The limestone in Marble Arch Caves formed around 330 million years ago during the Carboniferous geological period, when Ireland lay close to the equator. Located on the edge of a supercontinent, where sea-levels were higher, the area that we now call Ireland was covered by a shallow tropical sea. The limestones formed by the accumulation of lime-mud on the bottom of this ancient sea floor and from the remains of dead sea creatures that would have thrived in these waters. Limestone is defined as containing at least 50 per cent calcium carbonate (CaCO₃), a common mineral extracted from sea-water by organisms and included in their bodies as skeletons or shells. Limestone formation is a very slow process; layers and layers of limey deposits build up on the ocean floor and are compacted by the weight of the water over millions of years. Eventually the layers of sediments are consolidated into beds of limestone rock.
Over time beds of limestone will be laid down on top of one another and as a result beds of different character lie on top of each other. These breaks in the sequence of limestone deposition are marked by horizontal bedding planes which are very important in the early stages of cave formation. At first the beds and bedding planes lie parallel to the original sea floor, then over time movements within the earth fold and tilt the beds to form mountain ranges, exposing parts of the rock to weathering. The beds of rock become fractured through this movement and vertical fissures called joints appear.
Limestone is not a porous rock, it is actually impervious to water because its structure is made up of tightly interlocking crystals. The bedding planes and joints found in limestone are its weakness and when water gets into these it can dissolve part of the rock. As mentioned earlier, Limestone is defined as containing at least 50% calcium carbonate and it’s this CaCO3 which is easily dissolved by rain and soil water, both of which are slightly acidic.
Therefore, the most important factor in the continued growth of cave systems is water –the largest passages in the Marble Arch Caves have formed where there has been a constant source of water.