The growth of science is becoming so overwhelming that the old
subdivisions of human knowledge are no longer sufficient for the
purpose of dividing the labor of experts. It is scarcely possible now
for any man to become a naturalist, a chemist, or a physicist in the
full sense of either term; he must, if he aims at thoroughness, be
satisfied with a general knowledge of the great body of science, and
a special and a full acquaintance with only one or two of its minor
subdivisions. Thus geology, though but a branch of natural history,
and the youngest of its branches, has now become so extensive that its
ablest votaries are compelled to devote their best efforts to the study
of sections which but a few years ago were scarcely definable.
Glaciation is one of these, which now demands its own elementary
text-books over and above the monographs of original investigators.
This demand has been well supplied by Mr. James Geikie in the “The
Great Ice Age,”[15] of which a second edition has just been issued.
Every student of glacial phenomena owes to Mr. Geikie a heavy debt
of gratitude for the invaluable collection of facts and philosophy
which this work presents. It may now be fairly described as a standard
treatise on the subject which it treats.
One leading feature of the work offers a very aggressive invitation to
criticism. Scotchmen are commonly accused of looking upon the whole
universe through Scotch spectacles, and here we have a Scotchman
treating a subject which affects nearly the whole of the globe, and
devoting about half of his book to the details of Scottish glacial
deposits; while England has but one-third of the space allowed to
Scotland, Ireland but a thirtieth, Scandinavia less than a tenth, North
America a sixth, and so on with the rest of the world. Disproportionate
as this may appear at first glance, further acquaintance with the work
justifies the pre-eminence which Mr. Geikie gives to the Scotch glacial
deposits. Excepting Norway, there is no country in Europe which affords
so fine a field for the study of the vestiges of extinct glaciers as
Scotland, and Scotland has an advantage even over Norway in being much
better known in geological detail. Besides this, we must always permit
the expounder of any subject to select his own typical illustrations,
and welcome his ability to find them in a region which he himself has
directly explored.
Mr. Geikie’s connection with the geological survey of Scotland has
afforded him special facilities for making good use of Scottish typical
material, and he has turned these opportunities to such excellent
account that no student after reading “The Great Ice Age” will find
fault with its decided nationality.
The leading feature—the basis, in fact—of this work deserves especial
notice, as it gives it a peculiar and timely value of its own. This
feature is that the subject—as compared with its usual treatment by
other leading writers—is turned round and presented, so to speak,
bottom upwards. De Saussure, Charpentier, Agassiz, Humboldt, Forbes,
Hopkins, Whewell, Stark, Tyndall, etc., have studied the living
glaciers, and upon the data thus obtained have identified the work
of extinct glaciers. Chronologically speaking, they have proceeded
backwards, a method absolutely necessary in the early stages of the
inquiry, and which has yielded admirable results. Geikie, in the work
before us, proceeds exactly in the opposite order. Availing himself
of the means of identifying glacial deposits which the retrogressive
method affords, he plunges at once to the lowest and oldest of these
deposits, which he presents the most prominently, and then works
upwards and onwards to recent glaciation.
The best illustration I can offer of the timely advantage of this
reversed treatment is (with due apology for necessary egotism) to
state my own case. In 1841, when the “glacial hypothesis,” as it was
then called, was in its infancy, Professor Jamieson, although very
old and nearly at the end of his career, took up the subject with
great enthusiasm, and devoted to it a rather disproportionate number
of lectures during his course on Natural History. Like many of his
pupils, I became infected by his enthusiasm, and went from Edinburgh
to Switzerland, where I had the good fortune to find Agassiz and his
merry men at the “Hotel des Neufchatelois”—two tents raised upon a
magnificent boulder floating on the upper part of the Aar glacier.
After a short but very active sojourn there I “did,” not without
physical danger, many other glaciers in Switzerland and the Tyrol, and
afterwards practically studied the subject in Norway, North Wales, and
wherever else an opportunity offered, reading in the meantime much of
its special literature; but, like many others, confining my reading
chiefly to authors who start with living glaciers and describe their
doings most prominently. When, however, I read the first edition of
Mr. Geikie’s “Great Ice Age,” immediately after its publication, his
mode of presenting the phenomena, bottom upwards, suggested a number
of reflections that had never occurred before, leading to other than
the usual explanations of many glacial phenomena, and correcting some
errors into which I had fallen in searching for the vestiges of ancient
glaciers. As these suggestions and corrections may be interesting to
others, as they have been to myself, I will here state them in outline.
The most prominent and puzzling reflection or conclusion suggested
by reading Mr. Geikie’s description of the glacial deposits of
Scotland was, that the great bulk of them are quite different from the
deposits of existing glaciers. This reminded me of a previous puzzle
and disappointment that I had met in Norway, where I had observed
such abundance of striation, such universality of polished rocks and
rounded mountains, and so many striking examples of perched blocks,
with scarcely any decent vestiges of moraines. This was especially
the case in Arctic Norway. Coasting from Trondhjem to Hammerfest,
winding round glaciated islands, in and out of fjords banked with
glaciated rock-slopes, along more than a thousand miles of shore line,
displaying the outlets of a thousand ancient glacier valleys, scanning
eagerly throughout from sea to summit, landing at several stations,
and climbing the most commanding hills, I _saw only one ancient
moraine_—that at the Oxfjord station described in “Through Norway with
Ladies.”[16]
But this negative anomaly is not all. The ancient glacial deposits
are not only remarkable on account of the absence of the most
characteristic of modern glacial deposits, but in consisting mainly of
something which is quite different from any of the deposits actually
formed by any of the modern glaciers of Switzerland or any other
country within the temperate zones.
I have seen nothing either at the foot or the sides of any living
Alpine or Scandinavian glacier that even approximately represents the
“till” or “boulder clay,” nor any description of such a formation by
any other observer; and have met with no note of this very suggestive
anomaly by any writer on glaciers. Yet the till and boulder clay form
vast deposits, covering thousands of square miles even of the limited
area of the British Isles, and constitute the main evidence upon which
we base all our theories respecting the existence and the vast extent
and influence of the “Great Ice Age.”
Although so different from anything at present produced by the Alpine
or Scandinavian glaciers, this great deposit is unquestionably of
glacial origin. The evidences upon which this general conclusion
rests are fully stated by Mr. Geikie, and may safely be accepted as
incontrovertible. Whence, then, the great difference?
One of the suggestions to which I have already alluded as afforded
by reading Mr. Geikie’s book was a hypothetical solution of this
difficulty, but the verification of the hypothesis demanded a re-visit
to Norway. An opportunity for this was afforded in the summer of 1874,
during which I traveled round the coast from Stavanger to the Arctic
frontier of Russia, and through an interesting inland district. The
observations there made and strengthened by subsequent reflections,
have so far confirmed my original speculative hypothesis that I now
venture to state it briefly as follows:
That the period appropriately designated by Mr. Geikie as the “Great
Ice Age” includes at least two distinct periods or epochs—the first
of very great intensity or magnitude, during which the Arctic regions
of our globe were as completely glaciated as the Antarctic now are,
and the British islands and a large portion of Northern Europe were
glaciated as completely, and nearly in the same manner, as Greenland is
at the present time; that long after this, and immediately preceding
the present geological epoch, there was a minor glacial period, when
only the now existing valleys, favorably shaped and situated for
glacial accumulations, were partially or wholly filled with ice. There
may have been many intermediate fluctuations of climate and glaciation,
and probably were such, but as these do not affect my present argument
they need not be here considered.
So far I agree with the general conclusions of Mr. Geikie as I
understand them, and with the generally received hypotheses, but in
what follows I have ventured to diverge materially.
It appears to me that the existing Antarctic glaciers and some of the
glaciers of Greenland are essentially different in their conformation
from the present glaciers of the Alps, and from those now occupying
some of the fjelds and valleys of Norway; and that the glaciers of the
earlier or greater glacial epoch were similar to those now forming the
Antarctic barrier, while the glaciers of the later or minor glacial
epoch resembled those now existing in temperate climates, or were
intermediate between these and the Antarctic glaciers. The nature
of the difference which I suppose to exist between the two classes
of glaciers is this: The glaciers (properly so called) of temperate
climates are the overflow of the _nevé_ (the great reservoir of ice and
snow above the snow line). They are composed of ice which is protruded
below the snow-line into the region where the summer thaw exceeds the
winter snow-fall. This ice is necessarily subject to continual thinning
or wasting from its _upper_ or exposed surface, and thus finally
becomes liquefied, and is terminated by direct solar action.
Many of the characteristic phenomena of Alpine glaciers depend upon
this; among the more prominent of which are the superficial extrusion
of boulders or rock fragments that have been buried in the _nevé_ or
have fallen into the crevasses of the upper part of the true glacier,
and the final deposit of these same boulders of fragments at the foot
of the glaciers forming ordinary moraines.
But this is not all. The thawing which extrudes, and finally deposits
the larger fragments of rock, sifts from them the smaller particles,
the aggregate bulk of which usually exceeds very largely that of the
larger fragments. This fine silt or sand thus washed away is carried by
the turbid glacier torrent to considerable distances, and deposited as
an alluvium wherever the agitated waters find a resting-place.
Thus the _débris_ of the ordinary modern glacier is effectively
separated into two or more very distinct deposits; the moraine at the
glacier foot consisting of rock fragments of considerable size with
very little sand or clay or other fine deposit between them, and a
distant deposit of totally different character, consisting of gravel,
sand, clay, or mud, according to the length and conditions of its
journey. The “chips,” as they have been well called, are thus separated
from what I may designate the _filings_ or _sawdust_ of the glacier.
The filings from the existing glaciers of the Bernese Alps are
gradually filling up the lake-basins of Geneva and Constance,
repairing the breaches made by the erosive action of their gigantic
predecessors; those of the southern slope of the Alps are doing a large
share in filling up the Adriatic; while the chips of all merely rest
upon the glacier beds forming the comparatively insignificant terminal
moraine deposits.
The same in Scandinavia. The Storelv of the Jostedal is fed by the
melting of the Krondal, Nygaard, Bjornestegs, and soldal glaciers. It
has filled up a branch of the deep Sogne fjord, forming an extensive
fertile plain at the mouth of its wild valley, and is depositing
another subaqueous plain beyond, while the moraines of the glaciers
are but inconsiderable and comparatively insignificant heaps of
loose boulders, spread out on the present and former shores of the
above-named glaciers, which are overflows from one side of the great
_nevé_, the Jostedal Sneefond. All of these glaciers flow down small
lateral valleys, spread out, and disappear in the main valley, which
has now no glacier of its own, though it was formerly glaciated
throughout.
What must have been the condition of this and the other great
Scandinavian valleys when such was the case? To answer this question
rationally we must consider the meteorological conditions of that
period. Either the climate must have been much colder, or the amount of
precipitation vastly greater than at present, in order to produce the
general glaciation that rounded the mountains up to a height of some
thousands of feet above the present sea-level. Probably both factors
co-operated to effect this vast glaciation, the climate colder, and the
snow-fall also greater. The whole of Scandinavia, or as much as then
stood above the sea, must have been a _nevé_ or sneefond on which the
annual snow-fall exceeded the annual thaw.
This is the case at present on the largest _nevé_ of Europe, the 500
square miles of the great plateau of the Jostedals and Nordfjords
Sneefond, on all the overflowing _nevé_ or snow-fields of the Alps
above the snow-line; over the greater part of Greenland; and (as the
structure of the southern icebergs prove) everywhere within the great
Antarctic ice barrier.
What, then, must happen when the snow-line comes down, or nearly down,
to the sea-level? It is evident that the out-thrust glaciers, the
overflow down the valleys, cannot come to an end like the present Swiss
and Scandinavian glaciers, by the direct melting action of the sun.
They may be somewhat thinned from below by the heat of the earth, and
that generated by their own friction on the rocks, but these must be
quite inadequate to overcome the perpetual accumulation due to the
snow-fall upon their own surface and the vast overflow from the great
snow-fields above. They must go on and on, ever increasing, until they
meet some new condition of climate or some other powerful agent of
dissipation—something that can effectively melt them.
This agent is very near at hand in the case of the Scandinavian valleys
and those of Scotland. It is the sea. I think I may safely say that the
valley glaciers of these countries during the great ice age _must_ have
reached the sea, and there have terminated their existence, just as the
Antarctic glaciers terminate at the present Antarctic ice-wall.
What must happen when a glacier is thus thrust out to sea? This
question is usually answered by assuming that it slides along the
bottom until it reaches such a depth that flotation commences and
then it breaks off or “calves” as icebergs. This view is strongly
expressed by Mr. Geikie (p. 47) when he says that—“The seaward portion
of an Arctic glacier cannot by any possibility be floated up without
sundering its connection with the frozen mass behind. So long as the
bulk of the glacier much exceeds the depth of the sea, the ice will of
course rest upon the bed of the fjord or bay without being subjected to
any strain or tension. But when the glacier creeps outwards to greater
depths, then the superior specific gravity of the sea-water will tend
to press the ice upward. That ice, however, is a hard continuous mass,
with sufficient cohesion to oppose for a time this pressure, and hence
the glacier crawls on to a depth far beyond the point at which, had it
been free, it would have risen to the surface and floated. If at this
great depth the whole mass of the glacier could be buoyed up without
breaking off, it would certainly go to prove that the ice of Arctic
regions, unlike ice anywhere else, had the property of yielding to
mechanical strain without rupturing. But the great tension to which it
is subjected takes effect in the usual way, and the ice yields, not by
bending and stretching, but by breaking.” Mr. Geikie illustrates this
by a diagram showing the “calving” of an iceberg.
In spite of my respect for Mr. Geikie as a geological authority, I
have no hesitation in contradicting some of the physical assumptions
included in the above.
Ice has no such rigidity as here stated. It _does_ possess in a
high degree “the property of yielding to mechanical strain without
rupturing.” We need not go far for evidence of this. Everybody who has
skated or seen others skating on ice that is but just thick enough to
“bear” must have felt or seen it yield to the mechanical strain of the
skater’s weight. Under these conditions it not only bends under him,
but it afterwards yields to the reaction of the water below, rising
and falling in visible undulations, demonstrating most unequivocally a
considerable degree of flexibility. It may be said that in this case
the flexibility is due to the thinness of the ice; but this argument
is unsound, inasmuch as the manifestation of such flexibility does
not depend upon absolute thickness or thinness, but upon the relation
of thickness to superficial extension. If a thin sheet of ice can be
bent to a given arc, a thick sheet may be bent in the same degree, but
the thicker ice demands a greater radius and proportionate extension
of circumference. But we have direct evidence that ice of great
thickness—actual glaciers—may bend to a considerable curvature before
breaking. This is seen very strikingly when the uncrevassed ice-sheet
of a slightly inclined _nevé_ suddenly reaches a precipice and is
thrust over it. If Mr. Geikie were right, the projecting cornice thus
formed should stand straight out, and then, when the transverse strain
due to the weight of this rigid overhang exceeded the resistance of
tenacity, it should break off short, exposing a face at right angles
to the general surface of the supported body of ice. Had Mr. Geikie
ever seen and carefully observed such an overhang or cornice of ice, I
suspect that the above-quoted passage would not have been written.
Some very fine examples of such ice-cornices are well seen from the
ridge separating the Handspikjen Fjelde from the head of the Jostedal,
where a view of the great _nevé_ or sneefond is obtained. This side of
the _nevé_ terminates in precipitous rock-walls; at the foot of one of
these is a dreary lake, the Styggevand. The overflow of the _nevé_ here
forms great bending sheets that reach a short way down, and then break
off and drop as small icebergs into the lake.[17]
The ordinary course of glaciers affords abundant illustrations of
the plasticity of such masses of ice. They spread out where the
valley widens, contract where the valley narrows, and follow all
the convexities or concavities of the axial line of its bed. If the
bending thus enforced exceeds a certain degree of abruptness crevasses
are formed, but a considerable bending occurs before the rupture is
effected, and crevasses of considerable magnitude are commonly formed
without severing one part of a glacier from another. They are usually
=V=-shaped, in vertical section, and in many the rupture does not reach
the bottom of the glacier. Very rarely indeed does a crevasse cross the
whole breadth of a glacier in such a manner as to completely separate,
even temporarily, the lower from the upper part of the glacier.
If a glacier can thus bend _downwards_ without “sundering its
connection with the frozen mass behind,” surely it may bend upwards
in a corresponding degree, either with or without the formation of
crevasses, according to the thickness of the ice and the degree of
curvature.
A glacier reaching the sea by a very steep incline would probably
break off, in accordance with Mr. Geikie’s description, just as an
Alpine glacier is ruptured fairly across when it makes a cascade over
a suddenly precipitous bend of its path. One entering the sea at an
inclination somewhat less precipitous than the minor limit of the
effective rupture gradient would be crevassed in a contrary manner to
the crevassing of Alpine glaciers. Its crevasses would gape downwards
instead of upwards—have =Λ=-shaped instead of a =V=-shaped section.
With a still more moderate slope, the up-floating of the termination
of the glacier, and a concurrent general up-lifting or upbending of
the whole of its submerged portion might occur without even a partial
rupture or crevasse formation occurring.
Let us now follow out some of the necessary results of these conditions
of glacier existence and glacial prolongation. The first and most
notable, by its contrast with ordinary glaciers, is the absence of
lateral, medial, or terminal moraines. The larger masses of _débris_,
the chippings that may have fallen from the exposed escarpments of
the mountains upon the surface of the upper regions of the glacier,
instead of remaining on the surface of the ice and standing above its
general level by protecting the ice on which they rest from the general
snow-thaw, would become buried by the upward accretion of the ice due
to the unthawed stratum of each year’s snow-fall.
The thinning agency at work upon such glaciers during their journey
over the _terra firma_ being the outflow of terrestrial heat and
that due to their friction upon their beds, this thinning must all
take place from below, and thus, as the glaciers proceed downwards,
these rock fragments must be continually approaching the bottom
instead of continually approaching the top, as in the case of modern
Alpine glaciers flowing below the snow-line, and thawing from surface
downwards.
It follows, therefore, that such glaciers could not deposit any
moraines such as are in course of deposition by existing Alpine and
Scandinavian glaciers.
What, then, must become of the chips and filings of these outfloating
glaciers? They must be carried along with the ice _so long as that
ice rests upon the land_; for this _débris_ must consist partly of
fragments imbedded in the ice, and partly of ground and re-ground
excessively subdivided particles, that must either cake into what I may
call ice-mud, and become a part of the glacier, or flow as liquid mud
or turbid water beneath it, as with ordinary glaciers. The quantity of
water being relatively small under the supposed conditions, the greater
part would be carried forward to the sea by the ice rather than by the
water.
An important consequence of this must be that the erosive power of
these ancient glaciers was, _cæteris paribus_, greater than that of
modern Alpine glaciers, especially if we accept those theories which
ascribe an actual internal growth or regeneration of glaciers by the
relegation below of some of the water resulting from the surface-thaw.
As the glacier with its lower accumulation advances into deeper and
deeper water, its pressure upon its bed must progressively diminish
until it reaches a line where it would just graze the bottom with a
touch of feathery lightness. Somewhere before reaching this it would
begin to deposit its burden on the sea-bottom, the commencement of
this deposition being determined by the depth whereat the tenacity of
the deposit, or its friction against the sea-bottom, or both combined,
becomes sufficient to overpower the now-diminished pressure and forward
thrusting, or erosive power of the glacier.
Further forward, in deeper water, where the ice becomes fairly floated
above the original sea-bottom, a rapid under-thawing must occur by the
action of the sea-water, and if any communication exists between this
ice covered sea and the waters of warmer latitudes this thawing must
be increased by the currents that would necessarily be formed by the
interchange of water of varying specific gravities. Deposition would
thus take place in this deeper water, continually shallowing it or
bringing up the sea-bottom nearer to the ice-bottom.
This raising of the sea-bottom must occur not only here, but farther
back, _i.e._, from the limit at which deposition commenced. This
neutral ground, whereat the depth is just sufficient to allow the
ice to rest lightly on its own deposit and slide over it without
either sweeping it forward or depositing any more upon it, becomes an
interesting critical region, subject to continuous forward extension
during the lifetime of the glacier, as the deposition beyond it must
continually raise the sea-bottom until it reaches the critical depth
at which the deposition must cease. This would constitute what I may
designate the normal depth of the glaciated sea, or the depth towards
which it would be continually tending, during a great glacial epoch,
by the formation of a submarine bank or plain of glacier deposit,
over which the glacier would slide without either grinding it lower by
erosion or raising it higher by deposition.
But what must be the nature of this deposit? It is evident that it
cannot be a mere moraine consisting only of the larger fragments of
rock such as are now deposited at the foot of glaciers that die out
before reaching the sea. Neither can it correspond to the glacial silt
which is washed away and separated from these larger fragments by
glacial streams, and deposited at the outspreadings of glacier torrents
and rivers. It will correspond to neither the assorted gravel, sand,
nor mud of these alluvial deposits, but must be an agglomeration of all
the infusible solid matter the glacier is capable of carrying.
It must contain, in heterogeneous admixture, the great boulders, the
lesser rock fragments, the gravel chips, the sand, and the slimy mud;
these settling down quietly in the cold, gloomy waters, overshadowed by
the great ice-sheet, must form just such an agglomeration as we find in
the boulder clay and tills, and lie just in those places where these
deposits abound, provided the relative level of land and sea during the
glacial epoch were suitable.
I should make one additional remark relative to the composition of
this deposit, viz., that under the conditions supposed, the original
material detached from the rocks around the upper portions of the
glaciers would suffer a far greater degree of attrition at the glacier
bottom than it obtains in modern Alpine glaciers, inasmuch as in these
it is removed by the glacier torrent when it has attained a certain
degree of fineness, while in the greater glaciers of the glacial epoch
it would be carried much further in association with the solid ice, and
be subjected to more grinding and regrinding against the bottom. Hence
a larger proportion of slimy mud would be formed, capable of finally
induring into stiff clay such as forms the matrix of the till and
boulder clay.
The long journey of the bottom _débris_ stratum of the glacier, and its
final deposition when in a state of neutral equilibrium between its
own tendency to repose and the forward thrust of the glacier, would
obviously tend to arrange the larger fragments of rock in the manner
in which they are found imbedded in the till, _i.e._, the oblong
fragments lying with their longer axes and their best marked striæ in
the direction of the motion of the glacier. The “_striated pavements_”
of the till are thus easily explained; they are the surface upon which
the ice advanced when its deposits had reached the critical or neutral
height. Such a pavement would continually extend outwards.
The only sorting of the material likely to occur under these conditions
would be that due to the earlier deposition and entanglement of the
larger fragments, thus producing a more stony deposit nearer inland,
just as Mr. Geikie describes the actual deposits of till where,
“generally speaking, the stones are most numerous in the till of
hilly districts; while at the lower levels of the country the clayey
character of the mass is upon the whole more pronounced.” These “hilly
districts,” upon the supposition of greater submergence, would be the
near shore regions, and the lower levels the deeper sea where the
glacier floated freely.
The following is Mr. Geikie’s description of the distribution of the
till (page 13):—“It is in the lower-lying districts of the country
where till appears in greatest force. Wide areas of the central
counties are covered up with it continuously, to a depth varying
from two or three feet up to one hundred feet and more. But as we
follow it towards the mountain regions it becomes thinner and more
interrupted—the naked rock ever and anon peering through, until at
last we find only a few shreds and patches lying here and there in
sheltered hollows of the hills. Throughout the Northern Highlands it
occurs but rarely, and only in little isolated patches. It is not
until we get way from the steep rocky declivities and narrow glens and
gorges, and enter upon the broader valleys that open out from the base
of the highland mountains to the low-lying districts beyond, that we
meet with any considerable deposits of stony clay. The higher districts
of the Southern Uplands are almost equally free from any covering of
till.”
This description is precisely the same as I must have written, had I so
far continued my imaginary sketch of the results of ancient glaciation
as to picture what must remain after the glaciers had all melted away,
and the sea had receded sufficiently to expose their submarine deposits.
Throughout the above I have assumed a considerable submergence of the
land as compared with the present sea-level on the coasts of Scotland,
Scandinavia, etc.
The universality of the terraces in all the Norwegian valleys opening
westward proves a submergence of _at least_ 600 or 700 feet. When I
first visited Norway in 1856, I accepted the usual description of these
as alluvial deposits; was looking for glacial vestiges in the form of
moraines, and thus quite failed to observe the true nature of these
vast accumulations, which was obvious enough when I re-examined them in
the light of more recent information. Some few are alluvial, but they
are exceptional and of minor magnitude. As an example of such alluvial
terraces I may mention those near the mouth of the Romsdal, that are
well seen from the Aak Hotel, and which a Russian prince, or other
soldier merely endowed with military eyes, might easily mistake for
artificial earthworks erected for the defence of the valley.
In this case, as in the others where the terraces are alluvial, the
valley is a narrow one, occupied by a relatively wide river loaded with
recent glacial _débris_. It evidently filled the valley during the
period of glacial recession.
The ordinary wider valleys, with a river that has cut a narrow
channel through the outspread terrace-flats, display a different
formation. Near the mouth of such valleys I have seen cuttings of
more than a hundred feet in depth, through an unbroken terrace of
most characteristic till, with other traces rising above it. This
is the ordinary constitution of the _lower portions_ of most of the
Scandinavian terraces.
These terraces are commonly topped with quite a different stratum,
which at first I regarded as a subsequent alluvial or estuarine
deposit, but further examination suggested another explanation of the
origin of some portions of this superficial stratum, to which I shall
refer hereafter.
Such terraces prove a rise of sea or depression of land, during
the glacial epoch, to the extent of 600 feet as a _minimum_, while
the well-known deposits of Arctic shells at Moel Tryfaen and the
accompanying drift have led Prof. Ramsay to estimate “the probable
amount of submergence during some part of the glacial period at about
2300 feet.”[18]
It would be out of place here to reproduce the data upon which
geologists have based their rather divergent opinions respecting the
actual extent of the submergence of the western coast of North Europe.
All agree that a great submergence occurred, but differ only as to its
extent, their estimates varying between 1,000 and 3,000 feet.
There is one important consideration that must not be overlooked, viz.,
that—if my view of the submarine origin of the till be correct—the
mere submergence of the land at the glacial period does not measure the
difference between the depth of the sea at that and the present time,
seeing that the deposits from the glaciers must have shallowed it very
materially.
It is only after contemplating thoroughly the present form of the
granitic and metamorphic hills of Scandinavia,—hills that are always
angular when subjected only to subaerial weathering,—that one can form
an adequate conception of the magnitude of this shallowing deposit. The
rounding, shaving, grinding, planing, and universal abrasion everywhere
displayed appear to me to justify the conclusion that if the sea were
now raised to the level of the terraces, _i.e._, 600 feet higher than
at present, the mass of matter abraded from the original Scandinavian
mountains, and lying under the sea, would exceed the whole mass of
mountain left standing above it.
The first question suggested by reading Mr. Geikie’s book was whether
the terraces are wholly or partially formed of till, and more
especially whether their lower portions are thus composed. This, as
already stated, was easily answered by the almost unanimous reply of
all the many Norwegian valleys I traversed. Any tourist may verify
this. The next question was whether this same till extends below the
sea. This was not so easily answered by the means at my disposal, as I
travelled hastily round the coast from Stavanger via the North Cape to
the frontier of Russian Lapland in ordinary passenger steam-packets,
which made their stoppages to suit other requirements than mine. Still,
I was able to land at many stations, and found, wherever there was
a gently sloping strand at the mouth of an estuary, or of a valley
whose river had already deposited its suspended matter (a common case
hereabouts, where so many rivers terminate in long estuaries or open
out into bag-shaped lakes near the coast), and where the bottom had not
been modified by secondary glaciation, that the receding tide displayed
a sea-bottom of till, covered with a thin stratum of loose stones and
shells. In some cases the till was so bare that it appeared like a
stiff mud deposited but yesterday.
At Bodö, an arctic coast station on the north side of the mouth of the
Salten fjord (lat. 67° 20´), where the packets make a long halt, is
a very characteristic example of this; a deposit of very tough till
forming an extensive plain just on the sea-level. The tide rises over
this, and the waves break upon it, forming a sort of beach by washing
away some of the finer material, and leaving the stones behind. The
ground being so nearly level, the reach of the tide is very great, and
thus a large area is exposed at low tide. Continuous with this, and
beyond the limit of high tide, is an extensive inland plain covered
with coarse grass and weeds growing directly upon the surface of the
original flat pavement of till.
There is no river at Bodö; the sea is clear, leaves no appreciable
deposit, and the degree of denudation of the clayey matrix of the till
is very much smaller than might be expected. The limit of high water
is plainly shown by a beach of shells and stones, but at low tide the
ground over which the sea has receded is a bare and scarcely modified
surface of till. I have observed the same at low water at many other
arctic stations. In the Tromsö Sund there are shallows at some distance
from the shore which are just covered with water at low tide. I landed
and waded on these, and found the bottom to consist of till covered
with a thin layer of shells, odd fragments of earthenware, and other
rubbish thrown overboard from vessels. It is evident that breakers of
considerable magnitude are necessary for the loosening of this tough
compact deposit—that it is very slightly, if at all, affected by the
mere flow of running water.
I specify these instances as characteristic and easy of verification,
as the packets all stop at these stations; but a yachtsman sailing at
leisure amidst the glorious coast scenery of the Arctic Ocean might
multiply such observations a hundredfold by stopping wherever such
strands are indicated in passing. I saw a multitude of these in places
where I was unable to go ashore and examine them.
A further question in this direction suggested itself on the spot,
viz., what is the nature of the “_banks_” which constitute the
fishing-grounds of Norway, Iceland, Newfoundland, etc. They are
submarine plains unquestionably—they must have a high degree of
fertility in order to supply food for the hundreds of millions of
voracious cod-fish, coal-fish, haddocks, hallibut, etc., that people
them. These large fishes all _feed on the bottom_, their chief food
being mollusca and crustacea, which must find, either directly or
indirectly, some pasture of vegetable origin. The banks are, in fact,
great meadows or feeding grounds for the lower animals which support
the higher.
From the Lofoten bank alone twenty millions of cod-fish are taken
annually, besides those devoured by the vast multitude of sea-birds.
Now this bank is situated precisely where, according to the
above-stated view of the origin of the till, there should be a huge
deposit. It occupies the Vest fjord, _i.e._, the opening between the
mainland and the Lofoden Islands, extending from Moskenes, to Lodingen
on Hindö, just where the culminating masses of the Kjolen Mountains
must have poured their greatest glaciers into the sea by a westward
course, and these glaciers must have been met by another stream pouring
from the north, formed by the glaciers of Hindö and Senjenö, and both
must have coalesced with a third flood pouring through the Ofoten
fjord, the Tys fjord, etc., from the mainland. The Vest fjord is about
sixty miles wide at its mouth, and narrows northward till it terminates
in the Ofoten fjord, which forks into several branches eastward. A
glance at a good map will show that here, according to my explanation
of the origin of the till, there should be the greatest of all the
submarine plains of till which the ancient Scandinavian glaciers have
produced, and of which the plains of till I saw on the coast at Bodö
(which lies just to the mouth of the Vest fjord, where the Salten fjord
flows into it), are but the slightly inclined continuation.
Some idea of this bank may be formed from the fact that outside of
the Lofodens the sea is 100 to 200 fathoms in depth, that it suddenly
shoals up to 16 or 20 fathoms on the east side of these rocks, and this
shallow plain extends across the whole 50 or 60 miles between these
islands and the mainland.[19] It must not be supposed the fjords or
inlets of Scandinavia are _usually_ shallower than the open sea; the
contrary is commonly the case, especially with the narrowest and those
which run farthest inland. They are _very much_ deeper than the open
sea.
If space permitted I could show that the great Storregen bank, opposite
Aalesund and Molde, where the Stor fjord, Mold fjord, etc., were the
former outlets of the glaciers from the highest of all the Scandinavian
mountains, and the several banks of Finmark, etc., from which, in the
aggregate, are taken another 20 or 30 millions of cod-fish annually,
are all situated just where theoretically they ought to be found. The
same is the case with the great bank of Newfoundland and the banks
around Iceland, which are annually visited by large numbers of French
fishermen from Dunkerque, Boulogne, and other ports.
Whenever the packet halted over these banks during our coasting trip we
demonstrated their fertility by casting a line or two over the bulwark.
No bait was required, merely a double hook with a flat shank attached
to a heavy leaden plummet. The line was sunk till the lead touched the
bottom, a few jerks were given, and then a tug was felt: the line was
hauled in with a cod-fish or hallibut hooked, not inside the mouth,
but externally by the gill-plates, the back, the tail, or otherwise.
The mere jerking of a hook near the bottom was sufficient to bring it
in contact with some of the population. There is a very prolific bank
lying between the North Cape and Nordkyn, where the Porsanger and Laxe
fjords unite their openings. Here we were able, with only three lines,
to cover the fore-deck of the packet with struggling victims in the
course of short halts of fifteen to thirty minutes. Not having any
sounding apparatus by which to fairly test the nature of the sea-bottom
in these places, I cannot offer any direct proof that it was composed
of till. By dropping the lead I could _feel_ it sufficiently to be
certain that it was not rock in any case, but a soft deposit, and
the marks upon the bottom of the lead, so far as they went, afforded
evidence in favor of its clayey character. A further investigation of
this would be very interesting.
But the most striking—I may say astounding—evidence of the fertility
of these banks, one which appeals most powerfully to the senses, is the
marvelous colony of sea-birds at Sverholtklubben, the headland between
the two last-named fjords. I dare not estimate the numbers that rose
from the rocks and darkened the sky when we blew the steam-whistle in
passing. I doubt whether there is any other spot in the world where an
equal amount of animal life is permanently concentrated. All these feed
on fish, and an examination of the map will show why—in accordance
with the above speculations—they should have chosen Sverholtklubben as
the best fishing-ground on the arctic face of Europe.
I am fully conscious of the main difficulty that stands in the way of
my explanation of the formation of the till, viz., that of finding
sufficient water to float the ice, and should have given it up had I
accepted Mr. Geikie’s estimate of the thickness of the great ice-sheet
of the great ice age.
He says (page 186) that “The ice which covered the low grounds of
Scotland during the early cold stages of the glacial epoch was
certainly more than 2000 feet in thickness, and it must have been even
deeper than this between the mainland and the Outer Hebrides. To cause
such a mass to float, the sea around Scotland would require to become
deeper than now by 1400 or 1500 feet at least.”
I am unable to understand by what means Mr. Geikie measured this depth
of the ice which covered these low grounds, except by assuming that its
surface was level with that of the upper ice-marks of the hills beyond.
The following passage on page 63 seems to indicate that he really has
measured it thus:—
“Now the scratches may be traced from the islands and the coast-line up
to an elevation of at least 3,500 feet; so that ice must have covered
the country to that height at least. In the Highlands the tide of ice
streamed out from the central elevations down all the main straths and
glens; and by measuring the height attained by the smoothed and rounded
rocks we are enabled to estimate roughly the probable thickness of the
old ice-sheet. But it can only be a rough estimate, for so long a time
has elapsed since the ice disappeared, the rain and frost together have
so split up and worn down the rocks of these highland mountains that
much of the smoothing and polishing has vanished. But although the
finer marks of the ice-chisel have thus frequently been obliterated,
yet the broader effects remain conspicuous enough. From an extensive
examination of these we gather that the ice could not have been less,
and was probably more than 3,000 feet thick in its deepest parts.”
Page 80 he says: “Bearing in mind the vast thickness reached by the
Scotch ice-sheet, it becomes very evident that the ice would flow
along the bottom of the sea with as much ease as it poured across the
land, and every island would be surmounted and crushed, and scored and
polished just as readily as the hills of the mainland were.”
Mr. Geikie describes the Scandinavian ice-sheet in similar terms, but
ascribes to it a still greater thickness. He says (page 404)—“The
whole country has been moulded and rubbed and polished by an immense
sheet of ice, which could hardly have been less than 6,000 or even
7,000 feet thick,” and he maintains that this spread over the sea and
coalesced with the ice-sheet of Scotland.
My recollection of the Lofoden Islands, which from their position
afford an excellent crucial test of this question, led me to believe
that their configuration presented a direct refutation of Mr. Geikie’s
remarkable inference; but a mere recollection of scenery being too
vague, a second visit was especially desirable in reference to this
point. The result of the special observations I made during this second
visit fully confirmed the impression derived from memory.
I found in the first place that all along the coast from Stavanger to
the Varanger fjord every rock _near the shore_ is glaciated; among the
thousands of low-lying ridges that peer above the water to various
heights none near the mainland are angular. The general character of
these is shown in the sketch of “My Sea Serpent,” in the last edition
of “Through Norway with a Knapsack.”
The rocks which constitute the extreme outlying limits of the Lofoden
group, and which are between 60 and 70 miles from the shore, although
mineralogically corresponding with those near the shore, are totally
different in their conformation, as the sketch of three characteristic
specimens plainly shows. Mr. Everest very aptly compares them to
shark’s teeth. Proceeding northward, these rocks gradually progress
in magnitude, until they become mountains of 3,000 to 4,000 feet in
height; their outspread bases form large islands, and the Vest fjord
gradually narrows.
The remarkably angular and jagged character of these rocks when
weathered in the air renders it very easy to trace the limits of
glaciation on viewing them at a distance. The outermost and smallest
rocks show from a distance no signs of glaciation. If submerged, the
ice of the great ice age was then enough to float over without touching
them; if they stood above the sea, as at present, they suffered no
more glaciation than would be produced by such an ice-sheet as that of
the “paleocrystic” ice recently found by Captain Nares on the north
of Greenland. Progressing northward, the glaciation begins to become
visible, running up to about 100 feet above the sea-level on the
islands lying westward and southward of Ost Vaagen. Further northward
along the coast of Ost Vaagen and Hindö, the level gradually rises to
about 500 feet on the northern portion of Ost Vaagen, and up to more
than 1,000 feet on Hindö, while on the mainland it reaches 3,000 to
4,000 feet.
A remarkable case of such variation, or descent of ice-level, as the
ice-sheet proceeded seaward, is shown at Tromsö. This small oblong
island (lat. 69° 40´), on which is the capital town of Finmark, lies
between the mainland and the large mountainous island of Kvalö, with
a long sea-channel on each side, the Tromösund and the Sandesund; the
total width of these two channels and the island itself being about
four or five miles. The general line of glaciation from the mainland
crosses the broad side of these channels and the island, which has
evidently been buried and ground down to its present moderate height of
two or three hundred feet. Both of the channels are till-paved. On the
east or inland side the mountains near the coast are glaciated to their
summits—are simply _roches moutonnées_, over which the reindeer of
the Tromsdal Lapps range and feed. On the west the mountains are dark,
pyramidal, non-glaciated peaks, with long vertical snow-streaks marking
their angular masses.
The contrast is very striking when seen from the highest part of
the island, and is clearly due to a decline in the thickness of the
ice-sheet in the course of its journey across this narrow channel.
Speaking roughly from my estimation, I should say that this thinning
or lowering of the limits of glaciation exceeds 500 feet between the
opposite sides of the channel, which, allowing for the hill slopes,
is a distance of about 6 miles. This very small inclination would
bring a glacier of 3,000 feet in thickness on the shore down to the
sea-level in an outward course of 30 miles, or about half the distance
between the mainland and the outer rocks of the Lofodens shown in the
engraving.
I am quite at a loss to understand the reasoning upon which Mr. Geikie
bases his firm conviction respecting the depth of the ice-sheet on the
low grounds of Scotland and Scandinavia. He seems to assume that the
glaciers of the great ice age had little or no superficial down slope
corresponding to the inclination of the base on which they rested. I
have considerable hesitation in attributing this assumption to Mr.
Geikie, and would rather suppose that I have misunderstood him, as
it is a conclusion so completely refuted by all we know of glacier
phenomena and the physical laws concerned in their production; but the
passages I have quoted, and several others, are explicit and decided.
Those geologists who contend for the former existence of a great polar
ice-cap radiating outwards and spreading into the temperate zones,
might adopt this mode of measuring its thickness, but Mr. Geikie
rejects this hypothesis, and shows by his map of “The Principal Lines
of Glacial Erosion in Sweden, Norway, and Finland,” that the glaciation
of the extreme north of Europe proceeded from south to north; that the
ice was formed on land, and proceeded seawards in all directions.
I may add to this testimony that presented by the North Cape, Sverholt,
Nordkyn, and the rest of the magnificent precipitous headlands that
constitute the characteristic feature of the arctic-face of Europe.
They stand forth defiantly as a phalanx of giant heralds proclaiming
aloud the fallacy of this idea of southward glacial radiation; and
in concurrence with the structure and striation of the great glacier
troughs that lie between them, and the planed table-land at their
summits, they establish the fact that during the greatest glaciation of
the glacial epoch the ice-streams were formed on land and flowed out
to sea, just as they now do at Greenland, or other parts of the world
where the snow line touches or nearly approaches the level of the sea.
All such streams must have followed the slope of the hill-sides upon
which they rested and down which they flowed, and thus the upper limits
of glaciation afford no measure whatever of the thickness of the ice
upon “the low grounds of Scotland,” or of any other glaciated country.
As an example, I may refer to Mont Blanc. In climbing this mountain
the journey from the lower ice-wall of the Glacier de Bessons up to
the _bergschrund_ above the _Grand Plateau_ is over one continuous
ice-field, the level of the upper part of which is more than 10,000
feet above its terminal ice-wall. Thus, if we take the height of the
striations or smoothings of the upper _nevé_ above the low grounds on
which the ice-sheet rests, and adopt Mr. Geikie’s reasoning, the lower
ice-wall of the Glacier de Bessons should be 10,000 feet thick. Its
actual thickness, as nearly as I can remember, is about 10 or 12 feet.
Every other known glacier presents the same testimony. The drawing of a
Greenland glacier opposite page 47 of Mr. Geikie’s book shows the same
under arctic conditions, and where the ice-wall terminates in the sea.
I have not visited the Hebrides, but the curious analogy of their
position to that of the Lofodens suggests the desirability of similar
observations to those I have made in the latter. If the ice between
the mainland and the Outer Hebrides was, as Mr. Geikie maintains,
“certainly more than 2000 feet in thickness,” and this stretched
across to Ireland, besides uniting with the still thicker ice-sheet
of Scandinavia, these islands should all be glaciated, especially the
smaller rocks. If I am right, the smaller outlying islands, those south
of Barra, should, like the corresponding rocks of the Lofodens, display
no evidence of having been overswept by a deep “_mer de glace_.”
I admit the probability of an ice-sheet extending as Mr. Geikie
describes, but maintain that it thinned out rapidly seaward, and
there became a mere ice-floe, such as now impedes the navigation of
Smith’s Sound and other portions of the Arctic Ocean. The Orkneys and
Shetlands, with which I am also unacquainted, must afford similar
crucial instances, always taking into account the fact that the larger
islands may have been independently glaciated by the accumulations
due to their own glacial resources. It is the small rocks standing at
considerable distance from the shores of larger masses of land that
supply the required test-conditions.
From the above it will be seen that I agree with Mr. Geikie in
regarding the till as a “_moraine profonde_,” but differ as to the
mode and place of its deposition. He argues that it was formed under
glaciers of the thickness he describes, while their whole weight rested
upon it.
This appears to me to be physically impossible. If such glaciers are
capable of eroding solid rocks, the slimy mud of their own deposits
could not possibly have resisted them. The only case where this might
have happened is where a mountain-wall has blocked the further downward
progress of a glacier, or in pockets, or steep hollows which a glacier
might have bridged over and filled up; but such pockets are by no means
the characteristic localities of till, though the till of Switzerland
may possibly show examples of the first case. The great depth of the
inland lakes of Norway, their bottoms being usually far below that of
the present sea-bottom, is in direct contradiction of this.[20] They
should, before all places, be filled with till, if the till were a
ground moraine formed on land; but all we know of them confirms the
belief that the glaciers deepened them by erosion instead of shallowing
them by deposition.
Mr. Geikie’s able defence of Ramsay’s theory of lake-basin erosion
is curiously inconsistent with his arguments in favor of the ground
moraine.
I fully concur with Mr. Geikie’s arguments against the iceberg theory
of the formation of the till. This, I think, he has completely refuted.
Before concluding I must say a few words on those curious lenticular
beds of sand and gravel in the till which appear so very puzzling. A
simple explanation is suggested in connection with the above-sketched
view of the formation of the till. All glaciers, whether in arctic or
temperate climates, are washed by streamlets during summer, and these
commonly terminate in the form of a stream or cascade pouring down
a “_moulin_”—a well bored by themselves and reaching the bottom of
the glacier. Now what must be the action of such a downflow of water
upon my supposed submarine bed of till just grazing the bottom of the
glacier? Obviously, to wash away the fine clayey particles, and leave
behind the coarser sand or gravel. It must form just such a basin or
lenticular cavity as Mr. Geikie describes. The oblong shape of these,
their longer axis coinciding with the general course of the glacier,
would be produced by the onward progress of the moulin. The accordance
of their other features with this explanation will be seen on reading
Mr. Geikie’s description (pp. 18, 19, etc).
The general absence of marine animals and their occasional exceptional
occurrence in the intercalated beds is just what might be expected
under the conditions I have sketched. In the gloomy subglacial depths
of the sea, drenched with continual supplies of fresh water and cooled
below the freezing-point by the action of salt water on the ice,
ordinary marine life would be impossible; while, on the other hand, any
recession of the glacial limit would restore the conditions of arctic
animal life, to be again obliterated with the renewed outward growth of
the floating skirts of the inland ice-mantle.
But I must now refrain from the further discussion of these and other
collateral details, but hope to return to them in another paper.
In “Through Norway with Ladies” I have touched lightly upon some
of these, and have more particularly described some curious and
very extensive evidences of secondary glaciation that quite escaped
my attention on my first visit, and which, too, have been equally
overlooked by other observers. In the above I have endeavored to keep
as nearly as possible to the main subject of the origin of the till and
the character of the ancient ice-sheet.