The Highlands are separated into two disconnected and in some respects contrasted divisions by the depression of the Great glen, extending from Loch Linnhe to Inverness. In the north western section the highest ground is found along the At lantic coast, mounting steeply from the sea to an average height of 2,000 to 3,00o feet. The watershed consequently keeps close to the western seaboard, in some places not above 14 m. from it. From these hills, which catch the first downpour of the rains from the ocean, the ground falls eastward. Numerous eminences, however, prolong the mountainous features to the North sea and south-eastward to Glen More. The difference of the general level on the two sides of the waterparting is re flected in the length of their streams. On the west the drainage empties itself into the Atlantic after flowing only a very few miles, on the east it has to run 30 or 4o miles. At the head of Loch Nevis the western stream is but 3m. long, while the eastern has a course of some 18m. to the Great glen. Throughout the north-western region uniformity of features characterizes the scenery, betokening, even at a distance, the general monotony of structure. But the sameness is relieved along the western coast of the shires of Sutherland and Ross and Cromarty by groups of cones and stacks, and farther south by the terraced plateaux and abrupt conical hills of Skye, Rum and Mull.
The south-eastern region of the Highlands, having a more di versified geological structure, offers greater variety of scenery. Most of the valleys, lakes and sea lochs run in a south-westerly and north-easterly direction, a feature strikingly exhibited in west Argyllshire. But there are also several important transverse valleys, that of the Garry and Tay, already noticed, being the most conspicuous example. The watershed, too, is somewhat different. It first strikes eastwards round the head of Loch Laggan and then swings southwards, pursuing a sinuous course till it leaves the Highlands on the east side of Loch Lomond. The streams flowing westward, however, are still short, while those running to the north-east, east and south-east have long courses and drain wide areas. There is a marked contrast between the configuration of the north-eastern district, and the other parts of this region. In that area the Grampians rise to level or gently rounded summits, often more than 3,000, and in a few places exceeding 4,000ft. in height, and bounded by steep declivities and sometimes by precipices. Farther south-west, in the shires of Perth, Inverness and Argyll, they give place to the more typical hummocky crested ridges of Highland scenery which, in Ben Nevis and Aonach Beg, reach heights of over 4,000 ft. Geological struc ture alone does not account for this contrast, and one reason may lie in the heavier rainfall, and consequently stronger erosion, to which the western mountains, facing the Atlantic ocean, have been exposed. Long narrow strips of flat land occur in the more impor tant valleys. Most of the straths and glens have a floor of detritus which, spread out between the bases of the boundary hills, has been levelled into meadow land by the rivers and provides almost the sole arable ground in each district. It is appropriate here to notice certain terms common throughout Scottish topography in applica tion to types of valleys and low-lying land, for examples of most of them are found in the Highlands or on their borders.
Straths are broad expanses of low ground between hills, usually traversed by one main stream and its tributaries—e.g., Strath Tay, Strath Spey, Strath Conon. This name, however, has also been applied to wide tracts of lowland which embrace portions of several valleys, but are defined by lines of heights on each side; the best example is afforded by Strathmore—the "Great Strath" —between the southern margin of the Highlands and the line of the Sidlaw hills. This long wide depression, though it looks like
one great valley, includes portions of the valleys of the Tay, Isla, North Esk and South Esk, all of which cross it. Elsewhere in central Scotland such a wide depression is known as a hove, as in the Howe of Fife between the Ochil and Lomond hills. A glen is a narrower and steeper-sided valley than a strath, though the names have not always been applied with discrimination. Most of the Highland valleys are true glens, Glencoe being the best known example. The hills rise steeply on each side, sometimes in grassy slopes, sometimes in rocky bosses and precipitous cliffs, while the bottom is occupied by a lake. In the south of Scotland the larger streams flow in wide open valleys called dales, as in Clydesdale, Tweeddale, Teviotdale, Liddesdale, Eskdale, Niths dale. The strips of alluvial land bordering a river are known as . haughs, and where, along estuaries, they expand into wide plains they are termed carses. The carses of the Forth extend seawards as far as Bo'ness and consist chiefly of raised beaches. The Carse of Gowrie is the strip of low ground intervening between the Firth of Tay and the Sidlaw hills. Brae signifies the steep bank of a river, and so any slope or hill-side.
Scottish lochs (or lakes) are sometimes classified into four groups—glen lochs, rock-tarns, moraine-tarns, and lochs of the Lowlands, of which the first and most important are practically confined to the Highlands and the second and third are far more numerous there than elsewhere. The small rock-tarns, lying in rock-basins on the flanks of mountains, or the summit of ridges, or rocky plateaux, are by far the commonest, and especially so in the north-west. They almost invariably lie in strongly ice-worn platforms, and are held to occupy hollows produced by the gouging action of the ice-sheets in glacial times. Moraine tarns—small sheets of water dammed back by moraines left by retreating glaciers—are also numerous in the Highlands, nestling in the bottoms of corries. In the south-west, where the glaciers continued longest to reach sea-level, lakes retained by moraine barriers are found very little above the sea. More important, if less numerous than either of these categories, are the larger glen-lochs, which are associated with the finest inland scenery in the Highlands. These occupy depressions in the glens, not due to local heaping up of detritus, but true rock-basins, often of great depth. It is commonly but not invariably held that these depressions were formed by the erosive action of ice, since glaciers occupied the glens where they occur and wore down the rocks along the sides and bottom; but it is a point of difficulty in this theory whether ice could have eroded the deepest of the hollows. In any circumstances the lochs must be of recent geological date. Any such basins belonging to the time of the folding of the crystalline schists would have been filled up and effaced long ago, so rapid is the infilling by the torrents which sweep down detritus from the surrounding heights. Glen lakes are almost wholly confined to the western half of the Highlands. Hardly any lakes are to be seen east of a line drawn from Inverness to Perth. West of that line, however, they abound in both the longitudinal and the transverse valleys.