Celt will be driven or improved out of these islands, and will retire
to other lands, where, mingling and intermarrying with Teutons, he
will eventually disappear, but not without leavening the races amongst
which he is destined to vanish. And who will take his place in the
Long Island? Probably a few farmers, a few shepherds, and a sprinkling
of gamekeepers; and it is just possible that a few fishermen also may
be allowed to settle down here and there upon the coast. One may see
the process going on at present. Large tracts that once supported many
villages are now quite depopulated. The time will come when somebody in
Parliament will move for the reduction of the Civil Service estimates
by the amount of the sheriff-substitute's salary, and when the jail at
Lochmaddy will have nothing higher in the scale of being to imprison
than some refractory ram. One may be pardoned for wishing that he could
foretell for the islands another fate than this. It is sad to think
that a fine race of people is thus surely passing away from amongst
us, for, despite all that can be urged against them, they are what I
say. The fishermen of Lewis and Barra are bold, stalwart fellows, whom
it would be difficult to peer amongst any similar class of men on the
mainland. And all through the island one meets with equally excellent
specimens of our kind. Many a brave soldier who fought our battles in
the great French wars hailed from these outer islands. Pity it is that
no feasible plan to prevent the threatened scattering of the race has
yet been brought forward. Some day we may regret this, and come to
think that though mutton and wool in the Long Island are desirable, yet
islanders would have been better.
[Postscript.--On pages 153.4 I have described the second general
ice-sheet that overflowed the Outer Hebrides as having eventually
become resolved into a series of local ice-sheets and glaciers.
Subsequent research, however, has since led me to believe that the
district ice-sheets and local glaciers referred to were not the
direct descendants of the last great ice-sheet. They appear to
have come into existence long after that ice-sheet had entirely
disappeared. _See_ Article X.]
VI.
The Ice Age in Europe and North America.[K]
[K] Address to the Geological Society of Edinburgh, 1884.
In casting about for a subject upon which to address you this evening,
I thought I could hardly do better than give you the result of a
comparison which I have recently been able to make between the glacial
phenomena of Europe and North America. The subject of glaciation seems
to be now somewhat worn; but I gather from the fact that writers can
still be found who see in our superficial deposits strong evidence of
the Deluge, that a short outline of what we really do know may not be
unacceptable. In the short time at our disposal, it is obvious that
I cannot enter into much detail, and that many interesting questions
must remain untouched. It will be as well, therefore, that I should at
the outset define the limits of the present inquiry, and state clearly
what are the chief points to which I wish to direct your attention. My
main object, then, will be to bring into prominence such evidence as
seems to betoken in a special manner the uniformity of conditions that
obtained in the northern hemisphere during the Ice Age. In other words,
I shall confine myself to a description of certain characteristic and
representative phenomena which are common to Europe and North America,
with the view of showing that the physical conditions of the glacial
period were practically the same in both continents.
The phenomena which might be considered under this head embrace nearly
all the facts with which glacialists are familiar, but I purpose
restricting myself to three questions only, viz.:--
1st. _The extent of glaciation._
2nd. _Changes of climate during the Ice Age._
3rd. _The results of fluvio-glacial action._
The consideration of these questions, even if it were exhaustive
(which it cannot be on this occasion), would still leave the general
subject very incomplete, for we must forego the discussion of all
such interesting topics as the "connection between glaciation and
submergence," "the formation of rock-basins," and the "origin of the
geographical distribution of our faunas and floras." Confining my
inquiry within the limits just specified, I shall begin by sketching
broadly the general results obtained by glacialists in Europe, and
thereafter I shall proceed to give an outline of the corresponding
conclusions arrived at by American observers.
I.
_The Extent of Glaciation in Europe._
To what extent, then, let us ask, has Europe been glaciated? What areas
have been covered with perennial snow and ice? Owing to the fulness and
clearness of the evidence, we are able to give a very definite answer
to this question. It is hardly too much to say that we are as well
acquainted with the distribution of glacier-ice in Europe during the
Ice Age as we are with that of existing snow-fields and glaciers.
The nature of the evidence upon which our knowledge is based
is doubtless familiar to many whom I have the pleasure of now
addressing, but for the sake of those who have not such familiarity
with the subject I may be allowed to indicate very briefly its
general character. A rock-surface over which ice has flowed for any
considerable time exhibits either an abraded, worn, and smoothed
appearance, or the rocks are disrupted and broken, and larger or
smaller fragments are found to have been removed and carried forward
in the direction followed by the ice. Now, ice-worn and shattered
rock-surfaces of this description, such as can be seen underneath
existing glaciers, occur more or less abundantly over vast regions in
Europe. They are met with from the North Cape south as far as Leipzig,
and from the Outer Hebrides east to the valley of the Petchora and the
foot-slopes of the Ural Mountains. Nor are they confined to northern
Europe. They appear again and again in France and Spain and Italy,
and in the low-grounds of middle Europe, where they occupy positions
now far removed from the influence of glacial action. Such ice-worn
and disrupted rock-surfaces not only prove that glacier-ice formerly
covered large portions of our Continent, but they also indicate for
us the directions in which that enveloping ice moved. The smoother
surfaces in question are very frequently marked with coarse and fine
parallel scratches and grooves of precisely the same nature and origin
as the scratches and grooves which characterise the rocky bed of a
modern glacier. And these markings, having been produced by the sand,
grit, and stones which are pushed and dragged over the rocks by flowing
ice, necessarily discover for us the path of glacial movement. But
all rocks subjected to glacial action are not necessarily smoothed
and polished. Sometimes, owing to structural peculiarities, and for
various other reasons, rocks cannot resist the pressure of the ice,
but are crushed and broken, and the resulting fragments are rolled and
dragged forward in the direction of ice-flow. In this manner the path
of a glacier becomes strewed with débris which has from time to time
been forced from its rocky bed. There is really no mystery, therefore
in tracking the spoor of extinct glaciers; for we have two sets of
facts to aid us, either of which might suffice to indicate the extent
and direction of glaciation. Consider, however, for a moment, what one
observes in connection with rock-striation. We have, in the first
place, the rounding and smoothing, and the parallel ruts and striæ. Not
only so, but we frequently find that one side of prominent projecting
knolls and hills is more highly worn and abraded than the other. Often,
indeed, one side may show no trace whatsoever of abrasion. Here, again,
we have clear evidence of the direction of ice-flow. Who can doubt that
the worn and abraded rocks look towards the point whence the ice came,
and that the non-glaciated rocks in the rear have been sheltered by the
rocks in front? It is for this reason that in the mountainous regions
of northern Europe the striated and smoothed rock-surfaces invariably
look up the valleys, while the broken and unworn rock-ledges face in
the opposite direction.
Once more, note the manner in which the sub-glacial rock-rubbish,
consisting of clay, sand, grit, stones, and boulders, has been amassed.
In places where the ice must have moved more or less rapidly, as on
considerable slopes, no accumulation took place, while in the rear
of projecting crags and knobs of rock, sub-glacial materials often
gathered deeply. Again, over low-lying tracts, where the motion of
the ice would necessarily be retarded, clay, sand, and stones tended
to collect. And this particularly appears to have been the case in
those regions where the slow-creeping and gradually thinning ice-sheet
approached its terminal line. Hence it is that we encounter such thick
and wide-spread sheets of sub-glacial detritus upon the undulating
low-grounds and plains of southern Sweden, Denmark, Schleswig-Holstein,
Holland, northern Germany, Poland, and Russia.
The sub-glacial débris to which I specially refer is known as _Till_
or _Boulder-clay_ in this country, as _Krosstenslera_ in Sweden, as
_Geschiebelehm_ or _Geschiebemergel_ in Germany, and as _Grundmoräne_
or _Moraine profonde_ in Switzerland. Its general characters are too
well known to require more than the briefest summary. In general this
peculiar accumulation is an unstratified clay, containing, scattered
higgledy-piggledy through it, stones and boulders of all shapes and
sizes. Many of these rock-fragments are smoothed and striated, and
even the smallest particles, when viewed under the microscope, often
show delicate scratches. Frequently, too, the clay is excessively hard
and tough, and in many places it shows a kind of pseudo-lamination,
which is generally more or less crumpled, and often highly involved.
These appearances prove that the clay has not only been subjected to
intense pressure, but has actually been rolled over upon itself. I
need only refer to the plentiful occurrence of "slickensides" in such
clays--the joints by which the clay is often traversed showing such
polishing clearly on their faces. These, and many other facts which
time forbids me to mention, have received an explanation which has now
been generally adopted by European glacialists. The boulder-clay or
till is considered by them to represent the ground- or bottom-moraine
of glacier-ice. There used to be a notion prevalent amongst geologists
in our country that this clay was almost peculiar to these islands.
It occurs, however, in most countries of Europe. Vast regions in the
north are more or less continuously covered by it, and we meet with
it abundantly also upon the low-grounds of Switzerland, from which it
may be followed far down the great valley of the Rhone into the sunny
plains of France. The lower valleys of the Pyrenees and other Spanish
ranges show it well, and it is conspicuous likewise in northern Italy,
especially over the low tracts at the mouths of the great lake-valleys.
In all those places one can see boulder-clay of as pronounced a
character as any to be met with in Scotland.
Danish, Dutch, German, and Russian geologists have of late years
devoted much attention to the study of this clay, which is so
remarkably developed in their respective countries. It has been long
well known that a large proportion of the stones and boulders contained
in the till are of northern derivation, but it is only of recent years
that we have ascertained the particular routes by which those wanderers
or erratics have travelled. The rock-fragments in question have been
tracked back, as it were, to their parent masses, and thus, partly
in this way, and partly by the evidence of ice-worn surfaces, we have
been enabled to follow the spoor of the great northern ice-sheet in a
most satisfactory manner. Let one or two examples suffice. Boulders
derived from Lapland and Finland occur in the till at St. Petersburg,
and have been traced south-east to Moscow. Again, fragments carried
from Gottland, in the Baltic, are met with in the boulder-clay of east
Prussia, and have been followed south to beyond Berlin. In like manner
boulders of well-known Scanian rocks appear in the boulder-clay of
Leipzig. So also Swedish and Norwegian rock-fragments are seen in the
boulder-clay of Denmark, Hanover, and Holland.
Very wide areas in northern Germany are covered with an almost
continuous sheet of glacial detritus, so that it is only occasionally
that the underlying rocks crop out at the surface. Striated
rock-surfaces are therefore by no means so commonly exposed as in
regions like the Lowlands of Scotland. They are not wanting, however,
and their evidence is very striking. Thus, in the neighbourhood of
Leipzig and Dresden, we find glacial striæ impressed upon certain
highly-abraded and ice-worn hillocks of porphyry, the striæ being the
work of ice which flowed into Saxony from the north. Similar striæ;,
having a general southerly trend, occur at Rüdersdorf, near Berlin,
at Gommern, near Magdeburg, at Velpke in Brunswick, at Osnabrück in
Hanover, and at other places. Again, we encounter remarkable evidence
of the powerful pressure exerted by the ice in the displacement and
removal of huge blocks of strata. In Saxony, for example, the Tertiary
strata are turned up, pushed out of place, and involved in boulder-clay
to such an extent that the brown coals have often been mined for in
this strange position. Witness also the extensive displacements and
dislocations of the Cretaceous formation in the Danish islands of the
Baltic. So great are the contortions and displacements of the Chalk in
Moen, that these disturbances were formerly attributed to subterranean
action. Along the north-east coast of that island, cliffs 400 feet in
height exhibit the Cretaceous beds thrown upon end, twisted, bent,
and even inverted, boulder-clay being squeezed into and between the
disjointed and ruptured rock-masses.
From a study of these and similar phenomena, it has been demonstrated
that during the climax of the Ice Age a very large part of northern
Europe was buried under a thick covering of glacier-ice. And it has
been conclusively shown that this ice-sheet streamed outwards in all
directions from the high-grounds of Scandinavia, for which reason it is
often spoken of as the Scandinavian ice-sheet. But as it was fed, not
from the snow-fields of Scandinavia alone, but from the precipitation
of snow over its whole surface, it is better, I think, to speak of it
as the northern ice-sheet. In the extreme north of Scandinavia the ice
flowed northward into the Arctic Ocean, while south of the dominant
watershed of Lapland and Sweden its course in those high latitudes was
east and south-east. It filled up the depressions of the White Sea, the
Gulf of Bothnia, and the Baltic, extending east to the valley of the
Petchora and the base of the Ural Mountains, and south-east to Kazan,
some 200 miles east of Nijnii-Novgorod. From this point its terminal
front trended a little west of south, until it reached the fiftieth
parallel of latitude. Undulating a few miles south and north of this
parallel, it swept directly west through Russia into Galicia, till it
touched the foot-hills of the Carpathian range. After this we follow it
along the northern base of the Riesen Gebirge, the Erz Gebirge, and the
Harz, and thence westward through Hanover, and into the Low Countries,
as far south at least as the mouth of the Rhine. Throughout the vast
regions lying west and north of this terminal line, the track followed
by the ice has been well ascertained. It was east and south-east in
Russia, southerly in east Prussia, south-westerly in Denmark, Hanover,
and Holland.
The action of a mass of glacier-ice, reaching a thickness of several
thousand feet, must necessarily have resulted in extensive erosion of
the rocks over which it passed. Everywhere, therefore, throughout the
vast area just indicated, we meet with evidence of severe erosion.
But, as one should expect, such erosion is most marked in the hilly
regions--in those areas where steep slopes induced more rapid motion of
the ice, and where projecting crags and hills opposed the advance of
the eroding agent. All such prominent obstructions were energetically
assailed--abraded, rounded, worn, and smoothed, or crushed, shattered,
dislocated, and displaced. The high-grounds of Scandinavia and Finland,
formed for the most part of tough, crystalline rocks, or of more or
less durable strata, show everywhere _roches moutonnées_--smoothed and
rounded rocks--while innumerable rock-basins have been scooped out in
front of prominent crags and hills. In Denmark and other countries,
where less durable rocks prevail, the strata have often been broken
and disrupted, and pushed out of place. But as regions formed of such
rocks are generally gently-undulating, and seldom show abrupt crags and
hills, they oppose few obstructions to the advance of an ice-sheet.
When the northern ice-sheet flowed into Russia and Germany, it crept
over a low-lying and, for the most part, gently-undulating surface;
and although here and there the form of the ground favoured glacial
erosion and disruption, and extensive displacements of rock-masses
took place, yet, upon the whole the low-lying regions referred to
became areas of accumulation. The sub-glacial detritus--ground out
or wrenched away from the rough Scandinavian plateau and the uplands
of Finland--was dragged on underneath the ice, and spread over the
great plains lying to the south-east and south, as the gradually
attenuated ice-sheet crawled to its terminal line. My friend Dr. Amund
Helland, the well-known Norwegian geologist, has made an estimate of
the amount of rock-débris derived from Scandinavia and Finland which
lies scattered over the low-grounds of northern Europe. According to
him, the area in Denmark, Holland, Germany, and Russia (exclusive of
Finland), over which northern detritus is scattered, contains about
2,100,000 square kilometres, and the average thickness of the deposits
is about 150 feet, of which, however, only two-thirds, or 100 feet, are
of northern origin, the remaining third consisting of local materials.
Taking, then, 100 feet as fairly representing the average thickness
of the rock-rubbish derived from Finland and Scandinavia, the area of
which is given as 800,000 square kilometres, there is enough of this
material to raise the general surface of those lands by 255 feet. The
same amount of material would suffice to fill up all the numerous lakes
of Finland and Sweden sixteen or seventeen times over. Or, if tumbled
into the Baltic, it would fill the basin of that sea one and a half
times. In short, enough northern rock-débris lies upon the low-grounds
of northern Europe, which, were it restored to the countries from which
it has been taken, would obliterate all the lake-hollows of Finland
and Sweden, raise the level of those lands by 80 feet, and fill up the
entire basin of the Baltic, with all its bays. And yet this estimate
leaves out of account all the material which the ice-sheet carried away
from Norway and the British Islands.
Of the glaciation of our own land I need say very little. The
configuration of our country necessarily made it a centre of dispersion
during the Ice Age, and the ice which covered Ireland, Scotland, and
the major portion of England radiated outwards from the dominant
elevations of the land. But as the ice creeping outwards from those
centres became confluent, the directions which it followed were often
considerably modified, especially upon the low-grounds. We know that
the British ice-sheet not only covered the land up to near the tops of
our higher mountains, but filled up all our seas and extended into the
Atlantic beyond the coasts of Ireland and the Outer Hebrides--these
latter islands having been glaciated from the east by the ice that
flowed outwards from the mainland. Another point upon which we are now
well assured is the fact that the British and Scandinavian ice-sheets
coalesced, so that the basin of the North Sea completely brimmed over
with glacier-ice.
Finally, then, in contemplating the physical conditions that obtained
in northern Europe at the climax of the Ice Age, we have to picture to
ourselves the almost total obliteration under a vast ice-sheet of all
the land-features of the British Islands, Scandinavia, and Finland, and
the adjacent low-lying tracts of Denmark, Holland, Germany, Poland, and
Russia. If at that distant date a prehistoric man could have stood on
the summit of Snaehatten, he would have seen an apparently interminable
plain of snow and ice, bounded only by the visible horizon. Could
he have followed the plain southwards in hopes of escaping from it,
he would have descended its gently-sloping surface by imperceptible
gradations for a distance of 700 miles, before he reached its
termination at the foot of the mountains of middle Germany. Or, could
he have set out upon an easterly course, he would have crossed the Gulf
of Bothnia, buried several thousand feet beneath him, and touched the
foot-slopes of the Ural Mountains before he gained the terminal front
of the ice-cap, a distance of 1600 miles. On the other hand, had he
walked south-west in the direction of Ireland, he would have traversed
the area of the North Sea at a height of several thousand feet above
its bed, and, crossing the British area, would only have reached the
ice-front at a point some 50 miles beyond the coast of Ireland. Here he
would have seen the ice-sheet presenting a steep face to the assaults
of the Atlantic, and breaking away in massive tabular bergs, like those
which are calved by the ice-cap of the Antarctic regions.
I must now pass rapidly in review the facts relating to the glaciation
of the mountainous regions which lay outside of the area covered by
the northern ice-sheet. The glaciers of the Alps of Switzerland, about
which so much has been written, and the study of which first gave
Venetz, Charpentier, and Agassiz the clue to the meaning of striated
rocks, boulder-clay, and erratics, are, as is well known, the puny
descendants of former gigantic ice-flows. At the culmination of the Ice
Age all the mountain-valleys of Switzerland and northern Italy were
choked with glaciers that streamed out upon the low-grounds. Along
the northern slopes of the Alps, as in Bavaria and Würtemberg, these
glaciers coalesced to form a considerable ice-sheet, and so likewise
did the glaciers that descended from Switzerland, Savoy, and Dauphiny,
into the great valley of the Rhone. Even in north Italy the same was
the case with the glaciers that occupied the valleys in which now lie
Lakes Orta, Maggiore, Varese, Lugano, and Como--the united ice-flows of
those valleys forming a glacier which deployed upon the plains of the
Po, with a frontage of not less than 40 miles.
To the north of the Alps, the Vosges Mountains and the Black Forest,
the Harz, the Erz Gebirge, the Riesen Gebirge, and the Böhmer-Wald--all
had their perennial ice and glaciers, although none of those elevated
tracts now reaches the snow-line. It was the same with the Carpathians
and the Urals, amongst which we meet with relics of much larger
ice-streams than any that now exist in the Alps. Considerably further
south were the glaciers of the Despoto Dagh of Roumelia. Great glaciers
also in former times descended from the Caucasus, and in many hilly
regions of Asia Minor indubitable traces of similar large ice-flows
have been detected. The high-grounds of central France, and the
mountains of Beaujolais and Lyonnais supported considerable glaciers,
while from the Pyrenees numerous glaciers of the first class flowed out
upon the low-grounds of France, and considerable ice-streams occupied
the mountain-valleys on the Spanish side. Other Peninsular chains--the
Serra da Estrella, the Sierra Guadarama, and the Sierra Nevada--had
likewise their snow-fields and ice-streams. The same was the case
with the Apennines and the Apuan Alps of Italy, the traces of former
glacial action being conspicuous over a considerable part of Tuscany.
Even in Corsica we encounter the same evidence of glaciation--striated
rock-surfaces and moraines--which point to the former descent of
considerable glaciers from Monte Rotondo.