Most of Louth is underlain by rocks deposited in the Silurian period (shown in greenish-grey on the geological map below). These are part of the low-lying, long-eroded Longford-Down massif and explain some of the similarities in soil type and landscape between Louth and adjoining counties to the north (the brighter green rocks are of similar age and character). Some favoured parts of the county are underlain by the Carboniferous limestone (light blue below) that floors much of the midlands of Ireland. These two rock types, greywackes and limestone, are the main building rocks seen in the county, though there is some use of granite, granophyre and gabbro in the Cooley area.
Louth has a remarkably diverse and satisfying landscape for such a small county. This variety is a result of geological diversity. Most of Louth forms part of the low-lying Longford-Down massif and is underlain by sandy and shaley rocks deposited in the Silurian period (and some, older Ordovician rocks). These 420 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.
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, including near Dundalk and on the Cooley peninsula.
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-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? These mountains owe their origin to the opening, not the closing of an ocean. As Eurasia and North America went their separate ways, huge ruptures and cracks in the earth's crust allowed hot, molten rock reach close to or even onto the earth's surface. This magma forced its way up and into the existing 'country rocks', where it cooled and solidified, some 56 to 58 million years ago. This was the same time period as the formation of the world famous Giant's Causeway. 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.