The hard rocks of north Louth have defined a borderland for millennia but the Ulster border is also defined by much younger geological formations. Much of the Ulster border runs through the drumlin belt, a broad swathe of steep-sided, egg-shaped hills deposited by the last great ice sheet. The appearance of this belt is particularly noticeable viewed from Castle Roche, once on the edge of the English Pale. Most of Louth is gently rolling however, and being on the east coast, relatively dry. The combination of climate and open, level relief means that the county does not possess the vast array of inter-drumlin lakes, bogs and marshes found further west.
The interaction between the glacial deposits and the Irish sea has also given rise to several sites of regional, national and even international scientific importance along our coast. Almost all of Louth’s coast is low-lying, soft and at risk of erosion but also of great importance for wildlife. Many areas are designated as European (Natura 2000) sites (SACs and SPAs).
The county’s satisfying landscape is a result of geological diversity. Most of Louth forms part of the low-lying Longford-Down massif and is underlain by rocks deposited in the Silurian period. These 400 million year old shales and greywackes were formed from deep water sediments, laid down on the continental slopes and ocean floors of an ancient, now-vanished ocean (named the Iapetus, after the father of Atlas, for whom the current, Atlantic Ocean is named). This ocean lay between a proto-Europe and a proto-North America, which advanced upon each other, swallowing up the intervening ocean floor and slamming (ever so slowly) into each other along a line which runs across Ireland from the Shannon Estuary to Clogherhead. The continental collision was accompanied by volcanism and the formation of towering, Himalaya-sized mountains, now eroded back to sand, silt and mud and Louth’s fertile plains. [insert geological map]
Towards the south of the county, and along the continental suture, the land rises into low hills, such as Mount Oriel, from which wonderful views can be had, both south, into the Boyne Valley, and north over the rest of the county. The Boyne Valley, as with much of the rest of the midlands, is dominated by limestone and other rocks of Lower Carboniferous age, which also occur in a few other places.
The northern end of the county is, however, dominated in a very different, physical sense, by hard, resistant, igneous rocks. The Cooley peninsula has more in common with related igneous areas in South Armagh (Slieve Gullion, to the north-west) and to Down’s Mournes (to the north and north-east) than with the rest of Louth. Approaching from the south these three mountain ranges appear like a huge barricade, controlling access into Ulster from the rest of Ireland, or is it the other way around? In this area molten magma anciently forced its way up and into the existing ‘country rocks’, where it cooled and solidified, some 56 to 58 million years ago. Tens of millions of years of erosion, including several Ice Ages, have worn away the originally-overlying sedimentary rocks, exposing these bones of the landscape at the surface.
Between the Cooley Mountains and the southern hills, which separate the Boyne valley from the rest of the county, most of Louth is rolling farmland on a broad, well-watered coastal plain.,. In the north-west of the county however, the Drumlin Belt of southern Ulster, another result of the Ice Age, makes its appearance, creating a further barrier between the northern Province and the rest of the country. These rounded hills of clay, though not high, shelter lakes, bogs and swamps and long formed a further defence-in-depth along the ancient border of Ulster.
