. Many rock surfaces were moulded and striated by the passing ice.
In the lowlands, landforms derived from glacial deposition predominate. The widespread glacial drift material include large crescentic terminal moraines north of Lough Leane and irregular kame and kettle-hole deposits east of Muckross Lake, laid down by the ice when it reached the limestone lowlands and subsequently retreated. In addition to extensive sand and clay deposits, there areoccasional erratic rocks.
Deposits laid down beneath the ice and at its edge as it retreated also occur locally at the edge of and within the mountain area. Fretted cliffs and caves along the limestone lake shores are the results of limestone solution by lake waters, possibly combined with the earlier effects of glacial meltwaters and ice erosion. The same combination probably formed the basins of the lakes themselves.
A small lake in Dromyrourk townland, similar to turloughs elsewhere, has seasonal changes of water level controlled by water levels in underground drainage channels in the limestone.
The soils of the National Park are strongly influenced by the underlying rock formations. On the Devonian rocks the soils are generally acid and relatively infertile. Podsolised soils are widespread, with mor humus, usually sandy mineral soil of variable depth and often a pronounced iron pan.
In the uplands and in other non-wooded areas, high rainfall and acidity have led to the development of peaty soils, culminating in the formation of blanket bog over considerable areas. Locally there are peaty gleyed soils where winter flooding or waterlogging occurs, and gravelly skeletal soils where peat is absent at high altitudes.
Over Carboniferous rocks the pattern of soil types is closely related to parent material. On limestone there is often only a thin layer of skeletal soil beneath a carpet of moss, but there are pockets of slightly deeper reddish brown calcareous rendzina soils. Over chert layers and glacial drift, deeper moderately acid soils occur. These are commonly well-drained brown earth soils with mull humus. In low-lying level areas near Lough Leane there are waterlogged soils, often with a deep organic layer.
Climate, Weather and Air Quality
The moderating effect on temperatures of the moist warm air moving in from the Atlantic Ocean, and the upward movement of this air over the mountains causing precipitation, are the two most important factors governing the climate of Killarney National Park.
The climate is extremely oceanic, and is characterised by mild winters, cool summers and rainfall spread throughout the year.
Air temperature data from the Valentia weather station (65 km west of Killarney) for 1961-1990 show a range of only 8.2°C between the means of the coldest (February 6.6°C) and warmest (July 14.8°C) months.
Between the years 1990-98, the range was 8°C, the coldest month on average
being January at 7.5°C, and the warmest month on average being August at 15.5°C.
Extremes are almost unknown and since the establishment of a weather station at Muckross House, the lowest temperature recorded was approx -11C was exceeded during the winter of 2010-2011, while the highest was 30.1°C.
Soil temperatures remain relatively high throughout the year with few ground frosts, thus the growing season for plants is long.
In Killarney town, rain days average 223 days per annum. In the mountains, this rises to more than 250 days per year.
The 30-year mean (1961-1990) for annual rainfall at Muckross was 1,589 mm, with July being the driest (77 mm) and January the wettest (214 mm). This contrasts with a station on Mangerton Mountain (808 m altitude) with a total of 3,230 mm (168 mm in July and 402 mm in January).
Falls of snow, even on the higher hills, are usually confined to a few weeks in January and February. Even on the hills, except in very severe winters, snow rarely lies on the ground for more than a week or so.
Generally, precipitation exceeds evapo-transpiration over both winter and summer although a deficit in soil moisture may occur in very dry summers. Humidity is very high throughout the year.
Prevailing winds are from the south and south-west, with winds from the south-east being the least frequent. More than 50% of winds in the south-west quadrant are in excess of 10 knots (measured at Valentia).
Winds of more than 25 knots are infrequent while calms are recorded approximately 3% of the time. Wind directions and speeds are, of course, modified by local topography.
Because of the oceanic nature of the climate, with moist warm air off the Atlantic rising over the mountains to form cloud, sunshine totals are low. These vary from a mean of 0.9 hours/day in December to 4.5 hours/day for the period April/August, May being the sunniest month with 5.2 hours/day.
In view of the location of the National Park and the prevailing winds off the Atlantic Ocean, high standards of air quality with very low pollution levels are to be expected. The nearest air quality measurements, taken daily at Valentia, confirm this.