the Tees–Exe line. The upland areas include the Lake District, the Pennines, North York Moors, Exmoor and Dartmoor. The lowland areas are typically traversed by ranges of low hills, frequently composed of chalk, and flat plains. Scotland is the most mountainous country in the UK and its physical geography is distinguished by the Highland Boundary Fault which traverses the Scottish mainland from Helensburgh to Stonehaven. The faultline separates the two distinctively different regions of the Highlands to the north and west, and the Lowlands to the south and east. The Highlands are predominantly mountainous, containing the majority of Scotland's mountainous landscape, while the Lowlands contain flatter land, especially across the Central Lowlands, with upland and mountainous terrain located at the Southern Uplands. Wales is mostly mountainous, though south Wales is less mountainous than north and mid Wales. Northern Ireland consists of mostly hilly landscape and its geography includes the Mourne Mountains as well as Lough Neagh, at , the largest body of water in the UK. The overall geomorphology of the UK was shaped by a combination of forces including tectonics and climate change, in particular glaciation in northern and western areas. The tallest mountain in the UK (and British Isles) is Ben Nevis, in the Grampian Mountains, Scotland. The longest river is the River Severn which flows from Wales into England. The largest lake by surface area is Lough Neagh in Northern Ireland, though Scotland's Loch Ness has the largest volume. Geology The geology of the UK is complex and diverse, a result of it being subject to a variety of plate tectonic processes over a very extended period of time. Changing latitude and sea levels have been important factors in the nature of sedimentary sequences, whilst successive continental collisions have affected its geological structure with major faulting and folding being a legacy of each orogeny (mountain-building period), often associated with volcanic activity and the metamorphism of existing rock sequences. As a result of this eventful geological history, the UK shows a rich variety of landscapes. Precambrian The oldest rocks in the British Isles are the Lewisian gneisses, metamorphic rocks found in the far north-west of Scotland and in the Hebrides (with a few small outcrops elsewhere), which date from at least 2,700 Ma (Ma = million years ago). South and east of the gneisses are a complex mixture of rocks forming the North West Highlands and Grampian Highlands