Sir A. Geikie has well described what he saw from the top of Ben Nevis:—
"Much has been said and written about the wild, tumbled sea of the Highland Hills. But as he sits on his high perch, does it not strike the observer that there is after all a wonderful orderliness, and even monotony, in the waves of that wide sea? And when he has followed their undulations from north to south, all round the horizon, does it not seem to him that these mountain-tops and ridges tend somehow to rise to a general level; that, in short, there is not only on the great scale a marked similarity of contour about them, but a still more definite uniformity of average height? To many who have contented themselves with the bottom of the glen, and have looked with awe at the array of peaks and crags overhead, this statement will doubtless appear incredible. But let any one get fairly up to the summits and look along them, and he will not fail to see that the statement is nevertheless true. From the top of Ben Nevis this feature is impressively seen. Along the sky-line, the wide sweep of summits undulates up to a common level, varied here by a cone and there by the line of some strath or glen, but yet wonderfully persistent round the whole panorama. If, as sometimes happens in these airy regions, a bank of cloud with a level under-surface should descend upon the mountains, it will be seen to touch summit after summit, the long line of the cloud defining, like a great parallel ruler, the long level line of the ridges below. I have seen this feature brought out with picturesque vividness over the mountains of Knoydart and Glen Garry. Wreaths of filmy mist had been hovering in the upper air during the forenoon. Towards evening, under the influence of a cool breeze from the north, they gathered together into one long band that stretched for several miles straight as the sky-line of the distant sea, touching merely the higher summits and giving a horizon by which the general uniformity of level among the hills could be signally tested. Once or twice in a season one may be fortunate enough to get on the mountains above such a stratum of mist, which then seems to fill up the irregularities of the general platform of hill-tops, and to stretch out as a white phantom sea, from which the highest eminences rise up as little islets into the clear air of the morning.... Still more striking is the example furnished by the great central mass of the Grampians, comprising the Cairngorm Mountains and the great corries and precipices round the head of the Dee. This tract of rugged ground, when looked at from a distance, is found to present the character of a high, undulating plateau."[29]
This long level line of the Highland mountain-tops may be seen very well from the lower country outside; for example, from the isles of Skye and Eigg, where one may see the panorama between the heights of Applecross and the Point of Ardnamurchan showing very clearly the traces of the old table-land.
How are we to explain this curious fact, so opposed to our first impressions of a mountain region? It is quite clear that the old plateau thus marked out cannot be caused by the arrangement or position of the rocks of which the Highlands are composed. If these rocks were found to be lying pretty evenly in flat layers, or strata, undisturbed by great earth-movements, we could readily understand that they would form a plateau. But the reverse is the case: the rocks are everywhere thrown into folds, and frequently greatly displaced by "faults;" yet these important geological features have little or no connection with the external aspect of the country. It is therefore useless to look to internal structure for an explanation. We must look outside, and consider what has been for ages and ages taking place here.
As already pointed out, an enormous amount of solid rock has been removed from this region—thousands and thousands of feet. It was long ago planed down by the action of water, so that a table-land once existed of which the tops of the present mountains are isolated fragments. No other conclusion is possible. To the geologist every hill and valley throughout the whole length and breadth of the Highlands bears striking testimony to this enormous erosion. The explanation we are seeking may therefore be summed up in one word, "denudation." The valleys that now intersect the table-land have been carved out of it. If we could in imagination put back again onto the present surface what has been removed, we should have a mental picture of the Highlands as a wide, undulating table-land; and this rolling plain would suggest the bottom of the sea. The long flat surfaces of the Highland ridges, cut across the edges of inclined or even upright strata, are the fragments of a former base-line of erosion; that is, they represent the general submarine level to which the Highlands were reduced after exposure to the action of "rain and rivers," and finally of the sea. As the sea gradually spread over it, it planed down everything that had not been previously worn away, and so reduced the whole surface to one general level like the sea-bed of the present day. But it is not necessary to suppose that the whole region was under water at the same time, and it is probable that there were separate inland seas or lakes. In these the rocks of the Old Red Sandstone were formed; and they in their turn have suffered so much denudation that only patches and long strips of them are left on the borders of the Highlands.
Before we speak of individual mountains and their shapes, it is important to bear in mind another fact about mountain-chains; namely, that they are very low in proportion to their breadth and length. The great heights reached by some mountains produce such a powerful impression on our senses that we hardly realise how very insignificant they really are. It is only by drawing them on a true scale that we can realise this. The surface of the earth is so vast that even the highest mountains are in proportion but as the little roughnesses on the skin of an orange. Fig. 2 (see chap, vii., p. [236]) represents a section through the Highlands, drawn on the same scale for height as for length.
What has been said about the Highland plateau applies equally well to many other mountain-ranges. Mr. Ruskin observed something rather similar in the Alps. He says,—
"The longer I stayed in the Alps, and the more closely I examined them, the more I was struck by the one broad fact of there being a vast Alpine plateau, or mass of elevated land, upon which nearly all the highest peaks stood like children set upon a table, removed, in most cases, far back from the edge of the plateau, as if for fear of their falling; ... and for the most part the great peaks are not allowed to come to the edge of it, but remain like the keeps of castles, withdrawn, surrounded league beyond league by comparatively level fields of mountains, over which the lapping sheets of glaciers writhe and flow, foaming about the feet of the dark central crests like the surf of an enormous sea-breaker hurled over a rounded rock and islanding some fragment of it in the midst. And the result of this arrangement is a kind of division of the whole of Switzerland into an upper and a lower mountain world,—the lower world consisting of rich valleys, bordered by steep but easily accessible, wooded banks of mountain, more or less divided by ravines, through which glimpses are caught of the higher Alps; the upper world, reached after the first steep banks of three thousand or four thousand feet in height have been surmounted, consisting of comparatively level but most desolate tracts of moor and rock, half covered by glacier, and stretching to the feet of the true pinnacles of the chain."
He then points out the wisdom of this arrangement, and shows how it protects the inhabitants from falling blocks and avalanches; and moreover, the masses of snow, if cast down at once into the warmer air, would melt too fast and cause furious inundations.