In all countries the fundamental rock on which the boulder formation reposes, if it consists of granite, gneiss, marble, or other hard stone capable of permanently retaining any superficial markings which may have been imprinted upon it, is smoothed or polished, and exhibits parallel striae and furrows having a determinate direction. This prevailing direction, both in Europe and North America, is evidently connected with the course taken by the erratic blocks in the same district, and is very commonly from north to south, or if it be twenty or thirty or more degrees to the east or west of north, still always corresponds to the direction in which the large angular and rounded stones have travelled.

These stones themselves also are often furrowed and scratched on more than one side, like those already spoken of as occurring in the glacial drift of Bedford, and in that of Norfolk.

When we contemplate the area which is now exposed to the abrading action of ice, or which is the receptacle of moraine matter thrown down from melting glaciers or bergs, we at once perceive that the submarine area is the most extensive of the two. The number of large icebergs which float annually to great distances in the northern and southern hemispheres is extremely great, and the quant.i.ty of stone and mud which they carry about with them enormous. Some floating islands of ice have been met with from 2 to 5 miles in length, and from 100 to 225 feet in height above water, the submerged portion, according to the weight of ice relatively to sea water, being from six to eight times more considerable than the part which is visible. Such ma.s.ses, when they run aground on the bottom of the sea, must exert a prodigious mechanical power, and may polish and groove the subjacent rocks after the manner of glaciers on the land. Hence there will often be no small difficulty in distinguishing between the effects of the submarine and supramarine agency of ice.

SCANDINAVIA ONCE COVERED WITH ICE, AND A CENTRE OF DISPERSION OF ERRATICS.

In the north of Europe, along the borders of the Baltic, where the boulder formation is continuous for hundreds of miles east and west, it has been long known that the erratic blocks, often of very large size, are of northern origin. Some of them have come from Norway and Sweden, others from Finland, and their present distribution implies that they were carried southwards, for a part at least of their way, by floating ice, at a time when much of the area over which they are scattered was under water. But it appears from the observations of Boetlingk, in 1840, and those of more recent inquirers, that while many blocks have travelled to the south, others have been carried northwards, or to the sh.o.r.es of the Polar Sea, and others north-eastward, or to those of the White Sea. In fact, they have wandered towards all points of the compa.s.s, from the mountains of Scandinavia as a centre, and the rectilinear furrows imprinted by them on the polished surfaces of the mountains where the rocks are hard enough to retain such markings, radiate in all directions, or point outwards from the highest land, in a manner corresponding to the course of the erratics above mentioned.*

(* Sir R.I. Murchison, in his "Russia and the Ural Mountains" (1845) has indicated on a map not only the southern limits of the Scandinavian drift, but by arrows the direction in which "it proceeded eccentrically from a common central region.")

Before the glacial theory was adopted, the Swedish and Norwegian geologists speculated on a great flood, or the sudden rush of an enormous body of water charged with mud and stones, descending from the central heights or watershed into the adjoining lower lands. The erratic blocks were supposed in their downward pa.s.sage to have smoothed and striated the rock surfaces over which they were forced along.

It would be a waste of time, in the present state of science, to controvert this hypothesis, as it is now admitted that even if the rush of a diluvial current, invented for the occasion and wholly without a.n.a.logy in the known course of nature, be granted, it would be inadequate to explain the uniformity, parallelism, persistency, and rectilinearity of the so-called glacial furrows. It is moreover ascertained that heavy ma.s.ses of rock, not fixed in ice, and moving as freely as they do when simply swept along by a muddy current, do not give rise to such scratches and furrows.

M. Kjerulf of Christiania, in a paper lately communicated to the Geological Society of Berlin,* has objected, and perhaps with reason, to what he considers the undue extent to which I have, in some of my writings, supposed the mountains of northern Europe, to have been submerged during the glacial period.

(* "Zeitschrift der Deutschen Geologischen Gesellschaft"

Berlin 1860.)

He remarks that the signs of glacial action on the Scandinavian mountains ascend as high as 6000 feet, whereas fossil marine sh.e.l.ls of the same period never reach elevations exceeding 600 feet. The land, he says, may have been much higher than it now is, but it has evidently not been much lower since the commencement of the glacial period, or marine sh.e.l.ls would be traceable to more elevated points. In regard to the absence of marine sh.e.l.ls, I shall point out in the sequel how small is the dependence we can place on this kind of negative evidence, if we desire to test by it the extent to which the land has been submerged.

I cannot therefore consent to limit the probable depression and re-elevation of Scandinavia to 600 feet. But that the larger part of the glaciation of that country has been supramarine, I am willing to concede. In support of this view M. Kjerulf observes that the direction of the furrows and striae, produced by glacial abrasion, neither conforms to a general movement of floating ice from the Polar regions, nor to the shape of the existing valleys, as it would do if it had been caused by independent glaciers generated in the higher valleys after the land had acquired its actual shape. Their general arrangement and apparent irregularities are, he contends, much more in accordance with the hypothesis of there having been at one time a universal covering of ice over the whole of Norway and Sweden, like that now existing in Greenland, which, being annually recruited by fresh falls of snow, was continually pressing outwards and downwards to the coast and lower regions, after crossing many of the lower ridges, and having no relation to the minor depressions, which were all choked up with ice and reduced to one uniform level.

CONTINENTAL ICE OF GREENLAND.

In support of this view, he appeals to the admirable description of the continental ice of Greenland, lately published by Dr. H. Rink of Copenhagen,* who resided three or four years in the Danish settlements in Baffin's Bay, on the west coast of Greenland, between lat.i.tudes 69 and 73 degrees north.

(* "Journal of Royal Geographical Society" volume 23 1853 page 145.)

"In that country, the land," says Dr. Rink, "may be divided into two regions, the 'inland' and the 'outskirts.' The 'inland,' which is 800 miles from west to east, and of much greater length from north to south, is a vast unknown continent, buried under one continuous and colossal ma.s.s of permanent ice, which is always moving seaward, but a small proportion only of it in an easterly direction, since nearly the whole descends towards Baffin's Bay." At the heads of the fjords which intersect the coast, the ice is seen to rise somewhat abruptly from the level of the sea to the height of 2000 feet, beyond which the ice of the interior rises continuously as far as the eye can reach, and to an unknown alt.i.tude. All minor ridges and valleys are levelled and concealed, but here and there steep mountains protrude abruptly from the icy slope, and a few superficial lines of stones or moraines are visible at seasons when no recent snow has fallen. [23]

Although all the ice is moving seaward, the greatest quant.i.ty is discharged at the heads of certain large fjords, usually about 4 miles wide, which, if the climate were milder, would be the outlet of as many great rivers. Through these the ice is now protruded in huge blocks, several miles wide, and from 1000 to 1500 feet in height or thickness.

When these ma.s.ses reach the fjords, they do not melt or break up into fragments, but continue their course in a solid form in the salt water, grating along the rocky bottom, which they must polish and score at depths of hundreds and even of more than 1000 feet. At length, when there is water enough to float them, huge portions, having broken off, fill Baffin's Bay with icebergs of a size exceeding any which could be produced by ordinary valley glaciers. Stones, sand, and mud are sometimes included in these bergs which float down Baffin's Bay. At some points, where the ice of the interior of Greenland reaches the coast, Dr. Rink saw mighty springs of clayey water issuing from under the edge of the ice even in winter, showing the grinding action of the glacial ma.s.s mixed with sand on the subjacent surface of the rocks.

The "outskirts," where the Danish colonies are stationed, consist of numerous islands, of which Disco island is the largest in lat.i.tude 70 degrees north, and of many peninsulas, with fjords from 50 to 100 miles long, running into the land, and through which the ice above alluded to pa.s.ses on its way to the bay. This area is 30, 000 square miles in extent, and contains in it some mountains 4000 feet to 5000 feet high.

The perpetual snow usually begins at the height of 2000 feet, below which level the land is for the most part free from snow between June and August, and supports a vegetation of several hundred species of flowering plants, which ripen their seeds before the winter. There are even some places where phanerogamous plants have been found at an elevation of 4500 feet; a fact which, when we reflect on the immediate vicinity of so large and lofty a region of continental ice in the same lat.i.tude, well deserves the attention of the geologist, who should also bear in mind, that while the Danes are settled to the west in the "outskirts," there exists, due east of the most southern portion of this ice-covered continent, at the distance of about 1200 miles, the home of the Laplanders with their reindeer, bears, wolves, seals, walruses, and whales. If, therefore, there are geological grounds for suspecting that Scandinavia or Scotland or Wales was ever in the same glacial condition as Greenland now is, we must not imagine that the contemporaneous fauna and flora were everywhere poor and stunted, or that they may not, especially at the distance of a few hundred miles in a SOUTHWARD direction, have been very luxuriant. [24]

Another series of observations made by Captain Graah, during a survey of Greenland between 1823 and 1829, and by Dr. Pingel in 1830-32, adds not a little to the geological interest of the "outskirts," in their bearing on glacial phenomena of ancient date. Those Danish investigators, with one of whom, Dr. Pingel, I conversed at Copenhagen in 1834, ascertained that the whole coast from lat.i.tude 60 to about 70 degrees north has been subsiding for the last four centuries, so that some ancient piles driven into the beach to support the boats of the settlers have been gradually submerged, and wooden buildings have had to be repeatedly shifted farther inland.*

(* "Principles of Geology" chapter 30.)

In Norway and Sweden, instead of such a subsiding movement, the land is slowly rising; but we have only to suppose that formerly, when it was covered like Greenland with continental ice, it sank at the rate of several feet in a century, and we shall be able to explain why marine deposits are found above the level of the sea, and why these generally overlie polished and striated surfaces of rock.

We know that Greenland was not always covered with snow and ice, for when we examine the Tertiary strata of Disco Island (of the Upper Miocene period) we discover there a mult.i.tude of fossil plants, which demonstrate that, like many other parts of the arctic regions, it formerly enjoyed a mild and genial climate. Among the fossils brought from that island, lat.i.tude 70 degrees north, Professor Heer has recognised Sequoia Langsdorfii, a coniferous species which flourished throughout a great part of Europe in the Miocene period, and is very closely allied to the living Sequoia sempervirens of California. The same plant has been found fossil by Sir John Richardson within the arctic circle, far to the west on the Mackenzie River, near the entrance of Bear River, also by some Danish naturalists in Iceland to the east.

The Icelandic surturbrand, or lignite, of this age has also yielded a rich harvest of plants, more than thirty-one of them, according to Steenstrup and Heer, in a good state of preservation, and no less than fifteen specifically identical with Miocene plants of Europe. Thirteen of the number are arborescent; and amongst others is a tulip-tree (Liriodendron), with its fruit and characteristic leaves, a plane (Plata.n.u.s), a walnut, and a vine, affording unmistakable evidence of a climate in the parallel of the arctic circle which precludes the supposition of glaciers then existing in the neighbourhood, still less any general crust of continental ice, like that of Greenland.*

(* Heer, "Recherches sur la Vegetation du Pays tertiaire"

etc. 1861 page 178.)

As the older Pliocene flora of the Tertiary strata of Italy, like the sh.e.l.ls of the Coralline Crag, before adverted to, Chapter 12, indicate a temperature milder than that now prevailing in Europe, though not so warm as that of the Upper Miocene period, it is probable that the acc.u.mulation of snow and glaciers on the mountains and valleys of Greenland did not begin till after the commencement of the Pliocene period, and may not have reached its maximum until the close of that period.

Norway and Sweden appear to have pa.s.sed through all the successive phases of glaciation which Greenland has experienced, and others which that country will one day undergo, if the climate which it formerly enjoyed should ever be restored to it. There must have been first a period of separate glaciers in Scandinavia, then a Greenlandic state of continental ice, and thirdly, when that diminished, a second period of enormous separate glaciers filling many a valley now wooded with fir and birch. Lastly, under the influence of the Gulf Stream, and various changes in the height and extent of land in the arctic circle, a melting of nearly all the permanent ice between lat.i.tudes 60 and 70 north, corresponding to the parallels of the continental ice of Greenland, has occurred, so that we have now to go farther north than lat.i.tude 70 degrees before we encounter any glacier coming down to the sea coast.

Among other signs of the last retreat of the extinct glaciers, Kjerulf and other authors describe large transverse moraines left in many of the Norwegian and Swedish glens.

CHRONOLOGICAL RELATIONS OF THE HUMAN AND GLACIAL PERIODS IN SWEDEN.

We may now consider whether any, and what part, of these changes in Scandinavia may have been witnessed by Man. In Sweden, in the immediate neighbourhood of Upsala, I observed, in 1834, a ridge of stratified sand and gravel, in the midst of which occurs a layer of marl, evidently formed originally at the bottom of the Baltic, by the slow growth of the mussel, c.o.c.kle, and other marine sh.e.l.ls of living species intermixed with some proper to fresh water. The marine sh.e.l.ls are all of dwarfish size, like those now inhabiting the brackish waters of the Baltic; and the marl, in which myriads of them are embedded, is now raised more than 100 feet above the level of the Gulf of Bothnia. Upon the top of this ridge (one of those called osars in Sweden) repose several huge erratics consisting of gneiss, for the most part unrounded, from 9 to 16 feet in diameter, and which must have been brought into their present position since the time when the neighbouring gulf was already characterised by its peculiar fauna. Here, therefore, we have proof that the transport of erratics continued to take place, not merely when the sea was inhabited by the existing Testacea, but when the north of Europe had already a.s.sumed that remarkable feature of its physical geography, which separates the Baltic from the North Sea, and causes the Gulf of Bothnia to have only one-fourth of the saltness belonging to the ocean.

I cannot doubt that these large erratics of Upsala were brought into their present position during the Recent period, not only because of their moderate elevation above the sea-level in a country where the land is now rising every century, but because I observed signs of a great oscillation of level which had taken place at Sodertelje, south of Stockholm (about 45 miles distant from Upsala), after the country had been inhabited by Man. I described, in the "Philosophical Transactions"

for 1835, the section there laid open in digging a level in 1819, which showed that a subsidence followed by a re-elevation of land, each movement amounting to more than 60 feet, had occurred since the time when a rude hut had been built on the ancient sh.o.r.e. The wooden frame of the hut, with a ring of hearthstones on the floor, and much charcoal, were found, and over them marine strata, more than 60 feet thick, containing the dwarf variety of Mytilus edulis, and other brackish-water sh.e.l.ls of the Bothnian Gulf. Some vessels put together with wooden pegs, of anterior date to the use of metals, were also embedded in parts of the same marine formation, which has since been raised, so that the upper beds are more than 60 feet above the sea-level, the hut being thus restored to about its original position relatively to the sea.

We have seen in the account of the Danish kitchen-middens of the Recent period that even at the comparatively late period of their origin the waters of the Baltic had been rendered more salt than they are now. The Upsala erratics may belong to nearly the same era as these. But were we to go back to a long antecedent epoch, or to that of the Belgian and British caves with their extinct animals, and the signs they afford of a state of physical geography departing widely from the present, or to the era of the implement-bearing alluvium of St. Acheul, we might expect to find Scandinavia overwhelmed with glaciers, and the country uninhabitable by Man. At a much remoter period the same country was in the state in which Greenland now is, overspread with one uninterrupted coating of continental ice, which has left its peculiar markings on the highest mountains. This period, probably anterior to the earliest traces yet brought to light of the human race, may have coincided with the submergence of England, and the acc.u.mulation of the boulder-clay of Norfolk, Suffolk, and Bedfordshire, before mentioned. It has already been stated that the syenite and some other rocks of the Norfolk till seem to have come from Scandinavia, and there is no era when icebergs are so likely to have floated them so far south as when the whole of Sweden and Norway were enveloped in a ma.s.sive crust of ice; a state of things the existence of which is deduced from the direction of the glacial furrows, and their frequent unconformity to the shape of the minor valleys.

GLACIAL PERIOD IN SCOTLAND [25].

Professor Aga.s.siz, after his tour in Scotland in 1840, announced the opinion that erratic blocks had been dispersed from the Scottish mountains as from an independent centre, and that the capping of ice had been of extraordinary thickness.*

(* Aga.s.siz, "Proceedings of the Geological Society" 1840 and "Edinburgh Philosophical Journal" 49 page 79.)

Mr. Robert Chambers, after visiting Norway and Sweden, and comparing the signs of glacial action observed there with similar appearances in the Grampians, came to the conclusion that the Highlands both of Scandinavia and Scotland had once been "moulded in ice," and that the outward and downward movement and pressure of the frozen ma.s.s had not only smoothed, polished, and scratched the rocks, but had, in the course of ages, deepened and widened the valleys, and produced much of that denudation which has commonly been ascribed exclusively to aqueous action. The glaciation of the Scotch mountains was traced by him to the height of at least 3000 feet.*

(* "Ancient Sea Margins" Edinburgh 1848. Glacial Phenomena "Edinburgh New Philosophical Journal" April 1853 and January 1855.)

Mr. T.F. Jamieson, of Ellon, in Aberdeenshire, has recently brought forward an additional body of facts in support of this theory. According to him the Grampians were at the period of extreme cold enveloped "in one great winding sheet of snow and ice," which reached everywhere to the coast-line, the land being then more elevated than it is now. He describes the glacial furrows sculptured on the solid rocks as pointing in Aberdeenshire to the south-east, those of the valley of the Forth at Edinburgh, from west to east, and higher up the same valley at Stirling, from north-west to south-east, as they should do if the ice had followed the lines of what is now the princ.i.p.al drainage. The observations of Sir James Hall, Mr. Maclaren, Mr. Chambers, and Dr. Fleming, are cited by him in confirmation of this arrangement of the glacial markings, while in Sutherland and Ross-shire he shows that the glacial furrows along the north coast point northwards, and in Argyleshire westwards, always in accordance with the direction of the princ.i.p.al glens and fjords.

Another argument is also adduced by him in proof of the ice having exerted its mechanical force in a direction from the higher and more inland country to the lower region and sea-coast. Isolated hills and minor prominences of rock are often polished and striated on the land side, while they remain rough and jagged on the side fronting the sea.

This may be seen both on the east and west coast. Mention is also made of blocks of granite which have travelled from south to north in Aberdeenshire, of which there would have been no examples had the erratics been all brought by floating ice from the arctic regions when Scotland was submerged. It is also urged against the doctrine of attributing the general glaciation to submergence, that the glacial grooves, instead of radiating as they do from a centre, would, if they had been due to ice coming from the north, have been parallel to the coast-line, to which they are now often almost at right angles. The argument, moreover, which formerly had most weight in favour of floating ice, namely, that it explained why so many of the stones did not conform to the contour and direction of the minor hills and valleys, is now brought forward, and with no small effect, in favour of the doctrine of continental ice on the Greenlandic scale, which, after levelling up the lesser inequalities, would occasionally flow in mighty ice-currents, in directions often at a high angle to the smaller ridges and glens.

The application to Scandinavia and Scotland of this theory makes it necessary to reconsider the validity of the proofs formerly relied on as establishing the submergence of a great part of Scotland beneath the sea, at some period subsequent to the commencement of the glacial period. In all cases where marine sh.e.l.ls overlie till, or rest on polished and striated surfaces of rock, the evidence of the land having been under water, and having been since upheaved, remains unshaken; but this special proof rarely extends to heights exceeding 500 feet. In the basin of the Clyde we have already seen that Recent strata occur 25 feet above the sea-level, with existing species of marine testacea, and with buried canoes, and other works of art. At the higher level of 50 feet occurs the well-known raised beach of the western coast, which, according to Mr. Jamieson, contains, near Fort William and on Loch Fyne and elsewhere, an a.s.semblage of sh.e.l.ls implying a colder climate than that of the 25-foot terrace, or that of the present sea; just as, in the valley of the Somme, the higher-level gravels are supposed to belong to a colder period than the lower ones, and still more decidedly than that of the present era. At still greater elevations, older beds containing a still more arctic group of sh.e.l.ls have been observed at Airdrie, 14 miles south-east of Glasgow, 524 feet above the level of the sea. They were embedded in stratified clays, with the unstratified boulder till both above and below them, and in the overlying unstratified drift were some boulders of granite which must have come from distances of 60 miles at the least.*

(* Smith of Jordanhill, "Quarterly Journal of the Geological Society" volume 6 1850 page 387.)

The presence of Tellina calcarea, and several other northern sh.e.l.ls, implies a climate colder than that of the present Scottish seas. In the north of Scotland, marine sh.e.l.ls have been found in deposits of the same age in Caithness and in Aberdeenshire at heights of 250 feet, and on the sh.o.r.es of the Moray Firth, as at Gamrie in Banff, at an elevation of 350 feet; and the stratified sands and beds of pebbles which belong to the same formation ascend still higher--to heights of 500 feet at least.*

(* Prestwich, "Proceedings of the Geological Society" volume 2 page 545; Jamieson, "Quarterly Journal of the Geological Society" volume 16 1860.)

At much greater heights, stratified ma.s.ses of drift occur in which hitherto no organic remains, whether of marine or freshwater animals, have ever been found. It is still an undecided question whether the origin of all such deposits in the Grampians can be explained without the intervention of the sea. One of the most conspicuous examples has been described by Mr. Jamieson as resting on the flank of a hill called Meal Uaine, in Perthshire, on the east side of the valley of the Tummel, just below Killiecrankie. It consists of perfectly horizontal strata, the lowest portion of them 300 feet above the river and 600 feet above the sea. From this elevation to an alt.i.tude of nearly 1200 feet the same series of strata is traceable, continuously, up the slope of the mountain, and some patches are seen here and there even as high as 1550 feet above the sea. They are made up in great part of finely laminated silt, alternating with coa.r.s.er materials, through which stones from 4 to 5 feet in length are scattered. These large boulders, and some smaller ones, are polished on one or more sides, and marked with glacial striae.

The subjacent rocks, also, of gneiss, mica slate, and quartz, are everywhere grooved and polished as if by the pa.s.sage of a glacier.*

(* Jamieson, "Quarterly Journal of the Geological Society"

volume 16 1860 page 360.)

At one spot a vertical thickness of 130 feet of this series of strata is exposed to view by a mountain torrent, and in all more than 2000 layers of clay, sand, and gravel were counted, the whole evidently acc.u.mulated under water. Some beds consist of an impalpable mud, like putty, apparently derived from the grinding down of felspar, and resembling the mud produced by the grinding action of modern glaciers.

Mr. Jamieson, when he first gave an account of this drift, inferred, in spite of the absence of marine sh.e.l.ls, that it implied the submergence of Scotland beneath the ocean after the commencement of the glacial period, or after the era of continental ice indicated by the subjacent floor of polished and grooved rock. This conclusion would require a submergence of the land as far up as 1550 feet above the present sea-level, after which a great re-upheaval must have occurred. But the same author, having lately revisited the valley of the Tummel, suggests another possible, and I think probable, explanation of the same phenomena. The stratified drift in question is situated in a deep depression between two b.u.t.tresses of rock, and if an enormous glacier be supposed to have once filled the valley of the Tummel to the height of the stratified drift, it may have dammed up the mouth of a mountain torrent by a transverse barrier, giving rise to a deep pond, in which beds of clay and sand brought down by the waters of the torrent were deposited. Charpentier in his work on the Swiss glaciers has described many such receptacles of stratified matter now in progress, and due to such blockages, and he has pointed out the remnants of ancient and similar formations left by extinct glaciers of an earlier epoch. He specially notices that angular stones of various dimensions, often polished and striated, which rest on the glacier and are let fall when the torrent undermines the side of the moving ice, descend into the small lake and become interstratified with the gravel and fine sediment brought down by the torrent into the same.*