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THE LANDSCAPE — GLACIAL

THE LANDSCAPE OF JOYCE COUNTRY

A
GLACIAL
LANDSCAPE

about Joyce Country & Western Lakes Geopark mission

UPLAND GLACIATION

The Ice Ages had a dramatic influence on the Irish landscape and none more than the last Ice age that only ceased 11,700 years ago. The snow originally started accumulating on higher ground in mountain hollows that got enlarged in a circular fashion over time to become natural amphitheatres or corries (“coire” in Irish meaning ‘cauldron’), such as the Deircs south of Tourmakeady. In these hollows, snow accumulated to the point that, under its own weight, it became ice and started to flow downhill. The glaciers carved out U-shaped valleys out of their corries, such as in the Owenbrin valley south of Tourmakeady, the Lough Nafooey valley, the Maam valley and Glen Inagh, and met other glaciers in the plains to form ice sheets.

about Joyce Country & Western Lakes Geopark mission

As the climate got colder and wetter, the ice centres in Ireland transferred to the midlands as ice domes and almost all of the mountains of modern Ireland, including those in Joyce Country, were covered in ice. From evidence further afield, it is estimated that at its maximum the ice was 800 m thick, but some computer models estimate the ice could have been up to 2000 m thick.

As the ice moved towards the sea, it continued to carve, erode and drag large amounts of sediments across the landscape. The rocks that make up the mountains of our geopark project region are sandstones, conglomerates, schists and quartzite; hard rocks that are difficult to erode and their glacial legacy is the moulded shape seen today.

LOWLAND GLACIATION

In contrast, the limestone is a softer rock and was flattened by the movement of the ice and has been extensively quarried. The contrasting landscape of the uplands of Joyce Country and the lowlands of the Western Lakes and Irish Midlands illustrates the differences in bedrock and resistance to erosion.

The lowlands of our geopark also house numerous examples of drumlins (“droimnin” in Irish meaning ‘small ridge’), an international glacial geomorphological term for small glacial sediment ridges deposited under the ice sheet and shaped by its movement. The longer axis of the ridge indicates the orientation of ice flow. Many of these drumlins are found on either side of the lakes or as islands in the lakes, as seen around the lake shore at Clonbur.

Using the internal composition of the drumlin, which indicates ice flow direction, and striae (i.e. ice marks) found on bedrock outcrops, we can reconstruct the movement of the ice. In our geopark area, the ice flowed to the east and south east of the mountains towards the lakes where they veered northwards towards Clew Bay or Killala Bay. Ice also flowed into Killary Harbour where it is responsible for the over-deepening of the valley there and the creation of the fjord.

When the ice eventually melted, it left behind a number of ridges of sediments called moraines that indicated the former edges of the glacier or ice sheet. These moraines were sometimes called kame terraces and good example of them are visible in the Erriff valley. In the geopark area, these are found principally in the U-shaped valleys as the ice retreated towards the corries.

Some of these moraines acted as levees that kept the glaciers meltwater from flowing into the sea and so created glacial lakes. Due to the mounting pressure of the meltwater, these barriers eventually broke allowing the water to flow towards the sea and deposit some of the sediments it was carrying. The sediments were deposited in structures geologists call deltas, such as the one at the top of the Maam valley above the village of Leenane. However, sometimes the drainage of the lake can happen much faster, creating sharp v-shaped meltwater channels such as the ones either side of the Gabhlán valley.

Meltwater was sometimes flowing inside and underneath the ice sheets, carrying sediments along the way. These channels have left behind low and sharp gravel ridges called eskers, from the Irish “eiscir” meaning ‘a ridge that divides the landscape’. Some examples of eskers lie to the east and south of the geopark region.

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