The county's geological landscape is predominantly made up of Lower Carboniferous limestone, which underlies approximately 75% of the county. These were laid down following the erosion of mountain ranges which formed due to the closure of the Iapetus Ocean. The eroded mountains became basins in which limestone sediments and carbonate mud were deposited. The oldest rocks in the county are Ordovician in age and are found in thin layers near Slane and at Stamullen, while the youngest rocks are of Paleogene age, and were formed as a result of volcanic activity. These are found in small dykes and sills throughout the county. Crustal stretching beneath Ireland during the Carboniferous allowed fluids to infiltrate through faults in the rock, and extensive mineralisation occurred. Most notably, zinc-bearing Sphalerite and lead-bearing Galena were deposited in vast quantities, giving Ireland the highest concentration of zinc per square kilometre on Earth. The ubiquity of these minerals gave rise to the term "Irish-type" lead-zinc deposits, which is a descriptive term for lead-zinc deposits hosted in carbonate rocks.