Members Reports and Photos
SATURDAY 8 March 2014
LEADER: jack scott
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.
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.
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.
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.
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
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
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
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.
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.
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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