Donegal

Heritage Sites of Co. Donegal

Church of St John the Baptist (RC), Carrigart

Built between 1868 and 1886, this cruciform gabled Catholic church’s lasting impression, according to Alistair Rowan, is of the “extraordinarily gaily patterned ” squared rubble masonry, which he states is from at least four different quarries. The stone was sourced from Lackagh Bridge, close to Dunmore Head, in Donegal where there were a number of working quarries at the time that this church was erected. This pink/grey, even-grained, foliated stone, described by Kinahan as a granite or gneiss, is more correctly a psammatic schist. Pelitic schists and metadolerites, also situated at this locality, may too have been extracted for built stone purposes.

The external dressings are of native white marble, most likely sourced from Dunlewey in Co. Donegal. On the banks of Dunlewey, about thirty miles west of Derry, there was a vein of white, crystalline, saccaroid (dedolomite) marble surrounded by the Donegal granite. The Donegal granite was responsible for metamorphosing pure limestone into this marble. A bust of Henry Grattan in this stone was exhibited at the Great Exhibition in London in 1851 and although the marble displayed fractures due to improper blasting of the procured block, it demonstrated ‘the fine polish it is susceptible to’ and ‘resembled Parian marble in grain and finish’. Unfortunately, its coarsely crystalline nature ultimately rendered it unsuitable for statuary purposes, but it could be employed for non-elaborately carved ornamental work. It was mainly used for ordinary building. The quarry was only worked along the surface of the bed to a depth of approx. 20 ft.

The devotion to native materials continues inside with two impressively large columns of Donegal white marble flanking the entrance to the chancel. The external dressings of Donegal marble is also mirrored internally around the doorways. The ornately carved altars are of Carrara Marble with colonnettes of polished Cork red limestone.

Further reading: Rowan, Alistair 1979. The Buildings of Ireland: North West Ulster (Pevsner Guides)Penguin, London.

Geology of Co. Donegal

The geology of Co. Donegal most closely resembles that of Co. Mayo and the county contains the oldest rocks in Ireland, around 1780 million years old, exposed on the offshore island of Inishtrahull. 1000 million years ago [Ma] sediments were deposited in an ocean and an Ice Age that affected the Earth at this time produced glacial till of cobbles of rock set in a matrix of crushed rock. Between 470 and 395 Ma the whole area was subjected to a mountain building event called the Caledonian Orogeny and the rocks were metamorphosed or altered into gneiss, schists and quartzites, now known as the Dalradian Group.

Errigal Mountain is composed of this quartzite, which weathers to a ‘sugarloaf’ shape. The metamorphosed glacial deposits are called Tillites. In the late phase of the orogeny two continents collided and the north-east to south-west trend of the rocks in Donegal was produced. At the same time, around 405 Ma, a series of six granite masses were injected into the older rocks of which the Main Donegal Granite is the largest. Granite is an igneous rock, which crystallised as it cooled down and its constituent minerals, quartz (glassy), feldspar (white or pink) and mica (black or silvery), interlocked.

There are no Ordovician or Silurian rocks in Donegal and only a small patch of Devonian sandstones along the northern shore of Donegal Bay. In the Lower Carboniferous (350 Ma) a warm shallow ocean migrated northwards very slowly. When it reached Donegal it resulted, firstly, in the deposition of sands and muds carried south by rivers that drained the old continent. This material was laid down close to the shoreline and now forms the sandstones and mudstones at Doorin Point. These gave way to limestones that often contain corals and other fossils. The sea level fluctuated at this time and a shallowing of the sea saw further sandstones (such as the Mountcharles Sandstone) being deposited. Later, limestones were deposited in the deepening ocean in which some reefs rich in organisms, such as corals, cephalopods (squids) and bryozoans, also grew. The Carboniferous rocks are found in a semicircle around Donegal Bay.

Geological map of Donegal

Map legend: Geological Map of Donegal (based on GSI 1:1,000,000 map 2003). Pink: Precambrian Dalradian rocks; Turquoise: Precambrian Gneiss and Schists; Yellow: Precambrian Quartzite; Red: Granite; Beige: Devonian sandstones; Dark blue: Lower Carboniferous sandstones; Light blue: Lower Carboniferous limestone.