Heritage Sites of Co. Louth

Boyne Railway Viaduct

With the opening of the first railway in Ireland in 1834 there was a rapid expansion of the network that reached its greatest extent in the early years of the 1900s.  Many individual railways were established and it was not until 1945 that those in the Republic were consolidated into Córas Iompair Éireann (CIE).

The Dublin & Drogheda Railway opened in 1844 and had its Dublin terminus at Amiens Street, while the Ulster Railway, which opened four years later, commenced at Belfast.  By 1852 the northern line had been extended from Portadown to Drogheda.  For some time the River Boyne acted as a significant obstacle to connecting Dublin to Belfast by rail, and the linking of these two railway lines was only achieved when the Boyne Viaduct was opened in April 1855 by the Dublin & Belfast Junction Railway Company.

Construction commenced in 1851 to the design of the noted railway engineer Sir John Benjamin Macneill (1793–1880) who was knighted on the platform at Amiens Street Station (Connolly Station) in 1844. Soon after building started the contractor, William Evans of Cambridge, was confronted with serious difficulties, particularly in laying a secure foundation for one of the supporting piers that would carry the rail track 30 m above the river Boyne.  Excavations had to penetrate through 13 m of sediment before a firm base was located for the lowest part of the foundations. By October 1853 Evans had gone bankrupt and the scheme was completed by his replacement James Barton, an engineer already working with Macneill.

The structure consisted of twelve masonry arches on the south side and three to the north, connected in the middle with an iron structure in three parts supported on two stone piers rising from the riverbed.  The central span was 69 m in length and those on either side 43 m. The ironwork was replaced in 1932 with steel trusses to the designs of George Howden.

The piers and arches were built of local Mississippian limestones from Lagavoreen quarry just south of viaduct and from a quarry on Mornington Road in Drogheda.  These limestones, which are allocated by geologists to the Mornington Formation, are dark, contain a fair proportion of mud, and resemble the Calp Limestone that was frequently used in Georgian Dublin. In the railway viaduct this stone is largely left with a rough rock-finished surface.

The limestones of the voussiors of the arches, quoins of the piers and arches, supra-trackway extensions of piers, and the stringcourses required purer limestone so that it could be chisel dressed.  These paler limestones were transported to the Boyne by rail from Milverton quarry located near Skerries in north Co. Dublin and transported by cart and then by river from Ardbraccan in Co. Meath.  Superior limestone was also quarried at Ardbraccan; a pale limestone, it was favoured by some architects for high quality work and was used for facing Leinster House in the mid-1870s and for re-facing the National Library of Ireland on Kildare Street, Dublin in the 1930s. A carved plaque, in a pale cream-coloured limestone, inserted into the top of a pier shows the crest of the railway company and the date 1855.

Further reading:

Geology of Co. Louth

The imposing hills of the Cooley Peninsula in the north-east of the county rise steeply above Dundalk Bay and overlook a more subdued landscape stretching across the rest of Co. Louth.

The oldest rocks form a low range of hills just a few kilometres to the north-west of Drogheda, but they are poorly exposed. These Ordovician rocks, around 465 to 450 million years old, are dominated by volcanic lavas and ash falls erupted from volcanic islands and deposited on the ocean floor. Much of the low ground in the north of the county, between Dundalk and Ardee, is underlain by Silurian rocks, around 440 to 425 million years old. These were deposited on a deep ocean floor where layers of slowly deposited dark mud were periodically interrupted by influxes of muddy sand avalanching down into the ocean basin from shallower water. Later, as the continental plates either side of this ocean moved together, these interbedded layers of mudstone and sandstone became buckled and broken. Today, these ancient ocean floor sediments, tilted almost vertical, are superbly exposed in the coastal cliffs at Clogher Head.

There are a few rather small areas of Carboniferous limestone to the north of Dundalk, around Drogheda and to the west of Ardee. These rocks, deposited on an equatorial sea bed around 340 million years ago, for the most part go unnoticed although they were formerly worked in a large quarry at Tullyallen, just to the north of Drogheda.

In places the limestone exposed at Tullyallen has been deeply weathered. Various fissures and potholes exposed by quarrying were found to contain pockets of pale clay quite different from the glacial ‘boulder clay’ on the surface. The clays are thought to be from the Cenozoic Era, between 65 and about 3 million years ago, but attempts to date them using microfossils has failed.

Although the age of the clays at Tullyallen remains unproven, the rocks that form the hills of the Cooley Peninsula undoubtedly are Cenozoic. All are volcanic and are Paleocene in age, around 60 million years old. They include basalt lava flows that erupted onto the surface as well as more coarsely crystalline gabbros that cooled in a magma chamber beneath the volcano. These gabbros form the mountain of Slieve Foye, the highest point in the county.

Much of the low ground across Co. Louth is blanketed with glacial deposits left behind by glaciers from the last Ice Age. Glacial till, or ‘boulder clay’, forms some of the rapidly eroding coastal cliffs, particularly at Dunany Point on the south side of Dundalk Bay.

Map legend: Geological Map of Limerick (based on GSI 1:1,000,000 map 2003). Pink: Ordovician; Pale green: Silurian; Grey green: Ordovician & Silurian sediments; Red: Granite; Pale blue: Lower Carboniferous limestone; Dark purple: Paleogene gabbros and other intrusive rocks.