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5. What did Ireland look like during the last Ice Age?
During the last glaciation ice covered the entire country except perhaps for a small area in west Limerick and north Kerry. This ice was up to 1 km thick in places and was continually flowing. The erosive power of this ice sheet on the underlying landscape was phenomenal. Only our highest mountains peaks stuck up above the ice as nunataks (see photo below). Everything under this level was scoured, planed, smoothed or bulldozed, and in most cases was then covered by debris that left when the ice retreated.
Nunatak protruding above Vatnajokull
Ice Sheet, Iceland
The Irish ice sheet was not one great mass of ice but contained a number of independent domes and small ice caps that coalesced to form one overall ice sheet. These domes had ice that radiated out from their centres and moulded the countryside under them. As the ice domes started to melt they retreated in on themselves and huge amount of glacial meltwater was released. The centres themselves moulded the landscape in such a way that we can reconstruct their geometry by looking at certain ice flow indicators in the landscape.
Ice flow indicator 1: Striae
Striae are scratches into bedrock which are formed by small pieces of rock or soil protruding from the base of the ice scratching the rock underneath them and leaving a groove oriented in the direction of ice flow (pictured right is striated rock outcrop in Iceland). They are usually found on smooth outcrop surface where the rock has been polished by the abrasive ice. They are remarkably consistent features and many striae may be seen side by side on one small outcrop. They are also found on bedrock under subsoil. If you were to dig a hole deep enough to hit bedrock both the striae and the rocks that dug these grooves can be found. These glacial 'tools' were deposited as soil when the ice melted all those thousands of year ago!!Ice flow indicator 2: Roche moutonnees.
Slieve Snaught in the Derryveagh Mountains,
Donegal. The knob of rock on the right flank of the
mountain is a striking roche moutonnee. Ice flow was
from left to right.
Roche moutonnees are asymmetric bedrock bumps or hills with polished 'up-ice' faces and jagged 'down-ice' faces. The polished face is caused by abrasion on a bedrock obstruction at the base of the ice and the jagged face is a result of plucking. They occur at a variety of scales (from less than 1 metre to several hundred metres across). Many bedrock outcrops in Ireland look polished and on close inspection it is seen that they have jagged faces too. Therefore, with some detective work we can infer ice flow direction by simply examining bedrock outcrop at most localities.
Ice flow indicator 3: Drumlins
A drumlin at Aclare in Meath. Flow was right to left,
note the streamlined shape of the hill.
Drumlins are oval shaped hills that are often blunt at the 'up-ice' end and elongated at the 'down-ice' end, with a thinning tail (see photo above). They are thus streamlined with their long axes in line with ice flow direction. This characteristic shape gave the feature its name, which is derived from the Irish droimnin (small, round-backed hill). They occur mostly in clusters, or swarms, and are usually tens of metres wide and a few hundred metres long. Their long axes parallel ice flow direction. Sometimes they have rock cores. In many counties (e.g. Cavan and Monaghan) lakes often occupy poorly drained, interdrumlin areas.
