The Geology of Ireland

Reviewed by Tom Kennedy on 6 Jul 2009

Basalt in the north, granite in the Wicklow Mountains, and a big expanse of limestone stretching across the country before rising up to face the Atlantic in Clare. In such broad strokes we usually picture the geology of Ireland. However, there was not just one intrusion of granite, but several, and the gap between sedimentary deposits can represent the passing of a million or more years.

We often think of limestone as just one great mass of grey rock, but looked at through the eyes of a geologist, it can be classed into several systems, and sub-systems, each representing a particular episode in all probability each being peculiar to just one area. Considering the vast time scales involved, it should not surprise us that interpretation of the geological record is so complex. Perhaps we might understand just how complex it is by considering how hard it would be now to detect the traces of the 1940s bombing of Fairview and distingish them from the silt deposited from the Tolka flood of the late 1950s.

That's just a decade or two, but geologists are looking at what happened over a period of 400 million or so years. True, there were long periods when nothing much happened, and often these are the gaps of sameness that help to separate the periods of change, but even then, the world has such a violent past, that what was once up can end up down and half of Ireland is no longer south of the equator.

For more than 200 years geologists have been probing around Ireland, bit by bit, adding to the record of what is where and how it came to be there. The Geology of Ireland, edited by Charles Hepworth Holland and Ian Saunders brings that record right up to date, and the accumulation of knowledge it contains makes it indeed a heavy tome. No doubt professional geologists will value this book highly, it is very comprehensive, but the carefully worded text would be extremely difficult for a reader with a passing interest in geology to follow.

It is a book by experts for other experts, and this is undoubtedly its strength. More than a dozen of those who know everything there is to be known about the structure of Ireland have contributed chapters to this book, so it is not short of the sort of details and references that you would normally only find dispersed among a number of specialised journals. Pity the poor editor that had to make up and check the 60 closely spaced pages of references!

This is the second edition of of a book which was already regarded as a definitive text on Ireland's geology, and it has been considably enlarged and updated. In the ten years since the first edition, a lot has happened, including the Tellus survey in Northern Ireland, advances in palaeomagnetism, and of course, geologists are no longer confined to dry land. One of the chapters, by D Naylor and P M Shannon brings us right out to into the Atlantic and down around the Celtic Sea.

We also look back into the more recent past, with a chapter on the history of Irish geology by G L Herries Davies. Like now, people in the 19th century had different reasons for their interest in geology, and while the vulcanists and neptunians were fighting a holy war over the fossils, the learned members of the Dublin Society were taking a more pragmatic view. The industrial revolution in England, they argued quite rightly, was based on the exploitation of mineral wealth, and for that reason the Society started a museum to promote geology in Ireland.

That argument remains just as valid today, we literally pull wealth out of the ground, but I am not too sure that G L Herries Davies is altogether right in stating that geology is flourishing in Ireland's academic institutions. Just a few months ago, October last year (see Science Spin 31), I remember reporting that not enough geo graduates are coming out of Irish colleges to meet future needs, and that although geology is worth about €4 billion a year to the Irish economy, only five per cent of our research funding is going into this area.

Charting Ireland's past

Our lives, in geological terms, are so short, that we get the impression that everything, rocks and climate, always remain the same. Many of the features described in The Geology of Ireland, were the result of forces we can hardly begin to imagine.

How different Portrane in Dublin must have been when volcanic ash was raining down from a cone rising up out of the sea at Lambay. That's what the rocks there tell us actually happened some time in the early Ordovician Age about 400 million years ago. Lambay began as a volcano formed under the sea, and during this time of upheaval, lava also flowed over Balbriggan.

By themselves, volcanic rocks show us that there was an eruption, but to get the full story, geologists must go out over the ground, chip, drill, and map all the details. The fact that the submarine volcanic rocks have been observed to be overlain by thin beds of limestone with trilobite and other animal fossils from the Late Ordovician, indicates that, as the volcano became dormant, a fringing reef formed around it. Not that everything remained quiet and calm.

Other signs have been noted showing that tremors, possibly volcanically triggered, set off slides of broken rock fragments, known as breccias, and there are intrusions, where molten materials broke through the surface. Eventually, some of those intrusions, the porphyritic andesites, ended up as Neolithic stone axe heads.

Over at Portrane, as the volcanics died down, limestone containing fragments of pumice, brachiopods and other fossils, including the local trilobite, Tretaspis portrainensis, was laid down in shallow waters.

Those details mean a lot to geologists who are in tune with the terminology, and to my mind, a very useful exercise would be to go one step further in presenting a series of local summaries for the public. As the book shows, a lot of the details we would all like to know about are already there, but they are not really accessible, except to the specialist.

The book is a solid resource, more to be mined than read, and although big at 568 pages in full colour, it is quite expensive. €99 for the paperback and for some reason the hardback is almost double the price, €160.

576 pages, €90 paperback, €160 hardback.