Geology of Ireland longest known free-hanging stalactites. Geology of Ireland The geology of Ireland consists of the study of the rock formations in Ireland. The island contains rocks from every age from Proterozoic to Holocene and a large variety of different rock types is represented. The basalt columns of the Giant's Causeway together with geologically significant sections of the adjacent coast have been declared a World Heritage Site. The geological detail follows the major events in Ireland's past based on the geological timescale. The oldest known Irish rock is about 1.7 billion years old and is found on Inishtrahull Island off the north
Cave of the Crystals of the crystals. As the cave's accessibility is dependent on the mine's water pumps, when mineral exploitation is ended in the area it was likely pumping would stop and water level allowed to rise again. In February 2017, Penelope Boston reported to the BBC that mining operations had ceased and that the caves have re-flooded. Cave of the Crystals Cave of the Crystals or Giant Crystal Cave () is a cave connected to the Naica Mine at a depth of , in Naica, Chihuahua, Mexico. The main chamber contains giant selenite crystals (gypsum, CaSO·2 HO), some of the largest natural