EVENTS

Members Reports and Photos

SATURDAY 22 March  2014            

LEADER: professor Peter coxon

WICKLOW IN THE GRIP OF AN ICE AGE

Peter Coxon gave us an introduction to the solid geology of the south Dublin and County Wicklow area before we departed by bus from Bray Railway Station. The oldest rocks in the district belong to the Cambrian era (550-499 million years in age) belong to the ‘Bray Series’. Much of the area had a very large Caledonian batholithic implacement. Contact with this implacement resulted in the metamorphosis of adjacent rocks through heating as the large granite mass cooled. Subsequently the Palaeozoic sequences were uplifted and at a later stage eroded.  The Wicklow granite which now reaches the surface is much of Wicklow is approximately 400 million years old.

Pete introduced us to climate change as it has affected the planet and more locally Ireland through the process of successive glaciation. Evidence from deep sea drilling has shown that the Earth’s climate commenced to cool 25 million years ago and polar ice caps started to expand rapidly, for reasons that are not entirely clear. This cooling that began in the Tertiary Era culminated in a series of rapid step-like and sudden changes in climate conditions in the last 2.6 million years as the Earth’s climate began to oscillate from cool to warm and the polar ice caps began to expand and contract. This latter period has been designated the Quaternary Era. We are now considered to be in the Holocene Epoch which commenced 11,700 years ago.

Evidence that changes in earth’s climate were governed by astronomical cycles was first computed by Milankovitch, a Serbian mathematician, in the 1930s and 1940s. Three cycles relate to the precession of the equinoxes (21,000 year period), axial tilt (42,000 years) and eccentricity of orbit around the sun (96,000 years). But it wasn’t until the 1970s that ocean bed cores taken allowed the ratio of oxygen isotopes present in sediments to be determined. This analysis confirmed beyond reasonable doubt that Milankovitch’s theory and calculations were correct. Sunspot activities which have an 11 year cycle also have an influence on our weather/climate. Carbon dating has been a very powerful technique for the determination of the age of carbon containing organic materials.

Stop 4 at Glencree gave us an excellent view of a parabolic (U-) shaped glacial valley with a clear view of the steeper north side and a more gently south facing eroded slope shaped by many cycles of freezing and thawing.

Stop 5 at Upper Lough Bray Viewpoint gave us an excellent view of a glacially eroded corrie lake. The build-up of ice during cold stages lead to glaciers which were large enough to erode the underlying rock as it eventually deforms under its own weight.

Stop 6. The Liffey Head Bog. All were impressed by the depth of the blanket bog as demonstrated by 8 metres cores which was taken, and equally by the evidence of wood and charcoal at the bottom of the core. This is deemed to be evidence that the area was ‘originally’ forested and subsequently  deforested by Neolithic man who adopted a scorched earth policy. The resultant of the deposition of charcoal caused the soil to become impermeable and waterlogged. As a result peat accumulation  commenced.

On our travels over the military road we observed secondary evidence of bog bursts and landslides.

‘Stop 7’. View down the Upper Liffey Valley to Coronation Plantation.

Stop 8. Valley of Lough Tay. Very contrary weather limited our outdoor exposition and expeditions. But were able to get a view of the change from granite to schist and traces of the metamorphic aureole which formed as heat and pressure from the rising granite altered the surrounding country rock.

Stop 10. The confluence of the Glendasan and Glendalough glaciers (glacial trough) combined to form a medial moraine where Glendalough Monastery was latterly sited.

Stop 11. We viewed some of the spoil associated with the mining of minerals such as lead and gold at the granite-schist boundary. This facilitated the collection of trophy specimens with visible mineral seams.

Stop 12. Lough Nahangan below Turlough Hill. The corrie whose level was lowered as a result of the hydroelectric scheme displayed evidence of a glacial moraine. The corrie is similar in origin to Lough Bray and radiocarbon dating has confirmed the existence of a glacier here from 12,400 to 11,600 years ago. It dates from the Younger Dryas Period [Dryas octopetala is an Alpine plant now strangely flourishing in the Burren, Co Clare].

Stop 13. King’s River. This was part of the very large Glacial Lake of Blessington which was partially reflooded in the 20th Century to form the Poulaphouca reservoirs. The terraces with granitic gravel deposited from glacial melt waters are clearly visible alongside the current much reduced river.

Stop 14 Hollywood Glen. An impressive walk through Hollywood Glen which has its origin as a sub glacial water channel when the Glacial Lake Blessington melted.

We then proceeded across the Poulaphouca Dam and past the large Blessington Sandpits and back to Bray Station along the M50.

This was a fascinating and highly educational tour of Wicklow. Essential reading is Wicklow in the grip of an ice age - a very readable and well-illustrated booklet - by Peter Coxon, Fraser Mitchell and Patrick Wyse Jackson obtainable from the Irish Quaternary Association (http://www.iqua.ie/ ). See also the website:  http://www.countywicklowheritage.org/page_id__73_path__.aspx. We are most grateful to Professor Coxon for a very illuminating tour and recommend that any member, regardless of knowledge of glaciation, seize any opportunity to participate on a future tour of County Wicklow.  

 

 

Glencree u-shaped Valley  

Upper Lough Bray Glacially Eroded Corrie

     

Lough Tay - Granite to Schist                           Schist

 

Wind-swept Participants and Bus at Liffey Head Bog

  

Glendasan site of glacial trough

    

Mining Spoil Heaps at Glendasan

Lough Nahangan Corrie (from Younger Dryas period)

King's River: Terraces ex Blessington Glacial Lake

Holly Wood Glen Sub-glacial Melt Channel

Photos  ©  P Lenihan 

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