Highlands are formed essentially of Pre- Cambrian and Lower Palaeozoic rocks folded mainly in the Caledonian earth-movements. The structures are so complex and the lithology so variable yet so similar that structure is less important in the relief here than in the uplands and the lowlands. Over large areas of central Wales, the Southern Uplands and even parts of the Highlands of Scotland the relief is monotonous, uplifted, dissected plateau. The general impermeability of the rocks, allied with the heavy rainfall deriving from their westerly position, together with the general lack of limestone, results in infertile, leached, acidic soils. Having been the centres of the main ice caps of Britain the highlands bear signs of more severe glacial erosion than the other regions of Britain. Yet there are differences within and between the areas.
The most extensive of them, the Highlands and islands of Scotland, is by far the largest area of crystalline metamorphic rocks in Britain. Over the whole area occupied by the Moine and Dalradian rocks the relief is due mainly to fracturing and glaciation and not to lithology. This is the highest region in Britain, the summits usually exceeding 3,000 ft and often 4,000 ft. It is deeply dissected, often along fracture lines, by broad glacial troughs, many of them now occupied by lochs some of which, for example Ness and Morar, occupy greatly overdeepened basins. All the signs of severe highland glaciation are here: valley steps, roches moutonnées, hanging valleys, corries, recessional moraines, and streams rising either in corries or on imperceptible divides in through valleys. In this extensive area of foliated crystalline rocks few rocks have individual effects on the relief with the possible exception of the quartzites, which are held responsible for the more pointed form of some of the peaks, for example Schichallion.
Lithological diversity increases in importance near the west coast, where, beyond the thrust planes which terminate the outcrop of the Moine series, there are tracts of Lewisian gneiss, Torridonian sandstone and very much younger Tertiary igneous rocks. On the Lewisian is developed monotonous, lowlying, bare, hummocky glaciated relief, showing in many places numerous lakes and indeterminate drainage, especially on the mainland north of Ullapool and in the Outer Hebrides, notably in Uist and Benbecula. Along the north-west coast of the mainland the Torridonian mountains rise above the Lewisian in the greatest contrast, slabs of near-horizontal, ancient, resistant sediments often weathered, as on Stac Polly and Suilven, into steep and ragged ridges. It is curious how the Lewisian and also the Moine rocks on the coast farther south around Arisaig give the appearance of having been almost ground out of existence by the strength of the glaciation, while the Torridonian rocks, both in the mountains and in lowlying outliers on the Lewisian, do not.
The Tertiary igneous rocks, although so different in age, have many features in common with the Pre-Cambrian rocks: like them they are crystalline, while the relief on some of the major intrusions, especially the gabbro of the Cuillin Hills of Skye and the granite of northern Arran, provides superb examples of mountain glaciation. Sheets of plateau basalt, especially in northern Skye and the dyke swarms of Skye, Mull and Arran are distinctive elements in the relief, though both lava-sheets, for example the Devonian andesites south of Oban, and dykes of earlier ages are not unknown in the Highlands.