The

erosion rather than by underground movement. But in their case, the erosion has been carried by glacier-ice far below the level at which the original stream acted. softer nature of the rock which allowed the stream to widen its valley above the gorge would enable the ice to deepen it. As already suggested, many of the flat alluvial plains above gorges in the Highlands were probably at one time glen-lakes which have been gradually filled up. Large as is the number of existing lakes in the Highlands, it is a mere fraction of the number that once existed. Everywhere we see them being filled up by the sand and mud poured into them by their tributary streams. The shallow basins, of course, disappear first; those that are deep and have steep sides last longer, and except at the upper end, where the main stream enters, may show no sign of diminution. But they too are natural filters that receive the muddy water from the surrounding hillsides, and discharge it clear and bright at their lower ends. Every spate, therefore, helps to displace the water of the lake by an equivalent amount of sediment deposited on the bottom. Slowly but certainly each lake is diminishing in volume, and unless some new series of geological revolutions should begin, the result of the present operations of nature must inevitably be to convert every Highland lake into an alluvial plain.

R

CHAPTER XI

THE ANCIENT ICE-SHEETS AND GLACIERS OF THE

HIGHLANDS

IN the fourth chapter a brief outline was given of the effects produced on the surface of the land by the movement of sheets of ice across it. I propose to describe here the traces of the march of the ancient glaciers which have so profoundly affected the scenery of Scotland, and notably of the Highlands. It is now well ascertained that during a comparatively recent geological period, the climate of the northern hemisphere was much colder than at present, and that in the British Islands, as well as in other countries where glaciers are now unknown, the land was enveloped in snow and ice. This part of the geological record is known as the Ice Age or Glacial Period.

In following the track of the Scottish glaciers and icesheets, and noting how much they have modified the contours of the country, it must be borne in mind that the present great leading features of mountain and valley had been fixed before the snow and ice spread over them. The minor outlines of the surface, however, were in some respects unlike those which we now see. There was doubtless far more angularity and ruggedness about

the aspect of that ancient land. The long-continued passage of the ice across it did much to remove such irregularities and smooth the general surface. But though the ice abraded the valleys, it did not make them. Keeping in recollection, therefore, that hill and valley were grouped into their present arrangement before the ice began to settle down upon them, let us look for a little at the evidence from which this strange chapter in the country's history is deciphered.

FIG. 54.-Ice-worn rocks, Rispond, Sutherland.

The surface of Scotland, like that of Ireland and of the northern half of England, as well as the whole of Scandinavia and Northern Europe, is distinguished from more southern countries by a peculiar contour, visible almost everywhere, irrespective of the nature of the rock (Fig. 54). This contour consists in a rounding and smoothing of the hills and valleys into long flowing outlines. What were no

doubt once prominent crags have been ground down into undulating or pillow-shaped knolls, while deep hollows and gentler depressions have been worn in the solid rock. It may seem paradoxical to speak of the well-known rugged Highland mountains as showing traces of a general smoothing of their surface. But such is really the case. There may be places, indeed, where from height, or steepness, or some other cause, the smooth surface was never communicated; and there is everywhere a constantly progressing destruction of that peculiar outline: the rains, springs, and frosts are re-asserting their sway, and carving anew upon the country its ancient ruggedness. Nevertheless, to an eye which has learnt to distinguish the characteristic flowing lines, there are not many landscapes in the kingdom where they cannot be traced. Even in the wildest Highland scenery, where the casual tourist may see nothing but thunder-riven crags and precipices, and glens blocked up with their ruins—

'Precipitous black, jagged rocks,

For ever shattered and the same for ever,'

an eye trained to observe it can detect the same universal smoothing and moulding. Nay, it is precisely amid such scenes that the geologist is most vividly impressed with the fact that the general surface of the country has been ground down, for he there sees the natural outlines which the rocks assume when left to the ordinary attacks of the elements. The smooth undulating outlines are there replaced by craggy precipices and scars, here and there red and fresh, where the last winter's frosts have let loose masses of rock into the valleys below. He can trace how, in this way, the hand of nature is once more roughening the landscape, restoring to the hard rocks their cliffs and

ravines, and to each knoll and crag a renewal of its former angularity. Yet his eye rests continually upon little bosses of rock, or even upon whole hillsides where, owing to a covering of drift or soil, or to the enduring nature of the material, the change has gone on but slowly, and where he can still view the uneffaced traces of that wonderful process by which the surface of the country from Cape Wrath to the Solway has been worn and smoothed.

This widespread abrasion can be seen on hill and crag, hummock and knoll, from the shoulders of the mountains down to the level of the sea and below it. It is traceable upon all the little dimples and prominences on a freshly exposed surface of rock. The hardest materials usually show it best, and when the soil and superficial detritus are stripped from them, their faces may often be seen to be as smoothly dressed as if they had been cut in a mason's yard, and were meant to form part of the polished ashlarwork of a great building.

Further, not only have the rock-surfaces been thus planed down, they have been covered with long more or less parallel ruts and striæ, varying in depth and width from mere streaks, such as might be scratched with a grain of sand, up to grooves like those worn in old pavements by the cart-wheels of successive generations. The fine scratches may be seen descending into the hollows and mounting over the prominences of a rock, keeping all the while their general direction, with about as much regularity and persistence as they do over the most even surface. These markings have obviously been produced by some agent that moved across the face of the country, grinding down, scratching, and grooving the rocks as it passed along. No violent or transient debacle will account for them. They can only have been made in a quiet, deliberate way,