swept along as the icy mass crawls downwards.1 This mound is called a moraine. By the confluence of separate glens two glaciers are joined into one. If moraines have already formed at their sides, those at adjacent edges are combined.2 The solitary blocks also travel on with the ice. If they are flat in form, they become elevated on pedestals; for they act as parasols, and so the ice beneath them melts. less rapidly than the exposed surface of the glacier. Sometimes they travel to the end of the ice-stream; but if small, they are more often engulfed in a crevasse, where we leave them for the present.

When the bed of a valley has a fairly uniform slope, the glacier descends in a comparatively unbroken flow, till it melts away under the increasing temperature and is transformed into a river. In this case the medial moraines are also continuous, and produce great mounds, almost comparable with railway embankments. It must not, however, be supposed that these consist entirely of débris. The ice beneath them, as in the case of the glacier-table, is protected from the rays of the sun, and forms at last an inner ridge or core, on which sometimes the outer cover lies in very unstable equilibrium. The Unter Aar Glacier, on which the general gradient is only about

1 The Alpine glaciers advance on an average about a foot a day. Large glaciers move more quickly than small; from 20 to 40, and in one case 50 feet has been recorded for the great ice-streams of Greenland. See Prestwich, "Geology," Part II. chap. xxxiii.

2 This is called a medial moraine, the others being called lateral.

In them much

6 in 100, affords a good example of these unbroken moraines. Commonly, however, the bed of the valley is more step-like, one or two steeper descents interrupting the uniformity of its slope. As the glacier passes over these, it is rent into a wilderness of icy crags sundered by yawning crevasses. of the moraine is engulfed; part of the débris descends to the bottom of the glacier, part is arrested in the narrower fissures and becomes embedded in its mass. At the base of the icy cascade the chasms often close up again, and the glacier moves on as a comparatively smooth stream. But the moraine has now lost its banklike form, its materials have been scattered. Some of the blocks which were swallowed up in the shallower fissures appear again, in some cases disclosed by the melting of the ice, in others possibly extruded by an upward motion in the mass itself. Thus, as a rule, the moraine, before reaching the end of the glacier, has become spread out over the surface. Sometimes for a considerable distance the mass of scattered débris grit, gravel, and boulders of all sizes almost conceals the lower part of the icestream. The load at last is dropped at the end of the glacier, where it forms a bank called the terminal moraine. This bank is usually more or less a crescent in plan, the convexity pointing downward, in correspondence with the shape of the ice-stream; and if

1 The possibility of this supposition will be discussed in a later chapter.

its end remain for long at the same spot, the moraine attains considerable dimensions. If, however, the glacier be slowly shrinking, the mound is replaced by a wide scatter of boulders and débris. The same is true of the lateral moraines. These also occasionally form large banks, resting partly on the slope of the valley, partly on the edge of the ice. Sometimes they too may be, as it were, stranded, and become features no less definite than the terminal moraine; sometimes their materials also may be scattered, owing to a steady shrinkage of the glacier on the sloping sides of the valley. The volume of the moraine is dependent on a variety of circumstances, but the nature of the rocks in the adjacent crags and the arrangement of their divisional surfaces are always important factors. For instance, the amount of moraine stuff on the glaciers descending the western valleys of the New Zealand Alps around Aorangi is comparatively small, while on those of the eastern side it is very large, covering the icestreams for about a quarter of their whole length, and piled up in heaps or hillocks more than fifty feet high. The cause is this: the mountain range is largely composed of slabby slaty masses of stratified rock, which dip towards the west. On that side the valleys descend nearly along the slope of the beds; the cliffs are low, and débris does not roll far; while on the eastern face the rock structure lends itself to the formation of great precipices, from which masses

are continually dislodged, to fall as avalanches of stone on the ice beneath.

Solitary blocks-sometimes many cubic yards in volume may be seen, as we have said, travelling down the glacier; as it retreats, such blocks are occasionally stranded, either on the sloping flanks of the valley, or on some prominent hummock of rock which previously had been concealed beneath the ice, like a reef beneath the surface of a swollen river.

FIG. 1. Perched Block on ice-worn rock islet, Nerlungshavn, near Langesund, Norway.

They lie sometimes on the bare rock, almost unaccompanied by moraine débris, and are poised occasionally in not very stable positions. These are called perched blocks. Most invaders set up monuments to commemorate their advance; the ice-king makes them memorials of his defeat.

But a glacier leaves other traces of its presence. If we had examined the bare rock near the Concordia hut, or the surfaces which, as we mounted the glacier, could be seen here and there shelving beneath the

margin of the ice, we should have noticed how much they differed in aspect from the crags in some dale of the limestone district of Derbyshire or among the granite hills of Auvergne. Instead of rough angular ridges, jutting knobs and rude hollows, we find rounded, smoothed, in places even polished surfaces. The last task, however, has been ill performed, for the smoothed face of the rock is scratched and striated sometimes even rather deeply scored. This also is clearly the handiwork of the ice, and a little investigation will show how it is accomplished.

When a glacier moves over the rocky bed of a valley, the friction of so great a mass of ice must produce considerable effect. If it trespasses upon ground previously uncovered, it encounters rugged surfaces and jutting ledges. These it gradually wears away, making the rough places smooth, as angles are replaced by curves and craggy prominences by rounded knolls, in outline like the backs of sheep. From this resemblance such masses (of which we shall soon see more) are called roches moutonnées. (See Frontispiece.)

The

But it is not only the ice that abrades. fragments broken off and the dust worn away from the underlying rocks are carried onwards and form an armature to the lower surface of the glacier. The moraine stuff, engulfed in crevasses which happen to extend from the top to the bottom of the glacier, also augments its rasping power. Moreover, when