IGH 7 - Quaternary Theme
The Ice Ages over the past couple of million years have had a profound effect on our landscape. Over most of the country, the geology of the bedrock is either strongly sculpted by frost shattering and ice movements or else partially obscured by a blanket of glacial deposits. This geological heritage displays great variety and a larger number of sites will be necessary to exhibit it than for any other theme except perhaps for the Carboniferous bedrock geology which underlies about half of the land surface.
Ice accumulating in mountain slopes often developed sufficient mass to flow and carry away the rocks broken up by freeze thaw action on the joints and fractures, forming corries. Lough Doon, by the Conor Pass in the Dingle Peninsula of County Kerry exemplifies most corries with a small lake in the scooped out basin, steep back walls and superb scratches and gouges ground into the rock surfaces by the abrasive load of loose rocks carried in the slow flowing ice.
Where ice flows from corries combined or where massive accumulations flowed off high ground single glaciers could excavate ‘U’ shaped valleys. Extensive and repeated glaciation of upland areas in Ireland left us a legacy of landforms such as pyramidal peaks, aretes, ribbon or paternoster lakes, nunataks and others, especially in areas such as McGillycuddy’s Reeks, Killarney and the Dingle Peninsula in County Kerry. Contrasts in the landscape of glaciated uplands result from the different bedrock – granite mountains in Wicklow and Donegal look quite different to the sandstone mountains of Kerry.
Once ice sheets in lowland areas, or glaciers from high ground lost their power and motion, and remained stationary or began to recede, they could deposit their entrained load of broken and ground up rock and sediment. Till or boulder clay is the unsorted mixture that often resulted and is found widely. Often looking at the rock types within it helps track the movement of ice sheets as the rock fragments can be traced back to their source. It can also have important influence of the drainage of an area.
One of the most common landforms left by ice sheets across Ireland is the drumlin. Most people are familiar with the whaleback topography of these elongated and aligned small hillocks which dominate the landscape in large areas of Cavan, Monaghan and adjoining counties. Clew Bay in Mayo has a drumlin field which has been partly drowned by sea level rise, making small islands out of the individual drumlins.
The meltwater from glaciers often flowed in tunnels within the ice sheet. The deposition of a sand and gravel load from these high energy streams bequeathed our most famous glacial landform – the esker.
These are long, winding steep sided ridges of sand and gravel, which can run for many miles. Across large parts of the Irish Midlands they are relatively common, and were important lines of dry communication and travel through otherwise boggy ground. Although found widely in glaciated terrains, the international name comes from the Irish, ‘eiscir’ meaning a ridge separating two plains or depressed surfaces. The eskers of Ireland are also attractive easily used sources of sand and gravel, and the challenge of finding relatively intact examples for geological NHA status is not easy.