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In 1934 the London, Midland and Scottish railway company commissioned the celebrated artist Paul Henry to produce a series of posters in an attempt to encourage travellers to use their trains and visit Ireland. One of these posters featured Minard Castle which overlooks the small inlet at Kilmurry Bay (Beal na gCloc) just south of Annascaul. This castle was constructed of local stone in the 16th century, but was later besieged by Cromwell’s army in 1650 and left structurally unsound.
Kilmurry Bay is of great geological interest for two main reasons. Firstly, 380 million year old fossilised sand dunes can be seen in the cliffs. In the Devonian period rivers flowed south across a large desert, and carried sands and coarse sediments. The sand formed crescent-shaped sand dunes in the desert, and these were subsequently consolidated and cemented to form the pale yellow Kilmurry Sandstone. This is a unit of the Old Red Sandstone that forms much of the Slieve Mish mountains and those seen on the Iveragh Peninsula. We know that the rocks were deposited as sand-dunes as large as those in the Sahara Desert today, because they contain alternating layers of sediments in a pattern called ‘herring-bone cross-stratification’. This layering is quite easy to see in the rocks in the cliffs.
Secondly, over many hundreds and possibly thousands of years large blocks of Kilmurry Sandstone have become rounded by the action of the waves and from knocking against each other—they now resemble large eggs. It seems incredible given their size that the force of storm waves could move them, but they have been thrown to the back of the beach where they form a ridge called a storm beach. This is probably the finest example of this feature in Ireland. If you are sharp-eyed you may be lucky to fine a ‘D’ beautifully carved into the top of one of the sandstone ‘eggs’.
Unlike in other rocks on the Dingle Peninsula, the sandstones at Kilmurry Bay don’t contain any fossil shells. However, they have yielded trace fossils—markings on or in the rocks such as burrows and tracks that were made by animals, which unfortunately have themselves proved elusive.
In Minard Bay, the next inlet along the coast to the west, two different rock types of considerably different ages have been brought alongside each other on a fault line. A fault is a crack or line of weakness in the Earth’s crust and movement along it causes earthquakes. This particular fault runs from Minard Bay all the way to Caherconree Mountain 20km to the east. Fortunately this fault is no longer active (it was probably last so many millions of years ago) and in any case earthquakes have not been recorded on the Dingle Peninsula in living memory.
Patrick Wyse Jackson is a frequent visitor to the Dingle Peninsula and teaches geology at Trinity College, Dublin.
