The creation of the highlands was a long process, yet elevations, compared with European equivalents, are low, with the highest summit, Ben Nevis, only 4,406 feet (1,343 metres) above sea level. In addition, the really mountainous areas above 2,000 feet (600 metres) often form elevated plateaus with relatively smooth surfaces, reminders of the effects of former periods of erosion.
In comparison with the Scottish Highlands, the Southern Uplands of Scotland present a more subdued relief, with elevations that never exceed 2,800 feet (850 metres). The main hill masses are the Cheviots, which reach 2,676 feet (816 metres) in elevation, while only Merrick and Broad Law have elevations above the 2,700-foot (830-metre) contour line. Broad plateaus separated by numerous dales characterize these uplands, and in the west most of the rivers flow across the prevailing northeast-southwest trend, following the general slope of the plateau, toward the Solway Firth or the Firth of Clyde. Bold masses of granite and the rugged imprint of former glaciers occasionally engender mountainous scenery. In the east the valley network of the River Tweed and its many tributaries forms a broad lowland expanse between the Lammermuir and Cheviot hills.
The Midland Valley lies between great regular structural faults. The northern boundary with the Highlands is a wall-like escarpment, but the boundary with the Southern Uplands is sharp only near the coast. This vast trench is by no means a continuous plain, for high ground—often formed of sturdy, resistant masses of volcanic rock—meets the eye in all directions, rising above the low-lying areas that flank the rivers and the deeply penetrating estuaries of the Firth of Clyde and the Firth of Forth.
In Northern Ireland, structural extensions of the Scottish Highlands reappear in the generally rugged mountain scenery and in the peat-covered summits of the Sperrin Mountains, which reach an elevation of 2,241 feet (683 metres). The uplands in the historic counties Down and Armagh are the western continuation of Scotland’s Southern Uplands but reach elevations of more than 500 feet (150 metres) only in limited areas; the one important exception is the Mourne Mountains, a lovely cluster of granite summits the loftiest of which, Slieve Donard, rises to an elevation of 2,789 feet (850 metres) within 2 miles (3.2 km) of the sea. In the central region of Northern Ireland that corresponds to Scotland’s Midland Valley, an outpouring of basaltic lavas has formed a huge plateau, much of which is occupied by the shallow Lough Neagh, the largest freshwater lake in the British Isles.