Tom Barragry ©
Between 4 million and 10,000 years ago, Ireland was covered by ice and glaciers . About 50,000 years ago, as the climate began to warm up, the ice melted and by 10,000 years ago it was all gone The resulting glaciers of that era had a huge chiselling effect on the visible landscape .The glaciers were moving , were of high mass and density , and exerted colossal downward pressure,and shearing forces on the adjacent rock faces. They followed straightforward and predictable principles of gravity, physics, mathematics and chemistry. Freezing and thawing, masses of ice moving downhill under gravity ,abrasion of vertical surfaces as they tumbled ,with the resulting erosion and deep excavation of the hollow by shear forces.
These colossal forces are responsible for the dramatic landscape changes we can see , corrie lakes, ribbon lakes, valleys, all caused by erosion.
Corrie lakes are one such spectacular and beautiful result of such erosion and the resulting deposition of debris (moraine) at the bottom and around the edges of the iced -over hollow, gives corrie lakes a very typical appearance.
Many corrie lakes are to be seen in Wicklow and indeed all over Ireland and elsewhere. They are always high altitude mountainous lakes with a steep northerly facing back wall.
Classic corries are about one kilometer long and one kilometre wide.The are always situated high on a mountainside under a steep north east slope where they are protected from the suns energy and the prevailing winds.
Historically,during the ice age, these high mountain hollows, northfacing and sheltered from heat, encouraged the mass accummulation of snow and ice .Snow accummulated in these north facing hollows ,and eventually turned into large, dense ice packs , exerting enormous lateral and downward presure on the hollow that they filled.
As the ice moved down hill ,it scraped the back wall making it steeper and pulling debris with it to the base of the hollow. Further freezing and thawing cycles ,and the powerful downward gravitational shifting, removed more material from the back of the hollow createing a steep back wall (think of Tonlagee cliff face and Lough Oular) . Rotation and movement of the ice pack dragged rocky debris (morraine) along the base of the glacier deepening the floor of the basin by abrasion , and forming a deep rock basin . Some of the moraine was shifted out to the periphery of the hollow, where it formed a dam- like structure of rocks, or a rock lip enclosing the iced up hollow.The terminal lip was often heightened by the deposition of moraine. Moraines are formed from the debris pulled down the steep rock face by the glacier , and normally consist of rounded particles varying in size from large glacial boulders, to small stones and rocks. This deposition of scraped -off wall debris , is deposited in heaps at the edges of the hollow. Glaciers are a moving phenomena and over thousands of years during the ice age, these massive slabs exerted huge abrasive pressure on rocks, as they froze,melted, collapsed , moved downwards under huge forces of gravity ,and scraped down and abraded adjacent surfaces. They acted much like conveyor belts carrying debris from the top of the high walled hollow glacier to the bottom ,where it deposited the debris in terminal moraines ,acting as a boundary, lip face or dam like structure.
When the ice melted ,corrie lakes appeared with a very steep north facing back wall ,a characteristic curved shape, and a border of boulders ,giving the typical “string of pearls” appearance to the border of the corrie lake.
Corries and such geological hollows are variously decribed as cwms (Welsh for valley ), tarns ( Norse/British) or cirques.(French). A cirque (French, from the Latin word circus) is an amphitheatre-like valley formed by glacial erosion. Indeed most corrie lakes have an amphitheatre-like appearance about them, open at one end (the terminal moraine lip) and the three walls of the original mountain hollow surrounding them..one being especially steep.
Because the massive glaciers exerted potent excavating and abrasive effects on the hollow of the basin,by moving and melting ,collapsing and refreezing, corrie lakes are usually quite deep .
Frequently seen corrie lakes around Wicklow include Lough Nahangan,(under Camaderry/Turlough Hill), Lough Oular ,(under Tonlagee), Lough Bray (under the Eagles Crag) ,Art’s Lough under Clohernaugh,and Cleevaun Lough (under the northerly face of Mullaghclevaun massiff.). There are of course many more around Ireland. The steep high back wall overlooking the lake always faces north. ..the coldest side where the ice had formed.
Lough Oular under Tonalgee
Cleevaun Lough under Mullaghcleevaun
Arts Lough under Clohernaugh.
Peripheral “String of Pearls” (moraine)