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. Most of the coast is designated as European (Natura 2000) sites (SACs and SPAs).
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.