EVENTS

Members Reports and Photos

SATURDAY 8 March  2014            

bray head  

LEADER: jack scott

Geology                                                                               

The subject of the outing was the rock formations of the Head, which together with those of the neighbouring Sugar Loaf Mountains are the oldest in Wicklow. They date from the Cambrian Period, about 550 – 490 million years ago. These rocks, turbidites, originated from sediments in shallow water that were deposited, downslope under gravity, by turbidity currents into deep water in the Lapetus Ocean. The latter was south of the equator and was bordered by several paleocontinents such as Laurentia and Avalonia.  

The continents began to converge in the Ordovician Period, shrinking the Ocean by subducting its crust, before eventually colliding at the end of the Silurian Period, 420 million years ago. This resulted in the disappearance of the Lapetus Ocean leaving a geological fault zone known as the Lapetus Suture. The latter divides Ireland in two and runs from Limerick to Clogher Head. The part of Ireland northwest of the fault is Laurentian in origin, and was originally joined with Scotland, Greenland and most of North America. The southeast of Ireland originated in Avalonia along with Wales, England and Brittany.  

The rocks of the Cambrian Period, which are also found in Howth and South Wexford, consist of greywackes, shales and quartzites. We had the opportunity of examining some pieces of greywacke that we found on a picnic table. It was surmised that they had been broken off, using a geology hammer, by a school party on a field trip, even though the use of a geology hammer in the area is prohibited. 

Greywacke is composed of muddy sand and sedimentary particles that were deposited rapidly. On a greywacke rock face nearby, Jack pointed out a layer of shale and explained that it is a very soft, fine grained rock consisting mainly of clay minerals with small amount of quartz, felspar and mica grains whose colour can vary depending on its chemical composition. Those occurring in the Bray area are usually red or green.  

Beside the path, we also examined areas of quartzite whose light colour was not always apparent. Quartzite originated from the metamorphosis of sandstones deposited in the Lapetus Ocean. It is very hard and resistant to erosion and forms the pale coloured ridges and summits of Bray Head and the Sugar Loaf Mountains. Therefore, contrary to popular opinion, the pointed peak of the Great Sugar Loaf has not resulted from volcanic activity. Among a number of quartzite ridges visible from the Cliff Walk is one that lies above the railway’s “Brandy Hole Tunnel”; the name is derived from the fact that smugglers used the cave in this locality to store contraband.

Other interesting geologic features examined on our walk included slump zones and ripple marks. The former result from movements on faults that cause layers of sediments, before they are lithified, to slump and mix with undeformed sediments. Ripple marks are due to agitation by water and can be used to indicate the original position of strata and the depositional environment of sediment.  

In contrast to the glacial climate of the Precambrian Period, the climate in the early Cambrian became more conducive to the development of marine organisms because of a rise in temperature, increased oxygen levels, and the formation of shelf seas.

The rocks of Bray Head were once thought, erroneously, to contain evidence for the “Origin of Life” with the discovery of what was thought to be the oldest ever fossil. The latter, Oldhamia radiata, was named after Thomas Oldham, the Director of the Geological Survey, who discovered it in 1840.  Although burrows and grazing trails made in the soft sediment, including Oldhamia are preserved as trace fossils in the rocks, we did not encounter any of these during the course of our outing  

Further along the Cliff Walk on a windswept site with panoramic views over the sea, we passed the ruins of Lords Meath’s Lodge and Toll Gate. A charge of a penny was levied for entry to the path south of the gate which was part of the Earl of Meath’s estate. These buildings are also of geological interest because, except for the granite quoins and some redbrick, they were constructed from stones collected from the immediate locality.  

Considering the geology of the area, the decision in the mid-1800s to build the railway, linking Bray to Greystones, around Bray Head presented major engineering challenges. This route was chosen because Lord Meath, the local landlord, would not allow the track to go via the Glen of the Downs, an easier route, as it would have divided his estate. As a result, the Cliff Walk was developed in the mid-1800s to provide access for the construction of the railway. The single line and tunnels were built under the direction of Isambard Kingdom Brunel, and its financing and development was led by William Dargan. It remains the most costly stretch of railway track ever constructed in Ireland and even nowadays the railway area regularly needs expensive stabilization works because of erosion and rock falls.

Even though this was primarily a geological outing, the botanists among us were not disappointed because many plants were beginning to bud, while others had already just come into flower. Beside the path we noted Smyrnium olusatrum (Alexanders), Arum maculatum (Lords-and-Ladies), Cardamine hirsuta (Hairy Bittercress), and Ficaria verna (Lesser Celandine). The rocks had profuse growths of Silene uniflora (Sea Campion), and Umbilicus rupestris (Navelwort), which relishes damp environments. The stonewalls were spotted with ferns such as Polypodium vulgare (Common Polypody) with its bright orange spores, and Asplenium adiantum-nigrum (Black Spleenwort) characterized by its black stipes and shiny green leaves. 

The day ended in time for the rugby aficionados among us to catch the beginning of the Ireland versus Italy match. Metaphorically speaking, now that Jack had made a formidable debut as captain of such an informative and enjoyable outing, the sight of Beann Éadair in the distance prompted us to hope that he might be enticed to lead another fixture for us with a Cambrian connection. 

 

 

                        Bray Head Hotel                                                                                  Lord Meath's Lodge

   

Greywacke Piece                                                                     Greywacke                        Cross Quartzite

   

A.                                                                             B.                                                                    C.

A. Shale layer above greywacke B. Quartzite Outcrop  C. Harder Sand (bottom) Ripples (centre) & Shale( top)

        

Slump Zone                                          Ripple Marks

     

                     Tunnel Brandy Hole                Railway track and tunnel     Tunnel with protective cladding

     

Umbilicus rupestris                 Cardamine hirsuta                                                Ficaria verna               

   

              Polypodium vulgare              Silene uniflora              Asplenium adiantum-nigrum    

  

Photos and Report ©  P Lenihan 

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