Members Reports and Photos
SATURDAY 22 March 2014
LEADER: professor Peter coxon
IN THE GRIP OF AN ICE AGE
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.
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.
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.
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.
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
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.
our travels over the military road we observed secondary evidence of bog bursts
7’. View down the Upper Liffey Valley to Coronation Plantation.
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.
10. The confluence of the Glendasan and Glendalough glaciers (glacial trough) combined to form a
medial moraine where Glendalough Monastery was latterly sited.
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.
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].
It dates from the Younger Dryas Period [Dryas octopetala is an Alpine plant now strangely flourishing in the Burren, Co Clare].
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.
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.
then proceeded across the Poulaphouca Dam and past the large Blessington
Sandpits and back to Bray Station along the M50.
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
© P Lenihan
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