Thursday, 30 April 2015

A Field Trip at White Rock

White Rock at Killiney

I had been surprised at the mild weather throughout autumn in Ireland, compared to the UK, and I wanted to make the most of my last month and see some more places around Dublin. Now well into November, I made use of the DART again to visit White Rock, at Killiney Beach.

Mica schist and aplite veins
Situated right next to the Killiney station, this coastline has traditionally been very popular with bathers but, for the geologist, it also contains a wide variety of interesting rocks and features that makes it a well used field trip location – for undergraduates at Trinity College and University College, which both have a good reputation.

Although I only discovered this later, an excursion organised as part of the Education and Outreach services at the Geological Survey of Ireland, explores 500 million years of Earth’s history – from the Cambrian to the Quaternary periods – that can be found along this short stretch of coastline. Aimed at the general public, guided walks along this beach provide a good introduction to the rocks of Ireland.

My job, to survey this County Geological Site, required me only to look at the contact between the Leinster Granite, which forms the Wicklow Mountains, and the Ordovician rocks into which it was intruded in Devonian times.

Geology students from University College Dublin

Never knowing quite what to expect when seeing some new geology, the jagged outline of what I presume to be White Rock gleamed in the afternoon sunshine and forms a spectacular local landmark. Much to my surprise, I wasn’t the only person there and I had arrived at the same time as students from University College Dublin, who were looking closely at the rocky cliffs.

Mica schists and granite at Killiney Beach

It is an excellent example of contact metamorphism, with mica schists displaying several growths of the mineral andalusite along the plane of cleavage and larger joints - and other structural features that show the metamorphic fabric of the Ordovician rocks.

Mica schist with andalusite

Diverging upwards, away from the main body of the granite - comprising quartz, white feldspar and white mica - thick veins of aplite and pegmatite pervade the schist. Here, the different rates of the cooling of the magma, and how this affects the size of the crystals and textures, can easily be demonstrated. Although never being as well developed as in such places as Glendalough, lead and copper were once mined here and a disused and inaccessible old adit can be seen in the cliff.

A view from White Rock towards Killiney Beach

Wednesday, 29 April 2015

Granite in Co. Dublin

An aerial view of Dalkey Quarry

With the Dublin Stone Show only a few weeks away, I had plenty to keep me busy in the basement at the GSI, writing up my site reports from my tour of southern Ireland and incorporating a selection of my photographs into one of the display panels.

Dalkey castle
Knowing now that I had to prepare to come back to Rotherham, I now started to put in some overtime work and, at the weekends, I made good use of the DART railway to explore some of the County Geological Sites that can be found around Dublin.

At Dalkey Quarry, where the grey Devonian granite was used to build the harbour at Dún Laoghaire and - and for the embankments along the River Thames in London - there is a vast excavation in the Wicklow granite. Nowadays, it is quite overgrown, thickly wooded in place and it now forms part of Killiney Hill Park. It is a popular site for rock climbers and there are several well marked and well used paths, solid stairs and excellent viewing points, where Dublin and its southern suburbs can be appreciated.

Returning to Dalkey, I had a quick look at its castle before getting back on the DART and then stopping off again at Blackrock, where its granite breccia provides yet another example of some very interesting geology in Ireland. Here, embankments and sea defences have encroached upon the underlying geology and small outcrops that have not been covered are all that remain.

A general view of Dalkey Quarry

Whilst walking around, I didn’t stop to look closely at the petrological details of these rocks, but it is a good location for undertaking research and, as one of several easily accessible sites in this area, it is a good stop off point on a geology field trip.

A coastal exposure of granite breccia at Blackrock

Saturday, 25 April 2015

Granite in Co. Wicklow

The old quarry at Ballyknockan

On the last day of my short tour of southern Ireland, to survey some building stone quarries that were listed as County Geological Sites, my plan was to visit Ballyknockan and Ballybrew which, historically, were very important producers of granite.

A boundary wall at Ballyknockan Quarry
Staying overnight at Blessington, having driven from Kilkenny the previous day, I was situated on the edge of the Wicklow National Park, where the hard granite was  sculpted into spectacular landscapes by glaciers in the Quaternary Period.

Ballyknockan overlooks the Blessington Lakes, where the River Liffey has been dammed to provide a hydroelectric power station and a reservoir, and there are some good views of the foothills of the Wicklow Mountains.

Like in the UK, the production of granite in Ireland has declined considerably in recent years and, although there were some operational workshops at the Ballyknockan quarry, much of this sprawling site had long since been disused. 

Away from the current working faces, small exposures of the granite can be observed in a relatively safe environment and here, the old traditional buildings and plant and machinery also provide interest for the industrial archaeologist.

General views of Ballyknockan Quarry

Having had a good walk around the site, checking the boundaries and making a note of the points of interest, I headed off into the heart of the Wicklow Mountains. I wanted to stop and look around Glendalough and enjoy a drive through this landscape, but I was thwarted by the heaviest downpour of rain that I had encountered during my stay in Ireland.

General views of Ballybrew Quarry

At Ballybrew Quarry, which has now closed, the old quarry faces were not easily or safely accessible, with one part being filled with water, and fenced off, and the other being littered with large piles of off cuts and waste material, which would make visits by the general public quite hazardous. With my work for the week finished, I finally arrived back in Dublin...

Friday, 24 April 2015

Irish Blue Limestone

A Geological Map of Ireland

Looking at a geological map of Ireland, the Carboniferous limestone – coloured pale blue – is the most common rock and, in places like The Burren, forms spectacular landscapes.

Although much of this rock is buried under glacial till, it has also contributed significantly to Ireland’s economy in other ways: mined for its associated lead–zinc deposits and quarried for hard rock aggregates and good quality building stone.

Holdenrath Quarry

In Amsterdam, it has been used for large paving and urban landscaping projects and is considered to be more durable than similar limestones that have traditionally been imported from Belgium.

Several companies produce this Irish Blue Limestone and, on the last leg of my tour of southern Ireland, with only County Geological Sites in Co. Wicklow to survey before returning to Dublin, I arranged to visit a major supplier - to look at their production facilities and take away a few samples of stone that have finishes made by sophisticated machine tools.

A bridge saw

As the resident building stone specialist at the Geological Survey of Ireland, I was very interested to see the differences in production between Ireland and the UK, where this limestone has been mainly used either for walling and vernacular architecture or polished – to show off the beautiful corals, crinoids, brachiopods and bivalves

Samples of Irish Blue Limestone

Wednesday, 22 April 2015

Co. Kilkenny - St. Canice's Cathedral

Various Irish marbles at St. Canice's cathedral

The town of Kilkenny has had a long history of producing good quality limestones for general construction work and also the famous Kilkenny Black Marble, which was widely used for monuments. My principal task was to inspect two disused quarries, which had been shortlisted as County Geological Sites.

Various building stones at St. Canice's cathedral

Firstly, for my work for the Dublin Stone Show, I wanted to have a good look around St. Canice’s cathedral, which was founded in the 6th century and displays a wide variety of building and decorative stones.

Dundry limestone
As would be expected, the cathedral is largely built of the local Carboniferous limestone, but dolomitic varieties of this rock can be seen in the round tower and Carboniferous sandstone from Coolcullen has been used to restore the windows and dressings.

For some decorative elements, such as the carved bosses to the entrance doorways, Dundry limestone from England has been used.

Apart from the range of stones seen in the external fabric, there is a magnificent display of Irish marbles used to decorate the interior, including the Kilkenny Black, Cork Red and Connemara varieties – all of which are now largely obsolete.

With a good collection of photos from St. Canice’s cathedral, I made a very brief inspection of the Archersgrove and Black quarries. Long since disused, they had become largely overgrown and, being on private land, offered very limited value for education, as is often the case with old quarries. Finishing my work for the day at Dunmore Cave, which is protected for both its geology and history, I returned to Kilkenny for my second night - to enjoy some good food, a couple of pints of Guinness and some traditional Irish music.

Black Quarry and Archersgrove Quarry