The physical geography of the UK varies greatly. England consists of mostly lowland terrain, with upland or mountainous terrain only found north-west of the River Tees-River Exe line. The upland areas include the Lake District, the Pennines, the 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 goes across the Scottish mainland from Helensburgh to Stonehaven. The Royal Observatory, Greenwich is the is the defining point of the Prime Meridian. 