Brief Geological and Biological Histories of the Killarney Valley

Ilex aquifolium woodland in the Killarney Valley © Jon Fern 2011

Part One: A Brief Geological History, Incorporating the Most Recent Period of Glaciation, and its Impact on the Killarney Valley and Environs

The Killarney Valley is one of the Republic of Ireland's greatest natural assets. It also has a fascinating geological history. The main contributing rock types in the geology of the Killarney Valley are Old Red Sandstone from the Devonian Age (395-345 million years old) in the south and west of the Park, and Carboniferous Limestone (345-295 million years old) in the north. These two rock types meet at an ancient geological boundary within the Park, part of which runs beneath Lough Leane (Carruthers, 1998).

Beginning with the Pleistocene Age, which began around 2 million years ago, and lasting until around 16,000 years ago, the last major period of glaciation helped to shape the bedrock of the Killarney Valley. Ice eroded the bedrock and deposited eroded material as glacial tills, forming moraines or mounds of mixed gravel and stones, which would later become hills. The massive Templenoe glacier carved out many of the mountain features we see today, including Moll’s Gap (Quirke, 2001). The Gap of Dunloe was formed when an area of ice moved across a ridge, carving out the valley beyond (Mitchell et al., 2007). Mitchell et al. (2007) also point to the striated rocks around the Upper Lake as evidence of the erosion from the ice.

Carruthers (1998) identifies several cold stages affecting Kerry, interspersed with warmer interglacial periods. During the Pleistocene Epoch, massive ice sheets formed which caused the earth’s crust to sag; during the interglacial periods, the resultant upwarping of the ground sometimes left beaches higher than sea level; an example of this is found at Ballydavid on the Dingle peninsula. These interglacial periods deposited organic material as well as inorganic tills; peat and semi-organic silts may be found within the low cliffs around Spa, near Tralee bay. Pollen from these deposits included grains from species including fir and rhododendron. Uranium-thorium disequilibrium dating determined the age of these deposits to be between 123 000 and 114 000 years BP (Carruthers, T., 1990).

Since each of the glaciers that affected Killarney likely removed the traces of those previous, the most noticeable effects of glaciation in the area come from the most recent glaciation event (Quirke, 2001). This lasted from 115 000 years ago to 10 000 years ago. Freeze-thaw action shattered mountain peaks almost continuously. Mountains of Old Red Sandstone particularly were eroded by layers of shattered rock. Snow accumulated as temperatures plummeted, and masses of ice formed. They thickened and became glaciers.

Quirke (2001) describes the Templenoe glacier as being instrumental to the formation of the Killarney landscape. More so than the mountain ice on the Macgillycuddys Reeks, the ice cap carved the Earth into now-familiar formations: north-moving tongues of ice split from the ice cap and met the Reeks – the ice split further, with some deepening Ballaghbeama Pass, down the Caragh River valley, around Caragh Lake and on towards Dingle Bay (with Cromane Point being part of the terminal moraine from this ice flow). More ice moved east past Mangerton. It also forced its way through what is now Moll’s Gap and the Gap of Dunloe. These are called glacial breaches. The ice moved through Owenreagh and Gearhameen in the lowlands, but it also flowed over higher ground as it thickened. These effects can be seen in the smoothness and width of the valley. Glacially scoured lakes such as Upper Lake are evident, as are smoothed outcrops called roch moutonnées, such as Eagle’s Nest overlooking Glaisín na Marbh.

The ice flowed north of Torc and spread in all directions, over the area containing Lough Leane, Killarney town and River Laune. It met here the glacier pushing through the Gap of Dunloe. Debris carried by the ice was deposited in moraines, the Old Red Sandstone erratic deposited on the limestone bedrock of the Park can be used to determine the direction the ice flowed in (Quirke, 2001). Different types of moraine were created as the ice began to retreat. Terminal moraines were left in corries among the Reeks as the last re-advancing of the ice subsided. As the ice melted, precipitation increased and the limestone dissolved forming cliffs and caves along the Lower and Upper Lakes, and leaving limestone islands. The cracks in the limestone pavement formed. Soil became waterlogged and peat began to build up, and blanket bog accumulated.

Part Two: A Biological History of the Killarney Valley, From the Last Period of Glaciation Onwards

As the warming trend took hold at the end of the last period of glaciation, grasslands appeared (Larner, 1992) and those trees that had survived the ice age began to flourish. These included birch, willow and hazel, and they formed the first new forests (Quirke, 2001; Mitchell et al., 2007), following the juniper that colonised the area immediately after the retreat of the ice (Larner, 1992). Pine spread widely around 9 000 years ago (Mitchell et al., 2007), and oak grew in Killarney 8 000 years ago.

Elm also moved in alongside oak on nutrient-rich soils, leaving the poorer soils for the pines, until a continual cover of trees between 8 500 years ago to 7 000 years ago. Following this period, the climate became conducive to alder colonisation, and with these wetter conditions fen-woods appeared. At around 5 000 years ago, elms declined, possibly due to a disease event (Larner, 1992).

The red deer for which the Killarney National Park is notable can be traced back to 26 000 years ago in Ireland (Ryan, 1998). The earliest evidence for the red deer in Kerry is 4 000 years ago on Ventry Beach. It was the only deer species that co-existed with humans in Ireland until the Normans later introduced the fallow deer, according to Ryan (1998). The other deer species once native (the giant Irish deer, the European elk and the reindeer) had apparently been wiped out before humans arrived. Roe deer were briefly introduced in the 1800s, only to be killed off a few decades later, and sika deer were introduced to Killarney in 1865. The Great Famine served to reduce the herd of red deer significantly (Viney, 2003).

Humans may have been living in Killarney before the last period of glaciation (Quirke, 2001), but it was not until 7 000 years ago that evidence for Mesolithic people living in Ireland exists. Evidence for human habitation in Killarney during the Bronze Age 4 000 years ago has been found at Ross Island, where copper mining took place, and a stone circle can be seen at Lissivigeen. The woodland within the park were cleared several times from the Iron Age onwards, as agriculture increased (Mitchell et al., 2007).

After 4 000 BC, there is evidence that woodland is disturbed in the Killarney National Park area (Quirke, 2001). It is around this time that the Arbutus appears in the pollen data (Viney, 2003), although this may have been from seed migration via birds. Farming increased from 3 000 BC onwards, as shown by pollen data from this period, displaying tree clearance increasing. Some hill bogs still show enclosures from this period, which would have been used to protect livestock from wolves, bears, foxes and other people. Around 2 500 BC, tribal territories were formed and there are mountain cairns in Killarney from this era.

As food supply became steadier, the local population increased, and around 2 000 BC, Killarney saw the beginning of metal working (O’Brien, 2000). This metal working continued throughout the Bronze Age. In the Late Bronze Age, tribal conflicts became common, possibly due to pressure on food sources, when soil fertility began to be problematic. Many hill forts date from this period (Clinton, 2001).

Conflict continued after the Iron Age brought new technologies related to war (Quirke, 2001). There was a resurgence in agriculture around 500 BC, when ploughing became popular (Mitchell et al., 2007). Elm and ash, previously having suffered locally, recovered when arable land declined in the area. Around 400 BC, Roman influence lead to widespread clearing of woodland for intensive farming, despite the fact that the Romans themselves did not invade the country.

According to Mitchell et al. (2007), soils that would have been well-drained with woodland now became increasingly degraded to peaty podzols, a process that had been going on wherever trees were cleared. This now became more widespread, particularly in the uplands. Ploughing practices acted to leach more nutrients from the soil, as sods were continually broken and re-broken. This may have led to the expansion of heathers. Increases in annual rainfall at this time increased the leaching process. Cattle, pigs and sheep were raised for meat, with cattle grazing on grassland that had by now been developed over hundreds of years. Soil degradation and the clearance of woodland led to a decline in plant biodiversity which is borne out by the pollen record. These areas of farmland were never again significantly colonised by woodland.

The pines disappeared around this time (Viney, 2003), and oaks became the prevalent species. A pocket of yew exists, which probably became established around 5 000 years ago (Mitchell, 1990), and similarly shows signs of having been cleared and occupied. The oak woods remained in large part untouched until the 1500s, being exploited for firewood and construction in a more or less manageable way (Larner, 1992). However, during the Elizabethan times, much of this woodland was destroyed to facilitate the passage of English troops through the country. But more so than military destruction, the onslaught of industrialisation denuded Killarney of much of the oak woodland, mainly for timber, charcoal production for iron smelting, barrel-making and boat-building.

The introduction of Rhododendron ponticum in the late 1700s (Viney, 2003), or early 1800s depending on the source, decimated the natural flora of Killarney. By 1969, half the natural woodland had been colonised by the shrub, which is not eaten by anything in the Park, and outshades and outcompetes other plants for light and nutrients. Due to the steady rain that characterises the local climate, most gaps in the woodland become waterlogged, filled with tussocky grass, and cannot nurture acorns. The deer (sika and red) graze continually on saplings, which does little for the regeneration process.

As a result of this denudation, Viney (2003) recognises that the birdlife in Killarney is missing species that are present in Britain, despite being annual passage migrants in Ireland. The loss of habitat may account for the lack of biodiversity, such as the four tits native to Ireland, compared to the seven of Britain, although this could also be due to the differences in climate. For instance, the increased rain (an effect of the retreat of the ice age), may have reduced ground invertebrates upon which small birds such as the chaffinch, wren and goldcrest feed. Heavy grazing by sika deer on brambles and ivy also serves to remove part of their habitat.

However, Killarney is notable for being home to several rare species of butterfly, such as the purple hairstreak, as well as dragonflies, including the downy emerald and the northern emerald. Another invertebrate of note in Killarney is the Kerry slug (Carruthers, 1998). It is conjectured (Viney, 2003) that it was an open-country species that became adapted to tree-cover following the postglacial warming.

The red squirrel thrived in postglacial Scots pine woodland. When this declined, the squirrels adapted to stands of hazel and oak. In medieval times, an export levy was placed on their skins, which suggests it was in abundance (Viney, 2003). However, it was considered extinct in Ireland by the end of the 1700s. It was reintroduced from England in at least ten sites between 1815 and 1876 by early ecologists. However, in 1911, the grey squirrel was introduced. Grey squirrels are better able to digest unripe acorns than the red, which explains their ability to compete so successfully. The red squirrels are more at home in Scots pine, and can feed on the kernels of their cones throughout winter, so the Park is a stronghold for them, with its stands of naturalised conifers.

In conclusion, the trend following the end of glaciation has been towards a warmer, wetter climate, with human pressure on the landscape leading to deforestation and degradation of soil. The denudation of the landscape has been at times amended, such as with the replanting of Tomies and Derricunnihy woods in the 1800s (Quirke, 2001), but more often exacerbated, as with the introduction of R. ponticum. How much of the effect on the biodiversity is due to recolonisation by plants and animals following glaciation, and how much is due to human intervention is debatable; no doubt the special climate of Ireland has a large effect, as does its being an island.

The greatest threat to the biodiversity in Killarney National Park is the spread of Rhododendron ponticum. It remains to be seen whether humans can undo the error they made in encouraging its growth here; however, it looks as though reversing the exponential spread of this shrub will take efforts beyond the economic scope of the Park as it presently stands.

If you are interested in helping to eradicate Rhododendron ponticum from the Killarney National Park, please contact Groundwork at info@groundwork.ie, or visit their website.

References

Bolton, J., (2008). Antiquities of the Ring of Kerry, Bray, Wordwell.

Carruthers, T., (1998). Kerry: A Natural History, Cork, Collins Press.

Clinton, M., (2001). The Souterrains of Ireland, Wicklow, Wordwell.

Feehan, J., O’Donovan, G., (1996). The Bogs of Ireland, Dublin, UCD Environmental Institute.

Larner, J., (1992). The Oakwoods of Killarney, Dublin, The Stationery Office.

Larner, J., (2004). The Ross Island Mining Trail, NPWS.

Mitchell, F., (1990). The history and vegetation dynamics of a yew wood (Taxus baccata L.) in S.W. Ireland. New Phytologist, 115: 573-577.

Mitchell, F., Ryan, M., (2007). Reading the Irish Landscape, Dublin, TownHouse.

O’Brien, W., (2000). Ross Island and the Mining Heritage of Killarney, Galway, National University of Ireland, Galway.

Ryan, S., (1998). The Wild Red Deer of Killarney, Kerry, Mount Eagle Publications.

Quirke, B., (2001). Killarney National Park, A Place to Treasure, Cork, Collins Press.

Viney, M., (2003). Ireland, Belfast, Blackstaff Press.