Page images
PDF
EPUB

and extend up the valleys of the Whiteadder and the Leader into the heart of the Silurian uplands. In like manner Permian sandstones are well developed in the ancient hollows of Annandale and Nithsdale. Along the northern borders of the Southern Uplands we meet with similar evidence to show that even as early as the Old Red Sandstone period the ancient plateau along what is now its northern margin was penetrated by valleys that drained towards the north. But the main water-parting then, as now, lay not far south of this northern margin]; in other words, the surface of the ancient plateau, a few miles back from its northern boundary, sloped persistently towards the south-east. Now the strike or general trend of the strata throughout the whole of these Uplands is south-west and north-east. We cannot doubt, therefore, that when the ancient plain of erosion was uplifted, and so became a plateau, the surface would be marked by many more or less well-defined ridges and hollows, probably none very prominent, but all having a north-east and south-west trend. The average slope of the surface being towards south-east, the flow of the principal rivers would follow that direction, they would cut their channels across the outcrops of the strata. But the "corduroy" character of the plateau would now and again lead to occasional deflections, while some streams and rivers would be conducted for long distances parallel to the strike of the strata. In a word, two sets of principal valleys would tend to be formed, namely, transverse and longitudinal valleys. Examples of the former have already been cited, such as the Cree, the Ken, and the Nith, and amongst the better-known longitudinal valleys may be mentioned those of the Teviot, the Ettrick, and the Yarrow. But a glance at any good map of the region will show that all the more important streams have a tendency to flow either in a transverse or a longitudinal direction, while many run now in one of these directions and now in the other.

1 Many modifications of the drainage have been effected which cannot be referred to here. It may be pointed out, however, that the head-waters of the Nilh flow towards the north until they reach the broad Nithsdale, whence the drainage is directed south-east, so that Nithsdale may be said to cut right across the Uplands from north-west to south-east. This is probably a case of capture, the Nith, working back, having gradually invaded the northern drainage-area and captured such streams as the Afton and the Connel. The Clyde and the Doon are the only rivers of any size which have preserved their north-westerly course, and the head-waters of the former have just escaped capture by the Tweed.

The Southern Uplands thus prove to be merely a highly eroded plateau. Their geological structure shows that towards the close of Silurian times the greywackes and shales were buckled up, folded, and faulted, and doubtless appeared at first as a range of true mountains of elevation. Thereafter followed a prolonged period of erosion, interrupted, it is true, at successive stages by partial submergence, but resulting finally in the demolition of the old mountains of elevation and the conversion of the tract into a plain of erosion. Then came a final regional uplift, when that plain was converted into a plateau, which still exists, but in a highly denuded and eroded condition.

The Northern Highlands of Scotland might be cited as another plateau of erosion with a somewhat similar geological history. There, as in the south, there is evidence to show that vast earth-movements resulted, towards the close of Silurian times, in the formation of great mountains of elevation. The thrust-planes visible in the north-west part of that region are on a much more extensive scale than those met with in the Southern Uplands. Probably the mountains of elevation which appeared over the site of the present Highlands were loftier and bolder than the pre-Devonian heights of Southern Scotland. They may quite possibly have rivalled the Alps in grandeur, for the folding and general disturbance of the rocks are quite as remarkable as the confusion seen in the mountains of Switzerland. We may well believe that when the Highland mountains first uprose, their external form and internal structure would more or less closely coincide. No sooner had they come into existence, however, than the usual cycle of erosion would commence, and it is certain that after a prolonged interval they were to a large extent reduced to their base-level—much of the formerly elevated area acquiring the character of a plain of erosion. Subsidence next ensued, and that plain became gradually overspread with sediment, several thousand feet of Old Red Sandstone strata being deposited on the planed and abraded surface of the ancient rocks. At a subsequent date the whole region was uplifted and converted into dry land, forming a plateau country,

[graphic][ocr errors][merged small]

which, so far as we know, has never since been completely submerged, although it may well have experienced many oscillations of level.

It is out of that ancient plateau that the Highland mountains have been carved. The original surfaceslope is, as usual in such cases, indicated partly by the direction of the principal drainage-lines and partly by the summits of the mountains, which decline in elevation as they are followed outwards in the direction of the chief lines of drainage. Again, the main waterpartings separating the more extensive drainage-areas of the country mark out in like manner the dominant portions of the same old plateau-land. The waterparting of the North-west Highlands runs nearly north and south, keeping quite close to the western shore, so that nearly all the drainage of that region flows inland. The average inclination of that section of the Highlands is therefore easterly, towards Glenmore and the Moray Firth. In the region east of Glenmore the land slopes in the directions followed by the rivers Spey, Dee, and Tay. These two regions —the North-west and the South-east Highlands— are separated by the remarkable depression of Glenmore, running through Lochs Linnhe, Lochy, and Ness, and the further extension of which towards north-east is indicated by the straight coast-line of the Moray Firth as far as Tarbat Ness. This long depression marks a line of fracture and displacement of very great geological antiquity. The old plateau of the Highland area was fissured and split in two, that portion which lay to the north-west sinking along the line of fissure to a great but unascertained depth.1 Thus the waters that flowed down the slopes of the north-west portion of the fractured plateau were dammed by the long wall of rock that rose upon the south-east side of the fissure, and compelled to flow off to north-east and south-west along the line of displacement. The erosion thus induced sufficed in course of time to hollow out Glenmore and all the mountain-valleys that open upon it from the west.

The dominant portion of the ancient plateau east of the great fault is approximately indicated by a line drawn from Ben Nevis through the Cairngorm and Ben Muich Dhui Mountains to Kinnaird Point. North of that line the drainage is towards the Moray Firth; east of it the rivers discharge to the North Sea; while an irregular winding line, drawn from Ben Nevis eastward through the Moor of Rannoch, and southward to Ben Lomond, forms the water-parting between the North Sea and the Atlantic, and probably marks approximately another dominant area of the fractured table-land.

The geological structure of the Highlands agrees so far with that of the Southern Uplands, that the dominant "strike" of the strata is south-west and north-east. This, therefore, is the trend of the flexures and folds and of all the larger normal faults and great

1 It is probable that movements have taken place again and again at different periods along this line of weakness, and these movements may not always have been in one direction.

« PreviousContinue »