A Guide to Cork City's Geological Heritage

The geological folding of the Variscan Orogeny is starkly visible here at Leitrim Street.

Leitrim Street and Upper John Street outcrops The geological folding of the Variscan Orogeny is starkly visible here at Leitrim Street. At the fuel station and the private parking lot next to it we can vividly see impressive geological folds. Notice the smaller vertical planes (looking at the rocks, these planes look like lines or cracks, but are actually planar features) in the layers of the rocks. These planes are known as cleavage, and they formed due to the huge pressures the Variscan Orogeny (the continental collisions south of Ireland) put on the rocks in this area. Before the rocks were compressed, the layers were straight and flat. To give an idea of what happened, here is a household comparison: imagine compressing half-baked lasagne pasta. At first, small “cracks” form in the pasta dough before the lasagne starts folding and perhaps even breaks. A similar thing happened in the rocks during the Variscan Orogeny. As the flat rock layers were compressed, the rock responded by becoming shorter, initially by developing this cleavage. This shortened the rock layers by 40-50%. After the compression continued, there was no other way for the rocks to form than to bend and fold. This also caused the huge faults that many rivers in Cork City follow. St Fin Barre’s Cathedral You can’t summarise or illustrate Cork City’s skyline without including St Fin Barre’s Cathedral. It has interesting designs and decorative rocks on the outside and inside sourced both locally and from afar. The limestones decorating the outer walls of this impressive building sport numerous fossils of seashells, crinoids, sponges and corals. Glimpsing limestone fossils requires a close look, but when finding a sufficient tile on the cathedral, it becomes apparent how populated the tropical sea of Carboniferous Cork was. We have now mentioned many locations with limestone in the City, and again, it is the most widespread bedrock on our island. But it is still amazing to glance over, imagining the rich tropical coral reefs of the Carboniferous, and the bricks of the Cathedral allow for some detailed looks at these fossilised organisms. Gargoyles and statues also decorate the outer walls of the building, and these are also, of course, made of limestone. Do note that some of the pale limestone is not locally sourced but is from Galway.