EVENTS

Members Reports and Photos

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SATURDAY 25th May 2013 

GEOLOGY, SOILS and FLORA of HOWTH

Leaders: Peter Coxon and Declan Doogue

We commenced the field trip by visiting Bottle Quay near where the bedrock geology of Howth changes from carboniferous to quartzite (Bray Series of Cambrian origin). The fault line demarking this change extends across the “island” to Balscadden. Peter gave an overview of the geology of Howth in the context of Dublin Bay, the Wicklow and Dublin Mountains and paid particular attention to last glacial period (Ice Age). He pointed out to us the striations on the rocks next the cliffs which are evidence of the movement of ice and this was also visible within the ‘erratics’ in the exposed glacial till on the cliffs above the beach.

We moved on to Balscadden Bay and viewed the local geology and noted the rather precarious military position of the Martello tower built on consolidated delta sand which at one time stretched across to Kilrock. Peter expounded on our glacial history and on the evidence for changes in the earth’s climate due to the Milkankovitchian cyclical alterations to the earth’s attitude and movements relative to the sun. Inevitably the discussion turned to carbon dioxide levels in the earth’s atmosphere which have recently breached the 400 ppm mark, and to sea levels and future climate ….  A little bit of beach combing uncovered some rounded Ailsa Craig granite which had been brought by the ice sheet when moving down the Irish Sea.

At Red Rock quarries we looked at the Variegated Horsetail Equisetum variegatum – a lime loving plant – and pondered the pH of seepage at the bottom of the cliffs which are composed of acid rock. What mysterious path is followed by the emerging water?

This was a superb and very informative field trip.  Copies of Wicklow in the grip of an ice age (Peter Coxon, Fraser Mitchell and Patrick Wyse Jackson), published by the Irish Quaternary Association are obtainable from http://www.iqua.ie/. This publication covers much of Peter’s talk and more.