Ice-work, present and past

Front matter

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Preface

Though separate papers, and even compreheTtsive bookSj npon ice and its work are numerous in our mother-tongue, to say notliiug of other languages, a difficulty, to jud^e from my own experience, often attends the student They seem to be written, in most caseSj more with a view to advocating some particular interpretation of the facts than of describing the facts themBeivee. As regards these, it appears not seldom to be tacitly assumed that they must have some particular significaiicej from which it follows, as an almost inevitable result, that the hypothesis of one writer become b the theory of his successors. Accordingly, I have eudeavonred in the following pages to give greater prominence to those facts of glacial geology on which all inferences must he founded. For instance, a clay oon taming erratics consists of certain materials, has a certain stractnre, and stands in certain relations to other depodtg. These are facts, and their bearing on tihe qneetiou of its origin — whether it ho the ground-moraine of an

vii viii PREFACE

ice-sheet or such a deposit as is now accumulating on the Great Bank of Newfoundland — must be determined by the teaching of other facts, namely, by what can be learnt from regions the history of which is practically beyond doubt.

With this end in view, I have begun by giving a sketch of some such regions, and then have passed on to describe the phenomena which are the subject of more dispute. Here, in order to avoid the risk of misunderstanding the words of others, I have selected, as far as possible, cases which I have personally examined; these, however, so far as I have been able to ascertain from the literature of Glacial Geology, are fair samples of the whole series. To the descriptions — necessarily rather brief — I have appended a short statement of the interpretations which have been proposed, and have pointed out where they appear to me strong and where weak. In other words, I have endeavoured to follow the example of a judge rather than of an advocate ; that is, to sum up the evidence on each side of a case, and leave the verdict to the jury. Like any such official, I have my own view as to what that verdict should be, and this doubtless will be disclosed to those who can read between the lines, but I may claim that it has not been formed hastily or without

PREFACE ix

experience. I saw a glacier for the first time in 1856; my earlier geological papers dealt mainly with ice and its work, and though for nearly twenty years I have written mostly on petrological subjects, I have never obeyed the well-known dictum and wholly cast ofiF the old love.

In the later part of the book a method is indicated by which, I believe, we can approximate to the temperature at various localities during the Glacial Epoch, and the different explanations of this widespread refrigeration are stated and briefly discussed. To account for it, seems to me, in the present state of our knowledge, the most perplexing of all the problems which this Epoch presents.

Glacial geology is a large subject: the volumes of this series are comparatively small. Hence, in order to keep the present one within the prescribed limits, it has been necessary to pass rapidly over some points of considerable interest, and to omit others which have a less direct connection with an ice age. For instance, nothing is said of the coarse gravels on some upland plateaux and in certain river valleys, though many of them almost undoubtedly were formed when the climate was distinctly colder than it is at the present time, and some probably fall within the Glacial Epoch. But as their connection with ice is

X PREFACE

less obvious and the Glacial Epoch did not begin with a particular year, I have dealt only with those deposits to which it can lay an undisputed claim. For the same reason nothing has been said either as to the physics of ice or as to the relation of Man to the Glacial Epoch ; each of these topics having already formed the subject of a volume in this series.

A few of the illustrations are from my own rough sketches. For the photograph of the ice-worn rocks near the Grimsel I am indebted to my friend, Mr. J. Eccles, who took it when we were travelling together in Switzerland. I have also to thank other friends for kind help, among them Dr. Du Eiche Preller for aiding me in examining the deposits near Zurich, the Eev. E. Hill for co-operation in visiting those of Eastern England, and Miss C. A. Raisin for reading the first proofs of this book.

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Part I.— Existing Evidence

CHAP. PAQE

I. ALPINE GLACIERS, PAST AND PRESENT ... 3

IL ARCTIC AND ANTARCTIC ICE-SHEETS ... 38

PART IL— TRACES OF THE GLACIAL EPOCH

I. LAKE BASINS AND THEIR RELATION TO GLACIERS — THE PARALLEL ROADS OF QLENROY —

BSKERS, ETC 79

II. ICE-WORK IN GREAT BRITAIN AND IRELAND — THE

DEPOSITS AND THEIR SIGNIFICANCE , , I20

III. ICE-WORK IN EUROPE AND OTHER PARTS OF THE

PART IIL— THEORETICAL QUESTIONS

I. TEMPERATURE IN THE GLACL^L EPOCH . . 23I

II. POSSIBLE CAUSES OF A GLACIAL EPOCH . .247

IIL THE NUMBER OF GLACIAL BIOCHS . . 261

IV. GLACLAL DEPOSITS AND GENERAL PRINCIPLES OF

INTERPRETATION 270

xi

LIST OF ILLUSTRATIONS loo- worn Eockii in the Gorge of the Aar below the Griinsel

Hospice Fiviithi*i&:f>

I. Perched r;iot.'k on Ice-wom Rot^k Islet, Nerluii^'sbavn^

near LangtJSUJid, K or way 9

:;. Scratched Btone from the Till uf lit' stem , . . -IS

3. iSectioii ( Diagrammatic) of an Old MoraiDei near Reiche-

iiiiu, Switzerland . • , * . i. .. * ^6

4. Section of Tillj near Thalwyl, Lako oi Zurich . , » 3 ^

5. Till Testing on Old Gmvel of Limmat Valley, near Kill-

6. Map of Glacial Movements in Kraiioe and Switzerland

tit f me ptifji: J 5

7. Map jsbowing the Linen of Debris e^ctcnding from the AlpB

iDto tho Plains of the Po ...... j5

S. Map of Greenland , 4^

g. Malaapina GlacicT . * ,69

iol The Kanita of Maine and Seutb-Eastem Hew Hampishiri' 108

I I. Section of Kame near Dovorj New jrimi[ishire . . .110

12. Drujulins in the Vicinity of Boston . . . - ♦ n6

14. ContotiT and Glacial Map of the Eritiiih Isleii to/ttci} jfrnjii 12Q

15. Typical Section of T^ill in Seattle, AVasbin^^on Slate . ,135 (6, Erratics in Glacial Drift h . . ; » , * laS 17* Supposed Moraine between Sfjeetoa and Flam borough . 134

Kill f xiv LIST OF ILLUSTRATIONS

FIG. PAGE

1 8. Section in Mr. Green's Pit, N.E. of Sudbury . , .MS

19. Map of British Isles during Glacial Epoch, according to

Professor Car vill Lewis .... to face page 186

20. Map showing the Glaciated Area of Europe according to

J. Geikie, and the Moraines in Britain and Germany according to Lewis and Salisbury . . . . . 207

21. Map showing the Glacial Geology of the United States

to face page 216

22. Map showing the alleged Effect of the Glacial Dam at Cin-

cinnati 220

23. Map showing Old Channel and Mouth of the Hudson 223

24. Diagram showing Effect of Precession . . . .252 p

PART I

EXISTING EVIDENCE or THF *^

or

ICE'WOEK, PRESENT AND PAST

Chapter I

Among the liigheat peaks of the Bernese OberlaQd is a great cplaiid valley ^ almost a natural amphitheatre. It ia a gathering- ground of glaciers, Fonr of these, larger and more definite than the rest, descend from snowy saddles, which are separated by grand peaks — masj^ea of stoop ic© broken by precipices and seamed by outcropping ridges of dark rock. The white floor of the valley, at this meeting-place of the frozen waters, ia comparatively level. It lies some two thousand feet below the snowy saddles, and above them the peaks tower up about aa high again. Minor tributaries of frozen rivers issue from shallower recesses in the flanks of the peaks themselves: and through a single opening towards the south, aa through a portalj the anited ice-streams glide slowly forth, forming the Great Aletsch Gr lacier, the largest in the Alps- Such a hidden group of glaciers is far from rare in the great mountain chainsp To speak of the

Alps alone, the basins of the Gomer and the Fee Glaciers present a certain resemblance to that which we have described ; those of the Argentiere and the Northern Miage are yet more like it ; while *the Mer de Glace, among the aiguilles of Mont Blanc, is the nearest rival of this " Place de la Concorde of Nature," as it has been not inappropriately termed. One of the most striking prospects of this courtyard of the ice-king's palace is obtained from the Concordia hut, a rude chalet at the base of a rocky spur, in which the night is passed before making the ascent of the Jungfrau and sundry other peaks. From ben,eath this spot for a considerable distance the wide field of n^v^^ shelves so gently as to seem nearly a plain, and its surface is almost unbroken. It is only as the slope steepens, on approaching the base of the actual peaks or the foot of the curtain walls, that crevasses become frequent. Then the frozen snow, for here it can hardly be called ice, is riven into strange forras^ and usually one chasm — named the Bergschrund — wider and more persistent than the rest, severs the snow and ice of the glacier basin from the snow and ice of the actual peaks. Above this rift the solidified water is motionless, for it is fast frozen to the rocks ; below it the whole mass creeps slowly downwards towards the valley of the Rhone. The snow which

^ The name applied to the material of the upper part of a glacier, where it is more nearly in the cc»ndition of frozen snow. The German term \%jim.

ALPINE GLACIERS, I^RKSENT AND PAST 5

falls on the steep slopes of the peaks soon shoots down in avalanches to augment the accumulation below, and a thick layer gathers annually all over the wide expanse of the basin, for at this elevation rain is not common. So the glacier is bom, so it is nourished. I have described its birthplace, I have dwelt on its beginning in some little detail, because in the pages which follow it will be necessary mote than once to call up before our minds a similar scene. The ice, as has been said, is at first comparatively smooth and free from debris. Here and there, however, a solitary boulder forms a black spot on its pure surface, or a small scatter of them trails forth from the foot of some rocky spur which is more prominent and precipitous than its fellows. These boulders after a time become more frequent, till at last they begin to form a kind of selvage on the edge of the ice-stream. Obviously, as the valley which it descends becomes better defined and sinks deeper into the mountain mass, the crags and precipitous slopes on either hand rise higher and barer, and stones more often come thundering down from them or are swept along by avalanches of snow. Now and again a block, more headlong than the rest, closes its career by one vast leap which lands it out on the glacier some hundreds of feet away from the rocks; but the majority fall on or near the edge, so that on this a mound of boulders, large and small, together with grit and earth, gradually accumulates and is slowly swept along as the icy mass crawls downwards.* 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.^ 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 debris. 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

' 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, "Greology," Part II. chap, xxxiii.

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

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 pusses over tliese, it is vent into a wilderness of icy craga sundered by yawning crevasses. Tu tbem much of the moraine is engulfed ; part of the debris 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 m^s of scattered debris ^ grit, gravel, and boulders of all sizes — almost conceals the lower part of the ice-stream. 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 painting downward, in correspondence with the shape of the ice -stream ; and if

1 The poBsibiUty of ihk suppositiun will be discuBaeJ in a later ohnpter.

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 debris. 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 ice-streams 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 debris 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

Fia. I. — Perched Bloc^ on ice- worn rock islet, Nerlungshavn, near Langesund, Norway.

They lie sometimes on the bare rock, almost unaccompanied by moraine debris, 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 lo ICE-WORK, PRESENT AND PAST

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 crfiggy 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 rocJies moutonnSes, (See Frontispiece.)

But it is not only the ice that abrades. The 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

ALPINE GLACIERS, PRESENT AND PAST ii

the glacier makes its first advance, it may possibly sweep onwards loose material which has accumulated on the bed of the valley. This mixture of mud and stones, transported between the ice and the rock, is called ground moraine, or moraine profonde. As to its amount and importance there is much dispute, some holding that a very large quantity of materials travels in this way, while others think it to be comparatively smalL

Boulders and grit, as already stated, are engulfed in crevasses, and in some cases ultimately become embedded in the ice like currants in a cake. Their amount obviously depends partly on the quantity of surface moraine, partly on the number and depth of the crevasses. A smooth glacier may be almost buried under debris and yet swallow up but little; while one which is rent by crevasses may have its moraines comparatively small, because it engulfs them almost wholly. So also the amount of moraine stuff which makes its way from the top to the bottom of a glacier in some cases may be insignificant, while in others it may become large. But debris, varying in size from fine mud to huge boulders, is transported in greater or less proportions in these three ways: on the ice, in the ice, and under the ice.

Streams, fed by snow-beds or by hanging glaciers, occasionally descend the slopes on either side and plunge beneath the ice, bearing with them much debris and many partially rounded stones, which go to augment either the ground moraine or the material which is swept along the main channel of subglacial drainage. Boulders which travel under the ice and help in scoring the subjacent rocks must themselves suffer a like treatment. It is often "diamond cut diamond." The burnisher is burnished ; the scratcher is scratched in its turn. So while rocks which have travelled on or in the ice remain remarkably angular, those that have been dragged beneath it are smooth and worn, on some sides rather than on others, according as they are more easily retained in the grip of the ice. This causes them to assume a peculiar subangular form, the broader surfaces being marked by striations, which not seldom are irregular in direction.

Yet one other mark is made by a glacier. Its surface is melted by the sun ; the water trickles into rivulets, and the rivulets collect into streams, which quickly furrow a path for themselves; the ice is sculptured by a group of shallow channels like a model of a river basin on the land. But the course of the main stream of this drainage system is often cut short. If a crevasse opens across its path, the water plunges into the abyss and strikes on the rocks beneath. As the glacier advances, the friction of the falling water notches the ice in the opposite direction, so that the plunging action of the stream is limited to a comparatively small surface of the underlying rock ; for if the ice travel forward more rapidly than the stream can cut backward, a new chasm in all probability will open before long in the original position, and the cascade will begin to work afresh

Fig. 2.— Scratched stone from the till of Boston. Natural size about one . foot and a half long by ten inches wide. (From photograph.)

over : its former path.^ Boulders and debris from time to time slip into the stream and are swept down

^ These wells in the ice are called motilins.

the waterfall; they are hurled against the rock beneath, and whirled round and round before they are carried onward along the subglacial trench which the torrent has worn, so that at last a hollow begins to be formed. This, when once it can retain some of the fallen blocks, is quickly deepened. As in the bed of any mountain torrent, so here a pot-hole is ground out, but the peculiar mode of excavation usually makes the hollow beneath the glacier deeper in proportion to its breadth than in the other case. It is frequently several feet, occasionally some yards, in depth. During the retreat of the ice these hollows, called " giants' kettles," often become filled up with mud and stones.

Such, then, are the effects of a glacier: namely, perched blocks and moraines, terminal, lateral, and subglacial ; roches motUonn^es ; ~ and surfaces smoothed or polished and striated or channelled, pitted occasionally with giants' kettles. Where these occur, it may be inferred that a glacier formerly existed, even though at the present time the snow vanishes from every recess in the surrounding mountains long before the summer reaches its end. It must be remembered, of course, that rocks and stones may be worn or striated by other causes than the action of a glacier. A mound need not always be a moraine; a perched block may possibly be due to some other agency ; but if due care be taken to avoid mistakes, the aforesaid marks indicate the path of a glacier as certainly as our footprints on the sand show that some one has been walking on the sea-shore.

We must now proceed to examine the valleys below the ends of the glaciers. In such a mountainchain as the Alps, the evidence, with many variations in detail owing to local circumstances, is everywhere substantially the same. Take, for instance, the valley of the Aar in the neighbourhood of the Grimsel hospice. The little tarn beneath the precipices of the Nagelisgratli is surrounded by billowy hummocks of granitic rock, " like the backs of plunging dolphins." Far above the bed of the valley, up the steep crags leading to :the *' Dead Man's Lake," up the great mountain buttress round which the river bends sharply to the north, the rocks everywhere are rounded and smoothed and striated. (See Frontispiece^ All down the deep and narrow glen for some miles the story is the same: ice-worn, ice-scored rocks, perched blocks and scattered debris, prove that a vast glacier once welled up high against the precipitous slopes on either hand ; for the marks extend almost from the present level of the stream to some two or three thousand feet above it, and further examination shows that many a lateral glen, in which now perhaps only a small snow-bed lingers in some sheltered corrie, once gave birth to a tributary glacier.

Another thing, however, is obvious ; the dominant outlines of the valley are those indicative of the action of water, for it is V-shaped in section. It has been filled with ice, it has been modified by icje, but it has been blocked out by running water and the ordinary atmospheric forcea No one accustomed to travel in non-glaciated as well as in glaciated regions can fail to decipher the familiar characters of ordinary rain and river action, though these are sometimes blurred by the palimpsest writing of the ice- scribe. Moreover, with one or two slight exceptions, as at the Raterichsboden, the V is always acutely pointed. This fact, and the general freshness of the ice-worn surfaces, suggest that no very long time, as geologists reckon, has elapsed since the ice melted from this part of the valley. Below the Handeck Falls, on approaching Guttannen, the Haslithal widens a little, and the distinctive outlines are to some extent obscured; but no level space of any importance is found until the confluence of the Gadmenthal with the main valley is reached. Here is a grassy basin, the flat meadows of which suggest that it formerly may have been a small lake, which has been filled up by debris brought down by the torrent. Beyond this little oasis in the wild mountain-land the valley is barred by a limestone ridge, called the Kirchet, through which the Aar has cut a narrow gorge.-^ The side which faces the east is steep, almost precipitous, but up and over this, some 300 feet above the valley, the glacier has been

^ This cafion has been made accessible, and affords a wonderful study of the erosive action of a torrent, forced by the resistless pressure of the accumulating mass behind. Bocks, everywhere ice - worn, and sprinkled with boulders from the upper parts of the valleys, place this fact beyond question. On the western side of the Kirchet lies the level delta of the Lake of Brienz, over the site of which, as will presently be seen, the great ice-stream formerly passed on to the lowlands in the same general direction as is taken by the present river. Every valley in the Alps affords similar testimony, though it is not always written in characters equally distinct. In some districts the rocks disintegrate more easily, the valleys are wider, their floor is buried beneath gravel and alluvium, their sloping sides are covered by debris which has been swept down from the upper crags by avalanches or streams or local " cloud-bursta" In one place ice-worn rocks such as have been described are conspicuous; in another, erratics strewn over zones on the mountain-sides, or some solitary perched block, or an isolated terminal moraine, bear testimony which is equally valid.

On the steep slope leading from the valley of the Bhone into the Val d'lUiez, erratics, formerly many hundreds, if not thousands, in number, are strewn among the vines and under the shadow of the Spanish chestnuts. They are mostly of crystalline rock, while the valley itself is wholly excavated in limestones and slates. They have been derived from various places higher up in the valley of the Bhone, one of

B i8 ICE-WOKK, PRESENT AND PAST

the commonest rocks being a protogine. This beyond all question has come from the aiguilles which rise above the Glacier du Trient at the eastern end of the Mont Blanc range. Generally these blocks are scattered over a zone varying from about lOO to 250 yards wide; occasionally three or four may be seen practically in contact. Numbers have been broken up for building, road-metalling, and wayside posts. The volume of the largest is estimated at 60,480 cubic feet, and on its flat top a pavilion has been built. Similar boulders — though generally not so large — may be traced along the shore of Lake Leman, even down to Geneva Here and there may be found a peculiar mottled green and white rock, which has started on its journey from crags on the upper part of the AUalin Glacier in the SaasthaL^ Erratic blocks, however, are not restricted to the neighbourhood of the Lake of Geneva; they are scattered over the lowland between the Alps and the Jura ; they may be traced up the flanks of the latter mountains; on the Ohasseron they reach an elevation of fully 3000 feet above the lowland, and from that their upper limit descends in either direction.^ On the limestone slopes of the Jura above