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
St. Michael's Cave receiving almost 1,000,000 visitors a year. The cave was created by rainwater slowly seeping through the limestone rock, turning into a weak carbonic acid which gradually dissolved the rock. Through this process, tiny cracks in The Rock's geological fault grew into long passages and large caverns over thousands of years. The numerous stalactites and stalagmites in the cave are formed by an accumulation of traces of dissolved rock deposited by water dripping from the ground above. In 1974 a Neolithic bowl was discovered in the cave, one of many examples which prove that the cave was known to prehistoric humans.