The oldest rocks are 2.7 billion years old and are found in Ireland, Wales and the northwest of Scotland. During the Silurian period, the north-western regions collided with the south-east, which had been part of a separate continental landmass. The topography of the islands is modest in scale by global standards. Ben Nevis, the highest mountain, rises to only , and Lough Neagh, which is notably larger than other lakes in the island group, covers . The climate is temperate marine, with cool winters and warm summers. The North Atlantic drift brings significant moisture and raises temperatures above the global average for the latitude. This led to a landscape that was long dominated by temperate rainforest, although human activity has since cleared the vast majority of forest cover. The region was re-inhabited after the last glacial period of Quaternary glaciation, by 12,000 BC, when Great Britain was still part of a peninsula of the European continent. Ireland, which became an island by 12,000 BC, was not inhabited until after 8000 BC. Great Britain became an island by 5600 BC.