Heritage Sites of Co. Mayo

Blacksod Point Lighthouse, Fallmore, Mullet Peninsula

The lighthouse is unusual in that it consists of a building with a squat central tower square in plan rather than the more traditional circular tall tower.  A number of side rooms and a small yard extend off the tower, all set into a compact square enclosure.   The light topping the tower is slightly offset. Designed by John Swan Sloane (c. 1823–1886), the Superintendent of Works of the Ballast Board who had responsibility for lighthouses in Ireland until 1867, work on site commenced in 1864 and was completed by 1866, at a cost of £2,100, by a local builder Bryan Carey.

All the stonework is in Termon Granite, a pink fine-grained stone that imparts a warm look to this neat structure.  This was sourced at Altmore on the southern edge of Termon Hill from cliffside quarries owned by the local landlord Rev Sir William Palmer (1802–1869), 4th Baronet. He leased the site for the lighthouse to the Board for £1 per annum and charged £100 for his granite. The stone masons cut the stone into flat ashlar blocks, which when fitted together provide a flat surface to all of the buildings and the walling. The only relief from the flat stone ashlar is furnished by the window sills and the decorative utilitarian hooded mouldings above the windows on the tower, both of which extend slightly beyond the surface.

The choice of Termon Granite for this building is not surprising in that it was available locally, but more importantly it is medium-grained in texture with interlocking crystals of quartz, pink feldspars and black micas that became tightly interlocked as they cooled from a molten magma soon after it was injected into the Earth’s crust 404 million years ago.  This granite is one of the Caledonian suite formed at this time, others being the Donegal, Galway, Leinster and Newry granites.  The Blacksod Lighthouse has been subjected to pounding by adverse weather and wetting by salt-rich sea spray, and the stone has not decayed much in just over one hundred and fifty years. Elsewhere in Ireland a number of lighthouses have been built of granite: these include Carlow granite for Skellig Michael and Cornish granite for Fastnet.

Some time after completion of the lighthouse its location, adjacent to Termon or Blacksod Pier (built in the 1830s also of Termon Granite), caused friction between Palmer and lighthouse authorities and considerable correspondence passed between the two parties.  The quarry owner wanted to maintain access to the pier that was linked to their quarry via a small tramway and the route was now compromised.  It was his only means of transporting quarried stone for shipping to market. A new route was agreed and it, together with later realignments, was used by the Mayo Granite Company after it had taken over the quarry in the 1870s; this company was active in promoting its output, exhibiting at the 1887 Manchester and 1888 London Olympia expositions. Amongst their commissions was the supply of setts for the quay at Westport. Extraction was sporadic and quarry ownership varied until the early 1900s. Following a lull, pink granite is actively quarried again in the area.

In 1944 the weather forecast sent from Blacksod Lighthouse was instrumental in determining the timing of the D-Day offensive.  This historic event is marked by a plaque.

(Image courtesy of the National Inventory of Architectural Heritage)

Further reading:

Geology of Co. Mayo

Mayo has a very long and complex geological history, which geologists are still trying to understand. Many large areas are defined as terranes; these are sequences of rocks that were formed in one place and are now alongside other sequences that were originally formed a long way apart. Major faults in the Earth’s crust (perhaps like the San Andreas Fault in California) have brought them together over millions of years.

North Mayo has the oldest rocks in the county, and among the oldest in the country, with ancient metamorphic schists, gneisses and other facies. On the Belmullet Peninsula the Annagh Gneiss is around 1750 million years old. Other metamorphic rocks are younger with most of north Mayo composed of Dalradian metamorphic rocks, similar to those found in Connemara, Donegal and in the Highlands of Scotland.

South Mayo has sedimentary rocks including some spectacular boulder conglomerates of Ordovician age preserved in an enormous fold called the South Mayo Trough. Terrane faults separate it from rocks either side. Silurian rocks are faulted alongside also in three distinct sequences. Croagh Patrick is made of quartzite rock as one sequence, Louisburgh and Old Head have another different sequence, and the southern rocks of Joyces Country are distinct again.

During the Carboniferous the sea lapped against the shores of mountains and hills from the lowlands to the east of the county. Around Lough Mask and in the Clew Bay area there are some sandstones and other rocks that were deposited on a land surface by rivers and then in a coastal plain. Above them limestone bedrock is found. Around Corraun to Newport, and on the north coast at Downpatrick Head are good places to see these rocks.

The youngest solid rocks in the county are around 60Ma (Paleogene) and are volcanic in origin (they are not shown on map). Stretching of the Earth’s crust as the north Atlantic Ocean was opening allowed molten magma to move up through fractures in the rock. It cooled and solidified to form dykes of a dark crystalline rock called gabbro. A few of these have been found in the Mayo hills but the largest by far, almost 400 meters wide, can be seen on the shore on the west side of Killala Bay.

The lakeshores of Lough Mask and Lough Carra have a range of superb karst features caused by slightly acidic lake water dissolving the rock. One very strange feature is the cylindrical tubes, which are dissolved upwards on the bottom of limestone beds and boulders. There are also massive sinks where the lake goes underground to reappear at Cong in Lough Corrib.

The Clew Bay drumlin field shows how powerful ice is in shaping the landscape. Drumlins (named from the Irish) are humps of debris left behind by ice sheets. In Clew bay the sea rose and drowned the low ground making a hundred or so islands.

Geological map of Co. Mayo

Map legend: Geological Map of Limerick (based on GSI 1:1,000,000 map 2003). Pale purple: Precambrian Dalradian rocks; Pale yellow: Precambrian quartzites; Dark blue: Precambrian Gneiss and Schists; Pink: Ordovician; Green: Silurian; Red: Granite; Beige: Devonian sandstones and conglomerates; Blue gray: Lower Carboniferous sandstones; Light blue: Lower Carboniferous limestone; Brown: Upper Carboniferous shales.