_THE GREAT ICE AGE._
Scientific superstitions, understanding by this name the reception
of hypotheses of prominent men, and using these as fetishes to be
worshipped and to be employed in miraculous works, are scarcely less
common in our time than superstitions of another kind were in darker
ages. One of these which has been dominant for a long time in geology,
and has scarcely yet run its course, is that of the Great Ice Age,
with its accompaniments of Continental Glaciers and Polar Ice Cap.
The cause of this it is not difficult to discern. The covering of
till, gravel and travelled boulders which encumbers the surface of the
northern hemisphere from the Arctic regions more than half way to the
equator, had long been a puzzle to geologists, and this was increased
rather than diminished when the doctrine of appeal to recent causes on
the principle of uniformity became current. It was seen that it was
necessary to invoke the action of ice in some form to account for these
deposits, and it was at the same time perceived that there was much
evidence to prove that between the warm climate of the early Tertiary
and the more subdued mildness of the modern time there had intervened a
period of unusual and extreme cold. In this state of affairs attention
was attracted to the Alpine glaciers. Their movement, their erosion
of surfaces, their heaping up of moraines bearing some resemblance to
the widely extended boulder deposits, their former greater extension,
as indicated by old moraines at lower levels than those in process of
formation, were noted. Here was a modern cause capable of explaining
all the phenomena. Men's minds were taken by storm, and as always
happens in the case of new and important discoveries, the agency of
glaciers was pushed at once far beyond the possibilities of their
action under any known physical or climatal laws. This exaggerated idea
of the action of land ice in the form of glaciers is not yet exploded,
more especially in the United States, where official sanction has been
given to it by the Geological Survey, and where it has been introduced
even into school and college text-books. It affords also a telling
bit of scientific sensationalism, which can scarcely be resisted by a
certain class of popular writers. America has also afforded greater
facilities for extreme theories of this kind, owing to the wide and
uninterrupted distribution of glacial deposits, and the more simple
and less broken character of its great internal plateau, while the
influence of great leading minds, like those of the elder Agassiz and
of Dana, naturally held sway over the younger geologists. Fortunately
Canada, which possesses the larger and more northern half of the
North American continent; though numerically inferior, and therefore
overborne in the discussion, has, in the main, remained steadfast to
facts rather than to specious theories, and has been confirmed in this
position by the clearer testimony of nature in a region where many of
the features of the glacial age still persist.[156]
[156] I may refer here to the recent researches of Dr. G. M. Dawson,
Mr. R. Chalmers, Mr. McConnell and Dr. Ells.
The writer of these pages has, ever since the publication of the first
edition of his "Acadian Geology,"[157] steadily resisted the more
extreme views of glaciation, and has opposed the southward progress of
the great continental glacier. Though, figuratively speaking, overborne
and pressed back in the course of its extension, he has now, like
those primitive men who are imagined in the post-glacial age to have
followed up the retreat of the ice, the pleasure of seeing the once
formidable continental glacier broken up into great local glaciers on
the mountain ranges separated by intervening areas of submergence.
[157] 1855.
The questions relating to this subject are too numerous and varied
for treatment here. The question of the causes of the great lowering
of temperature in the glacial age I shall leave for consideration in
the next chapter, and merely state here that I believe changes of
distribution of sea and land and of ocean currents are sufficient to
account for all the refrigeration of which there is good evidence. I
content myself with a comparison of the glacial phenomena of Mont Blanc
and of the Gulf of St. Lawrence from my own observation,[158] and some
general deductions as to glacier possibilities.
[158] Published in 1867.
A scientific voyager carries with him a species of questioning peculiar
to himself. Not content with vacantly gazing at the sea, scrutinizing
his fellow passengers, noting the changes of the weather and the length
of the day's run, he recognises in the sea one of the great features
of the earth, and questions it daily as to its present and its past
The present features of the sea include much of surpassing interest,
but the questions which relate to its origin and early history are
still more attractive. Some of these questions are likely to interest
a voyager from Canada entering the Atlantic by one of its greatest
tributaries, the St. Lawrence.
In doing so, we approach the ocean not at a right angle, but along a
line only slightly inclined to its western side, and we find ourselves
in a broad estuary or trough, having on its north-western side rugged
hills of old crystalline rocks, the Laurentian, ridged up in great
folds or earth waves parallel to the river. On the south-east or
right-hand side we have a lower barrier of earth waves composed of
sedimentary rocks somewhat later in date, but still geologically very
ancient. We are thus introduced to a remarkable feature of the west
side of the North Atlantic, namely, that its border is made up of
very old rocks folded into mountain ridges thrown up at an ancient
period, and approximately parallel to the coast. The Lower St. Lawrence
occupies a furrow between two of these ridges.
Here, however, a more modern feature attracts our attention. The sides
of the bounding hills are cut in a succession of terraces, rising one
above another from the level of the sea to a height of 500 feet or
more, capped with long ranges of the white houses and barns of the
Canadian habitants, and furnishing level lines for the "concession
roads" which run along the coast. These terraces are really old sea
margins indicating the stages of the elevation of the land out of the
sea immediately before the modern period. On these terraces, and in the
clays and sands which form the plateaus extending in some places in
front of them, are sea shells of the same kinds with those now living
in the Gulf of St. Lawrence, and occasionally we find bones of whales
which have been stranded on the old beaches.
These terraces are, of course, indications of change of level in very
modern times. They show that in what we call the Pleistocene age the
land was lower than at present, and we shall find that in the Lower
St. Lawrence there is evidence of a depression extending to over 1,000
feet, carrying the sea far up the valley, so that sea shells are found
in the clays as far up as Kingston and Ottawa, and stranded skeletons
of whales as far west as Smith's Falls, in Ontario.
If we examine the shores more minutely, we shall find all along the
south coast a belt of boulders which are often as much as eight to ten
feet in diameter, and consist largely of rocks found only in the hills
of the northern coast, more than thirty miles distant, from which
they must have been drifted to their present position. This boulder
belt, which extends from the lowest tide mark about fifty feet or
more upward, is sometimes piled in ridges and sometimes flattened out
into a rude pavement. It is a product of the modern field ice, which,
attaining a great thickness in winter, has boulders frozen into its
bottom, and floating up and down with the tide, deposits these on the
shore. At Little Metis, two hundred miles below Quebec, where I have a
summer residence, I have from year to year cleared a passage through
the boulder belt for bathing and for launching boats, and nearly every
spring I find that boulders have been thrown into the cleared space
by the ice, while one can notice from year to year differences in the
position of very large boulders.
If we pass inland from the shore belt of boulders, we shall find
similar appearances on the inland terraces at various heights, up to
at least 400 feet. These are inland boulder belts belonging to old
shores now elevated. Like the modern boulder belt these inland belts
and patches consist partly of Laurentian rocks from the North Shore,
partly of sandstones and conglomerates in place near to their present
sites. In some places the stones are smaller than those of the present
beach, in other places of gigantic size. These boulders lie not only
on the bare rock striated in places with ice grooves pointing to the
north-north-east; but on the old till or boulder clay, which also
abounds with boulders, and which is more ancient than the superficial
boulder drift. Locally we find here and there masses of fossiliferous
limestone which must have been derived from the high ground to the
south of the St. Lawrence, and which have been borne northward either
by drift ice or by local glaciers.
If now we study the polished and scored surfaces of rocks in the St.
Lawrence valley and the bounding hills, we shall find that while the
former testify to a great movement of ice and boulders up the river
from the north-east, the latter show evident signs of the movement
of local glaciers down the valleys of the Laurentide hills to the
south, and on the continuation of the Appalachians south of the river
similar evidence of the movement of land ice to the north. Thus we
have evidence of the combined action of local glaciers and floating
ice. To add to all this, we can find on the flat tops of the hard
sandstone boulders on the beach the scratches made by the ice of last
winter, often in the same north-easterly direction with those of the
Pleistocene time.
In addition to the ice formed in winter in the St. Lawrence itself,
the snow-clad hills of Greenland send down to the sea great glaciers,
which in the bays and fiords of that inhospitable region form at their
extremities huge cliffs of everlasting ice, and annually "calve," as
the seamen say, or give off a great progeny of ice islands, which,
slowly drifted to the southward by the arctic current, pass along the
American coast, diffusing a cold and bleak atmosphere, until they melt
in the warm waters of the Gulf Stream. Many of these bergs enter the
Straits of Belle-Isle, for the Arctic current clings closely to the
coast, and a part of it seems to be deflected into the Gulf of St.
Lawrence through this passage, carrying with it many large bergs. The
voyager passing through this strait in clear weather may see numbers
of these ice islands glistening in snowy whiteness, or showing deep
green cliffs and pinnacles--sometimes with layers of earthy matter
and stones, or dotted with numerous sea birds, which rest upon them
when gorged with the food afforded by shoals of fish and others marine
animals which haunt these cold seas. In early summer the bergs are
massive in form, often with flat tops, but as the summer advances they
become eroded by the sun and warm winds, till they present the most
grotesque forms of rude towers and spires rising from broad foundations
little elevated above the water.
Mr. Vaughan, late superintendent of the Lighthouse at Belle-Isle, has
kept a register of icebergs for several years. He states that for ten
which enter the straits, fifty drift to the southward, and that most
of those which enter pass inward on the north side of the island,
drift toward the western end of the straits, and then pass out on the
south side of the island, so that the straits seem to be merely a sort
of eddy in the course of the bergs. The number in the straits varies
much in different seasons of the year. The greatest number are seen
in spring, especially in May and June; and toward autumn and in the
winter very few remain. Those which remain until autumn are reduced
to mere skeletons; but if they survive until winter, they again grow
in dimensions, owing to the accumulations upon them of snow and new
ice. Those that we saw early in July were large and massive in their
proportions. The few that remained when we returned in September were
smaller in size, and cut into fantastic and toppling pinnacles. Vaughan
records that on the 30th of May, 1858, he counted in the Straits of
Belle-Isle 496 bergs, the least of them sixty feet in height, some of
them half a mile long and 200 feet high. Only one-eighth of the volume
of floating ice appears above water, and many of these great bergs may
thus touch the ground in a depth of thirty fathoms or more, so that if
we imagine four hundred of them moving up and down under the influence
of the current, oscillating slowly with the motion of the sea, and
grinding on the rocks and stone-covered bottom at all depths from the
centre of the channel, we may form some conception of the effects of
these huge polishers of the sea floor.
Of the bergs which pass outside of the straits, many ground on the
banks off Belle-Isle. Vaughan has seen a hundred large bergs aground
at one time on the banks, and they ground on various parts of the
banks of Newfoundland, and all along the coast of that island. As they
are borne by the deep-seated cold current, and are scarcely at all
affected by the wind, they move somewhat uniformly in a direction from
north-east to south-west, and when they touch the bottom, the striation
or grooving which they produce must be in that direction.
In passing through the straits in July, I have seen great numbers of
bergs, some low and flat-topped, with perpendicular sides, others
convex or roof-shaped, like great tents pitched on the sea; others
rounded in outline or rising into towers and pinnacles. Most of them
were of a pure dead white, like loaf sugar, shaded with pale bluish
green in the great rents and recent fractures. One of them seemed as
if it had grounded and then overturned, presenting a flat and scored
surface covered with sand and earthy matter.
At present we wish to regard the icebergs of Belle-Isle in their
character of geological agents. Viewed in this aspect, they are in
the first place parts of the cosmical arrangements for equalizing
temperature, and for dispersing the great accumulations of ice in the
Arctic regions, which might otherwise unsettle the climatic and even
the static equilibrium of our globe, as they are believed by some
imaginative physicists and geologists to have done in the so-called
glacial period. If the ice islands in the Atlantic, like lumps of ice
in a pitcher of water, chill our climate in spring, they are at the
same time agents in preventing a still more serious secular chilling
which might result from the growth without limit of the Arctic snow
and ice. They are also constantly employed in wearing down the Arctic
land, and aided by the great northern current from Davis's Straits, in
scattering stones, boulders and sand over the banks along the American
coast. Incidentally to this work, they smooth and level the higher
parts of the sea bottom, and mark it with furrows and striæ indicative
of the direction of their own motion.
When we examine a chart of the American coast, and observe the deep
channel and hollow submarine valleys of the Arctic current, and the
sandbanks which extend parallel to this channel from the great bank
of Newfoundland to Cape Cod, we cannot avoid the conclusion that the
Arctic current and its ice have great power both of excavation and
deposition. On the one hand, deep hollows are cut out where the current
flows over the bottom, and on the other, great banks are heaped up
where the ice thaws and the force of the current is abated. I have
been much struck with the worn and abraded appearance of stones and
dead shells taken up from the banks off the American coast, and am
convinced that an erosive power comparable to that of a river carrying
sand over its bed, and materially aided by the grinding action of ice,
is constantly in action under the waters of the Arctic current.[159]
The unequal pressure resulting from this deposition and abrasion is
not improbably connected with the slight earthquakes experienced in
Eastern America, and also with the slow depression of the coast; and
if we go back to that earliest of all geological periods when the
Laurentian rocks of Sir Wm. Logan, constituting the Labrador coast and
the Laurentide Hills, were alone above water, we may even attribute in
no small degree to the Arctic current of that old time the heaping up
of those thousands of feet of deposits which now constitute the great
range of the Alleghany and Appalachian mountains, and form the breast
bone of the American continent. In those ancient times also large
stones were floated southward, and enter into the composition of very
old conglomerates.
[159] At the time when this was written I had only studied stones
brought up accidentally by fishermen and others from the banks of
Newfoundland and elsewhere. At a later date Murray of the _Challenger_
has given more ample material. He states that the bottom in the
Labrador current, 100 miles from land, was found to be blue mud with
60 per cent, of sand and stones; and mentions a block of syenite
weighing 490 lbs. taken up in 1,340 fathoms, and stones and pebbles
of quartzite, limestone, dolomite, mica schist and serpentine, one
of which was glaciated. This is the modern boulder clay produced by
Greenland glaciers and the field ice of Baffin's Bay and the Labrador
coast.
But such large speculations might soon carry us far from Belle-Isle,
and to bring us back to the American coast and to the domain of common
things, we may note that a vast variety of marine life exists in the
cold waters of the Arctic current, and that this is one of the reasons
of the great and valuable fisheries of Labrador, Newfoundland and Nova
Scotia, regions in which the sea thus becomes the harvest field of much
of the human population. On the Arctic current and its ice also floats
to the southward the game of the sealers of St. John and the whalers of
Gaspé.
We may now proceed to connect these statements as to the distribution
of icebergs, with the glaciated condition of our continents, with the
remarkable fact that the same effects now produced by the ice and
the Arctic current in the Strait of Belle-Isle and the deep-current
channel off the American coast, are visible all over the North American
and European land north of forty degrees of latitude, and that there
is evidence that the St. Lawrence valley itself was once a gigantic
Belle-Isle, in which thousands of bergs worked perhaps for thousands
of years, grinding and striating its rocks, cutting out its deeper
parts, and heaping up in it quantities of northern _débris_. Out of
this fact of the so-called glaciated condition of the surface of our
continents has, however, arisen one of the great controversies of
modern geology. While all admit the action of ice in distributing and
arranging the materials which constitute the last coating which has
been laid upon the surface of our continents, some maintain that land
glaciers have done the work, others, that sea-borne ice has been the
main agent employed. As in some other controversies, the truth seems to
lie between the extremes. Glaciers are slow, inactive, and limited in
their sphere. Floating ice is locomotive and far-travelled, extending
its action to great distances from its sources. So far, the advantages
are in favour of the flotation. But the work which the glacier does is
done thoroughly, and, time and facilities being given, it may be done
over wide areas. Again, the iceberg is the child of the glacier, and
therefore the agency of the one is indirectly that of the other. Thus,
in any view we must plough with both of these geological oxen, and the
controversy becomes like that old one of the Neptunists and Plutonists,
which has been settled by admitting both water and heat to have been
instrumental in the formation of rocks.
In the midst of these controversies a geologist resident in Great
Britain or Canada should have some certain doctrine as to the
question whether at that period, geologically recent, which we call
the Pleistocene period, the land was raised to a great height above
the sea, and covered like Greenland with a mantle of perpetual ice,
or whether it was, like the strait of Belle-Isle and the banks of
Newfoundland, under water, and annually ground over by icebergs,
or whether, as now seems more probable, it was in part composed of
elevated ridges covered with snow and sending down glaciers, and partly
depressed under the level of ice-laden straits and seas.
A great advocate of the glacier theory has said that we cannot
properly appreciate his view without exploring thoroughly the present
glaciers of Greenland and ascertaining their effects. This I have not
had opportunity to do, but I have endeavoured to do the next best thing
by passing as rapidly as possible from the icebergs of Belle-Isle to
the glaciers of Mont Blanc, and by asking the question whether Canada
was in the Pleistocene period like the present Belle-Isle or the
present Mont Blanc, or whether it partook of the character of both? and
taking advantage of these two most salient points in order to elicit a
reply.
Transporting ourselves, then, to the monarch of the Alps, let us
suppose we stand upon the Flegere, a spur of the mountains fronting
Mont Blanc, and commanding a view of the entire group. From this point
the western end of the range presents the rounded summit of Mont Blanc
proper, flanked by the lower eminences of the Dome and Aiguille de
Gouté, which rise from a broad and uneven plateau of _nevé_ or hard
snow, sending down to the plain two great glaciers or streams of ice,
the Bossons and Tacony glaciers. Eastward of Mont Blanc the _nevé_
or snow plateau is penetrated by a series of sharp points of rock or
aiguilles, which stretch along in a row of serried peaks, and then
give place to a deep notch, through which flows the greatest of all
the glaciers of this side of Mont Blanc, the celebrated Mer de Glace,
directly in front of our standpoint. To the left of this is another
mass of aiguilles, culminating in the Aiguille Verte. This second group
of needles descends into the long and narrow Glacier of Argentiere,
and beyond this we see in the distance the Glacier and Aiguille de
Tour. As seen from this point, it is evident that the whole system of
the Mont Blanc glaciers originates in a vast mantle of snow capping
the ridge of the chain, and extending about twenty miles in length,
with a breadth of about five miles. This mass of snow being above the
limits of perpetual frost, would go on increasing from year to year,
except so far as it might be diminished by the fall of avalanches from
its sides, were it not that its plasticity is sufficient to enable the
frozen mass to glide slowly down the valleys, changing in its progress
into an icy stream, which, descending to the plain, melts at its base
and discharges itself in a torrent of white muddy water. The Mont Blanc
chain sends forth about a dozen of large glaciers of this kind, besides
many smaller ones. Crossing the valley of Chamouni, and ascending
the Montanvert to a height of about 6,000 feet, let us look more
particularly at one of these glaciers, the Mer de Glace. It is a long
valley with steep sides, about half a mile wide, and filled with ice,
which presents a general level or slightly inclined surface, traversed
with innumerable transverse cracks or crevasses, penetrating apparently
to the bottom of the glacier, and with slippery sloping edges of moist
ice threatening at every step to plunge the traveller into the depths
below. Still the treacherous surface is daily crossed by parties of
travellers, apparently without any accident. The whole of the ice is
moving steadily along the slope on which it rests, at the rate of eight
to ten inches daily--the rate of motion is less in winter and greater
in summer; and farther down, where the glacier goes by the name of the
Glacier du Bois, and descends a steeper slope, its rapidity is greater;
and at the same time by the opening of immense crevasses its surface
projects in fantastic ridges and pinnacles. The movements and changes
in the ice of these glaciers are in truth very remarkable, and show a
mobility and plasticity which at first sight we should not have been
prepared to expect in a solid like ice.[160] The crevasses become open
or closed, curved upwards or downwards, perpendicular or inclined,
according to the surface upon which the glacier is moving, and the
whole mass is crushed upward or flattens out, its particles evidently
moving on each other with much the same result as would take place in
a mass of thick mud similarly moving. On the surface of the ice there
are a few boulders and many stones, and in places these accumulate in
long irregular bands indicating the lines of junction of the minor ice
streams coming in from above to join the main glacier. At the sides
are two great mounds of rubbish, much higher than the present surface
of the glacier. They are called the lateral moraines, and consist of
boulders, stones, gravel and sand, confusedly intermingled, and for the
most part retaining their sharp angles. This mass of rubbish is moved
downward by the glacier, and with the stones constituting the central
moraine, is discharged at the lower end, accumulating there in the
mass of detritus known as the terminal moraine.
[160] I need scarcely say that I adopt the explanation of glacier
motion given by Forbes. "The fuller consideration of the physical
properties of glacier ice leads essentially to the same conclusions
as those to which Forbes was led forty-one years ago by the study of
the larger phenomena of glacier motion, that is, that the motion is
that of a slightly viscous mass, partly sliding upon its bed, partly
shearing upon itself under the influence of gravity."--Trotter, _Proc.
Royal Society of London_, xxxviii. 107.
Glaciers have been termed rivers of ice; but there is one respect in
which they differ remarkably from rivers. They are broad above and
narrow below, or rather, their width above corresponds to the drainage
area of a river. This is well seen in a map of the Mer de Glace. From
its termination in the Glacier du Bois to the top of the Mer de Glace
proper, a distance of about three and a half miles, its breadth does
not exceed half a mile, but above this point it spreads out into three
great glaciers, the Geant, the Du Chaud, and the Talefre, the aggregate
width of which is six or seven miles. The snow and ice of a large
interior table-land or series of wide valleys are thus emptied into
one narrow ravine, and pour their whole accumulations through the Mer
de Glace. Leaving, however, the many interesting phenomena connected
with the motion of glaciers, and which have been so well interpreted
by Saussure, Agassiz, Forbes, Hopkins, Tyndall, and others, we may
consider their effects on the mountain valleys in which they operate.
1. They carry quantities of _débris_ from the hill tops and mountain
valleys downward into the plains. From every peak, cliff and ridge the
frost and thaw are constantly loosening stones and other matters which
are swept by avalanches to the surface of the glacier, and constitute
lateral moraines. When two or more glaciers unite into one, these
become medial moraines, and at length are spread over and through the
whole mass of the ice. Eventually all this material, including stones
of immense size, as well as fine sand and mud, is deposited in the
terminal moraine, or carried off by the streams.
2. They are mills for grinding and triturating rock. The pieces of rock
in the moraine are, in the course of their movement, crushed against
one another and the sides of the valley, and are cracked and ground
as if in a crushing mill. Further the stones on the surface of the
glacier are ever falling into crevasses, and thus reach the bottom of
the ice, where they are further ground one against another and the
floor of rock. In the movement of the glacier these stones seem in
some cases to come again to the surface, and their remains are finally
discharged in the terminal moraine, which is the waste-heap of this
great mill: The fine material which has been produced, the flour of the
mill, so to speak, becomes diffused in the water which is constantly
flowing from beneath the glacier, and for this reason all the streams
flowing from glaciers are turbid with whitish sand and mud.
The Arve, which drains the glaciers of the north side of Mont Blanc,
carries its burden of mud into the Rhone, which sweeps it, with the
similar material of many other Alpine streams, into the Mediterranean,
to aid in filling up the bottom of that sea, whose blue waters
it discolours for miles from the shore, and to increase its own
ever-enlarging delta, which encroaches on the sea at the rate of about
half a mile per century. The upper waters of the Rhone, laden with
similar material, are filling up the Lake of Geneva; and the great
deposit of "loess" in the alluvial plain of the Rhine, about which Gaul
and German have contended since the dawn of European history, is of
similar origin. The mass of material which has thus been carried off
from the Alps, would suffice to build up a great mountain chain. Thus,
by the action of ice and water--
"The mountain falling cometh to naught,
And the rock is removed out of its place."
Many observers who have commented on these facts have taken it for
granted that the mud thus sent off from glaciers, and which is so
much greater in amount than the matter remaining in their moraines,
must be ground from the bottom of the glacier valleys, and hence have
attributed to these glaciers great power of cutting out and deepening
their valleys. But this is evidently an error, just as it would be
an error to suppose the flour of a grist mill ground out of the mill
stones. Glaciers, it is true, groove and striate and polish the rocks
over which they move, and especially those of projecting points and
slight elevations in their beds; but the material which they grind up
is principally derived from the exposed frost-bitten rocks above them,
and the rocky floor under the glacier is merely the nether mill stone
against which those loose stones are crushed. The glaciers, in short,
can scarcely be regarded as cutting agents at all, in so far as the
sides and bottoms of their beds are concerned, and in the valleys which
the old glaciers have abandoned, it is evident that the torrents which
have succeeded them have far greater cutting power.
The glaciers have their periods of advance and of recession. A series
of wet and cool summers causes them to advance and encroach on the
plains, pushing before them their moraines, and even forests and human
habitations. In dry and warm summers they shrink and recede. Such
changes seem to have occurred in bygone times on a gigantic scale. All
the valleys below the present glaciers present traces of former glacier
action. Even the Jura mountains seem at one time to have had glaciers.
Large blocks from the Alps have been carried across the intervening
valley and lodged at great heights, on the slopes of the Jura, leading
the majority of the Swiss and Italian geologists to believe that even
this great valley and the basin of Lake Leman were once filled with
glacier ice. But, unless we can suppose that the Alps were then vastly
higher than at present, this seems scarcely to be physically possible,
and it seems more likely that the conditions were just the reverse
of those supposed, namely, that the low land was submerged, and that
the valley of Lake Leman was a strait like Belle-Isle, traversed by
powerful currents and receiving icebergs from both Jurassic and Alpine
glaciers, and probably from farther north. One or other supposition
is required to account for the appearances, which may be explained on
either view. The European hills may have been higher and colder, and
changes of level elsewhere may have combined with this to give a cold
climate with moisture; or a great submergence may have left the hills
as islands, and may have so reduced the temperature by the influx of
arctic currents and ice, as to enable the Alpine glaciers to descend to
the level of the sea. Now, we have evidence of such submergence in the
beds of sea-shells and travelled boulders scattered over Europe, while
we also have evidence of contemporaneous glaciers, in their traces on
the hills of Wales and Scotland and elsewhere, where they do not now
occur.
I have long maintained that in America all the observed facts imply
a climate no colder than that which would have resulted from the
subsidence which we know to have occurred in the temperate latitudes
in the Pleistocene period, and though I would not desire to speak so
positively about Europe, I confess to a strong impression that the
same is the case there, and that the casing of glacier ice imagined
by many geologists, as well as the various hypotheses which have been
devised to account for it, and to avoid the mechanical, meteorological,
and astronomical difficulties attending it, are alike gratuitous and
chimerical, as not being at all required to account for observed facts,
and being contradictory, when carefully considered, to known physical
laws as well as geological phenomena.[161]
[161] _Canadian Naturalist_, vols. viii. and ix. _Geological Magazine_,
December, 1865.
Carrying with me a knowledge of the phenomena of the glacial drift as
they exist in North America, and of the modern ice drift on its shores,
I was continually asking myself the question To what extent do the
phenomena of glacier drift and erosion resemble these? and standing on
the moraine of the Bosson glacier, which struck me as more like boulder
clay than anything else I saw in the Alps, with the exception of some
recent avalanches, I jotted down what appeared to me to be the most
important points of difference. They stand thus:--
1. Glaciers heap up their _débris_ in abrupt ridges. Floating ice
sometimes does this, but more usually spreads its load in a more or
less uniform sheet.[162]
[162] Under floating ice I include floe, pack, and bordage ice as well
as bergs.
2. The material of moraines is all local. Floating ice carries its
deposits often to great distances from their sources.
3. The stones carried by glaciers are mostly angular, except where they
have been acted on by torrents. Those moved by floating ice are more
often rounded, being acted on by the waves and by the abrading action
of sand drifted by currents.
4. In the marine glacial deposits mud is mixed with stones and
boulders. In the case of land glaciers, most of this mud is carried off
by streams and deposited elsewhere.
5. The deposits from floating ice may contain marine shells. Those
of glaciers cannot, except where, as in Greenland and Spitzbergen,
glaciers push their moraines out into the sea.
6. It is of the nature of glaciers to flow in the deepest ravines they
can find, and such ravines drain the ice of extensive areas of mountain
land. Floating ice, on the contrary, acts with greatest ease on flat
surfaces or slight elevations in the sea bottom.
7. Glaciers must descend slopes and must be backed by large supplies of
perennial snow. Floating ice acts independently, and being water-borne
may work up slopes and on level surfaces.
8. Glaciers striate the sides and bottoms of their ravines very
unequally, acting with great force and effect only on those places
where their weight impinges most heavily. Floating ice, on the
contrary, being carried by constant currents and over comparatively
flat surfaces, must striate and grind more regularly over large areas,
and with less reference to local inequalities of surface.
9. The direction of the striæ and grooves produced by glaciers depends
on the direction of valleys. That of floating ice, on the contrary,
depends upon the direction of marine currents, which is not determined
by the outline of the surface, but is influenced by the large and wide
depressions of the sea bottom.
10. When subsidence of the land is in progress, floating ice may carry
boulders from lower to higher levels. Glaciers cannot do this under any
circumstances, though in their progress they may leave blocks perched
on the tops of peaks and ridges.
I believe that in all these points of difference the boulder clay and
drift on the lower lands of Canada and other parts of North America,
correspond rather with the action of floating ice than of land ice;
though certainly with glaciers on such land as existed at the different
stages of the submergence, and these glaciers drifting stones and
earthy matter in different directions from higher land toward the sea.
More especially is this the case in the character of the striated
surfaces, the bedded distribution of the deposits, the transport of
material up the natural slope, the presence of marine shells, and the
mechanical and chemical characters of the boulder clay. In short, those
who regard the Canadian boulder clay as a glacier deposit, can only do
so by overlooking essential points of difference between it and modern
accumulations of this kind.
I would wish it here to be distinctly understood, that I do not doubt
that at the time of the greatest Pleistocene submergence of Eastern
America, at which time I believe the greater part of the boulder clay
was formed, and the more important striation effected, the higher
hills then standing as islands would be capped with perpetual snow,
and through a great part of the year surrounded with heavy field and
barrier ice, and that in those hills there might be glaciers of greater
or less extent. Further, it should be understood that I regard the
boulder clays of the St. Lawrence valley as of different ages, ranging
from those of the early Pleistocene to that now forming in the Gulf
of St. Lawrence; and that during these periods great changes of level
occurred. Further, that this boulder clay shows in every place where I
have been able to examine it, evidence of subaqueous accumulation, in
the presence of marine shells or in the unweathered state of the rocks
and minerals enclosed in it; conditions which, in my view, preclude
any reference of it to glacier action, except possibly in some cases
to that of glaciers stretching from the land over the margin of the
sea, and forming under water a deposit equivalent in character to the
_bone glaciare_ of the bottom of the Swiss glaciers. But such a deposit
must have been local, and would not be easily distinguishable from
the marine boulder clay. It is of some interest to compare Canadian
deposits with those of Scotland,[163] which in character and relations
so closely resemble those of Canada; but I confess several of the facts
lead me to infer that much of what has been regarded as of subaërial
origin in that country must really be marine, though whether deposited
by icebergs or by the fronts of glaciers terminating in the sea, I do
not pretend to determine.[164] It must, however, be observed that the
antecedent probability of a glaciated condition is much greater in
the case of Scotland than in that of Canada, from the high northern
latitude of the former, its hilly and maritime character, and the fact
that its present exemption from glaciers is due to what may be termed
exceptional and accidental geographical conditions; more especially
to the distribution of the waters of the Gulf Stream, which might be
changed by a comparatively small subsidence in Central America. To
assume the former existence of glaciers in a country in north latitude
56°, and with its highest hills, under the present exceptionally
favourable conditions, snow-capped during most of the year, is a very
different thing from assuming a covering of continental ice over wide
plains more than ten degrees farther south, and in which, even under
very unfavourable geographical accidents, no snow can endure the summer
sun, even in mountains several thousand feet high. Were the plains
of North America submerged and invaded by the cold arctic currents,
the Gulf Stream being at the same time turned into the Pacific, the
temperature of the remaining North American land would be greatly
diminished; but under these circumstances the climate of Scotland
would necessarily be reduced to the same condition with that of South
Greenland or Northern Labrador. As we know such a submergence of
America to have occurred in the Pleistocene period, it does not seem
necessary to have recourse to any other cause for either side of the
Atlantic. It would, however, be a very interesting point to determine,
whether in the Pleistocene period the greatest submergence of America
coincided with the greatest submergence of Europe, or otherwise. It
is quite possible that more accurate information on this point might
remove some present difficulties. I think it much to be desired that
the many able observers now engaged on the Pleistocene of Europe,
would at least keep before their minds the probable effects of the
geographical conditions above referred to, and inquire whether a due
consideration of these would not allow them to dispense altogether with
the somewhat extravagant theories of glaciation now agitated.
[163] _Journal of Geological Society._ Papers by Jamieson, Bryce,
Crosskey, and Geikie.
[164] Geikie, _Trans. Royal Society of Edin._ Geikie assigns a more
complicated structure than appears to be present in Canada; but there
are Canadian equivalents of the principal glacial periods which he
assumes.
The preceding pages give the substance of my conclusions of
twenty-four years ago. I give those of to-day from a paper of
1891,[165] relating to Eastern Canada only:--
[165] Supplement to 4th edition of "Acadian Geology," 1891.
These conclusions have, in my judgment, been confirmed, and their
bearing extended, more especially by the researches of Mr. Chalmers,
who has shown in the most convincing way that glaciers proceeding from
local centres along with sea-borne ice, may have been the agents in
glaciating surfaces and transporting boulders in Nova Scotia and New
Brunswick. Taken in connection with the observations of Dr. Dawson
and Mr. McConnell in the Cordillera region of the west, and those of
Dr. Bell, Dr. Ells, Mr. Low, and others in the Laurentian country
north of the St. Lawrence, and in the Province of Quebec, we may now
be said to know that there was not, even at the height of the glacial
refrigeration of America, a continental ice sheet, but rather several
distinct centres of ice action,--one in the Cordillera of the West,
one on the Laurentian V-shaped axis, and one on the Appalachians, with
subordinate centres on isolated masses like the Adirondacks, and at
certain periods even on minor hills like those of Nova Scotia. It would
further seem that, in the west at least, elevation of the mountain
ridges coincided with depression of the plains. In Newfoundland also,
it would appear from the observations of Captain Kerr, with which those
of Mr. Murray are in harmony,[166] though they have been differently
interpreted, that the gathering ground of ice was in the interior
of the island, and that glaciers moved thence to the coasts, but
principally to the east coast, as was natural from the conformation of
the land and the greater supply of moisture from the Atlantic.
[166] _Trans. Royal Society of Canada_, vol. i.
The labours of Murray in Newfoundland, of Matthew, Chalmers, Bailey,
and others, in Nova Scotia and New Brunswick, have considerably
enlarged our knowledge of Pleistocene fossils, showing, however, that
the marine fauna is the same with that of the beds of like age in the
St. Lawrence valley, and with the existing fauna of the Labrador coast
and colder portions of the Gulf and River St. Lawrence, as ascertained
by Prickard, Whiteaves, and the writer. It would seem that throughout
this region, the 60 feet and the 600 feet terraces were the most
important with reference to these marine remains, and that their chief
repository is in the Upper Leda Clay, a marine deposit intermediate
between the Lower and Upper boulder drift, and corresponding to the
interglacial beds of the interior of America.
The general conditions of the period may be thus summarized:--
In this district, and the eastern part of North America generally, it
is, I think, universally admitted that the later Pliocene period was
one of continental elevation, and probably of temperate climate. The
evidence of this is too well known to require re-statement here. It is
also evident, from the raised beaches holding marine shells, extending
to elevations of 600 feet, and from drift boulders reaching to a far
greater height, that extensive submergence occurred in the middle and
later Pleistocene. This was the age of the beds I have named the _Leda_
clays and _Saxicava_ sands, found at heights of 600 feet above the sea
in the St. Lawrence valley, nearly as far west as Lake Ontario.
It is reasonable to conclude that the till or boulder clay, under
the Leda clay, belongs to the earliest period of probably gradual
subsidence, accompanied with a severe climate, and with snow and
glaciers on all the higher grounds, sending glaciated stones into
the sea. This deduction agrees with the marine shells, polyzoa, and
cirripedes found in the boulder deposits on the lower St. Lawrence,
with the unoxidized character of the mass, which proves subaqueous
deposition, with the fact that it contains soft boulders, which would
have crumbled if exposed to the air, with its limitation to the lower
levels and absence on the hillsides, and with the prevalent direction
of striation and boulder drift from the north-east.[167]
[167] Notes on the Post-Pliocene _Canadian Naturalist_, _op. cit._;
also Paper by the author on Boulder Drift at Metis, _Canadian Record of
Science_, vol. ii., 1886, p. 36, _et seq._
All these indications coincide with the conditions of the modern
boulder drift on the lower St. Lawrence and in the Arctic regions,
where the great belts and ridges of boulders accumulated by the coast
ice would, if the coast were sinking, climb upward and be filled in
with mud, forming a continuous sheet of boulder deposit similar to that
which has accumulated and is accumulating on the shores of Smith's
Sound and elsewhere in the Arctic, and which, like the older boulder
clay, is known to contain both marine shells and driftwood.[168]
[168] For references see "Royal Society's Arctic Manual," London, 1875,
_op. cit._
The conditions of the deposit of "till" diminished in intensity as
the subsidence continued. The gathering ground of local glaciers was
lessened, the ice was no longer limited to narrow sounds, but had a
wider scope, as well as a freer drift to the southward, and the climate
seems to have been improved. The clays deposited had few boulders and
many marine shells, and to the west and north there were land-producing
plants akin to those of the temperate regions; and in places only
slightly elevated above the water, peaty deposits accumulated. The
shells of the Leda clay indicate depths of less than 100 fathoms.
The numerous Foraminifera, so far as have been observed, belong to
this range, and I have never seen in this clay the assemblage of
foraminiferal forms now dredged from 200 to 300 fathoms in the Gulf of
St. Lawrence.
I infer that the subsidence of the Leda clay period and of the
interglacial beds of Ontario belongs to the time of the sea beaches
from 450 to 600 feet in height, which are so marked and extensive as
to indicate a period of repose. In this period there were marine
conditions in the lower and middle St. Lawrence and in the Ottawa
valley, and swamps and lakes on the upper Ottawa and the western end
of Lake Ontario. It is quite probable, nay, certain, that during this
interglacial period re-elevation had set in, since the upper Leda
clay and the Saxicava sand indicate shallowing water, and during this
re-elevation the plant-covered surface would extend to lower levels.
This, however, must have been followed by a second subsidence, since
the water-worn gravels and loose, far-travelled boulders of the later
drift rose to heights never reached by the till or the Leda clay, and
attained to the tops of the highest hills of the St. Lawrence valley,
1,200 feet in height, and elsewhere to still greater elevations. This
second boulder drift must have been wholly marine, and probably not
of long duration. It shows no evidence of colder climate than that
now prevalent, nor of extensive glaciers on the mountains; and it was
followed by a paroxysmal elevation in successive stages till the land
attained even more than its present height, as subsidence is known to
have been proceeding in modern times.
I am quite aware that the above sequence and the causes assumed are
somewhat different from those held by many geologists with reference to
regions south of Canada; but must hold that they are the only rational
conclusions which can be propounded with reference to the facts
observed from the parallel of 45° to the Arctic Ocean.
My own observations have been chiefly in the eastern part of North
America. My son, Dr. G. M. Dawson, has much more ably and thoroughly
explored those of the west; and after describing the immense
Cordilleran ice mass which extended for a length of 1,200 miles along
the mountains of British Columbia and discharged large glaciers to the
north, as well as to the west and south, and stating his reasons for
believing in that differential elevation and depression which caused
the greatest height of the mountains to coincide with the greatest
depression of the plains, and _vice versâ_, and showing the Cordilleran
glacier must have been separated by a water area from that of the
Laurentide hills on the east, thus concludes:--
"It is now distinctly known, as the result of work done under the
auspices of the Geological Survey of Canada, and more particularly
of observations by the writer and his colleagues, Messrs. McConnell
and Tyrrell, that the extreme margins of the western and eastern
glaciated areas of the continent barely overlap, and then only to a
very limited extent, while the two great centres of dispersion were
entirely distinct. For numerous reasons which cannot be here entered
into, the writer does not consider it probable, or even possible,
that the great confluent glacier of the north-eastern part of the
continent extended at any time far into the area of the great plains;
but erratics and drift derived from this ice mass did so extend,
and are found between the 49th and 50th parallels, stranded on the
surface of moraines produced by the large local glaciers of the Rocky
Mountains. Recognising, however, the essential separateness of the
western and eastern confluent ice masses, and the fact that it is no
longer appropriate to designate one of these the "continental glacier,"
the writer ventures to propose that the eastern _mer de glace_ may
appropriately be named the great _Laurentide glacier_, while its
western fellow is known as the "_Cordilleran glacier_." It may be
added that there is good evidence to show that both the Laurentide and
Cordilleran glaciers discharged into open water to the north."
These conclusions, based on a large induction of facts applying to a
very large area of the North American Continent, coincide with my own
observations in the east, and with the inferences deducible from the
present condition of Greenland and Arctic America.
When extreme glacialists point to Greenland and ask us to believe
that in the Glacial age the whole continent of North America, as far
south as the latitude of 40°, was covered with a continuous glacier,
having a wide front, and thousands of feet thick, we may well ask,
first, what evidence there is that Greenland or even the Antarctic
continent is at present in such a condition; and, secondly, whether
there exists a possibility that the interior of a great continent could
ever receive so large an amount of precipitation as that required. So
far as present knowledge exists, it is certain that the meteorologist
and the physicist must answer both questions in the negative. In
short, perpetual snow and glaciers must be local, and cannot be
continental, because of the vast amount of evaporation and condensation
required. These can only be possible where comparatively Warm seas
supply moisture to cold and elevated land, and this supply cannot, in
the nature of things, penetrate far inland. The actual condition of
interior Asia and interior America in the higher northern latitudes
affords positive proof of this. In a state of partial submergence
of our northern continents, we can readily imagine glaciation by
the combined action of local glaciers and great ice floes; but in
whatever way the phenomena of the boulder clay and of the so-called
"terminal moraines" are to be accounted for, the theory of a continuous
continental glacier must be given up.
The great interior plain of western Canada, between the Laurentian
axis on the east and the Rocky Mountains on the west, is seven hundred
miles in breadth, and is covered with glacial drift, presenting one
of the greatest examples of this deposit in the world. Proceeding
eastward from the base of the Rocky Mountains, the surface, at first
more than 4,000 feet above the sea level, descends by successive steps
to 2,500 feet, and is based on Cretaceous and Laramie rocks, covered
with boulder clay and sand, in some places from one hundred to two
hundred feet in depth, and filling up pre-existing hollows, though
itself sometimes piled into ridges. Near the Rocky Mountains the
bottom of the drift consists of gravel not glaciated. This extends
to about one hundred miles east of the mountains, and must have been
swept by water out of their valleys. The boulder clay resting on this
deposit is largely made up of local _débris_, in so far as its paste
is concerned. It contains many glaciated boulders and stones from
the Laurentian region to the east, and also smaller pebbles from the
Rocky Mountains, so that at the time of its formation there must have
been driftage of large stones for seven hundred miles or more from
the east, and of smaller stones from a less distance on the west. The
former kind of material extends to the base of the mountains, and to a
height of more than 4,000 feet. One boulder is mentioned as being 42 ×
40 × 20 feet in dimensions. The highest Laurentian boulders seen were
at an elevation of 4,660 feet on the base of the Rocky Mountains. The
boulder clay, when thick, can be seen to be rudely stratified, and at
one place includes beds of laminated clay with compressed peat, similar
to the forest beds described by Worthen and Andrews in Illinois, and
the so-called interglacial beds described by Hinde on Lake Ontario.
The leaf beds on the Ottawa river, and the drift trunks found in the
boulder clay of Manitoba, belong to the same category, and indicate in
the midst of the Glacial period many forest oases far to the north,
having a temperate rather than an arctic flora. In the valleys of
the Rocky Mountains opening on these plains there are evidences of
large local glaciers now extinct, and similar evidences exist on the
Laurentian highlands on the east. A recent paper of Dr. G. M. Dawson on
the Palæography of the Rocky Mountains illustrates in a most convincing
manner the changes which have occurred in the Cordillera of North
America, and the differential elevation and depression which have
affected its climate in the later geological periods.[169]
[169] _Transactions Royal Society of Canada_, 1890.
Perhaps the most remarkable feature of the western drift region is
that immense series of ridges of drift piled against an escarpment of
Laramie and Cretaceous rocks, at an elevation of about 2,500 feet, and
known as the "Missouri Coteau." It is in some places 30 miles broad
and 180 feet in height above the plain at its foot, and extends north
and south for a great distance: being, in fact, the northern extension
of those great ridges of drift which have been traced south of the
great lakes, and through Pennsylvania and New Jersey, and which figure
on the geological maps as the edge of the continental glacier--an
explanation obviously inapplicable in those western regions where
they attain their greatest development. It is plain that in the north
it marks the western limit of the deep-water of a glacial sea, which
at some periods extended much farther west, perhaps with a greater
proportionate depression in going westward, and on which heavy ice from
the Laurentian districts on the east was wafted south-westward by the
arctic currents, while lighter ice from the Rocky Mountains was being
borne eastward from these mountains by the prevailing westerly winds.
We thus have in the west, on a very wide scale, the same phenomena of
varying submergence, cold currents, great ice floes and local glaciers
producing icebergs, to which I have attributed the boulder clay and
upper boulder drift of eastern Canada. In short, we arrive at the
conclusion that there never has been a continental glacier, properly so
called, but that in the extreme Glacial period there have been great
centres of snow and glacial action, in the Cordillera of the west, in
the Laurentian plateau of the north, and in the northern Appalachians,
and the Adirondacks, while the lower lands have been either submerged,
or enjoying a climate habitable by hardy animals and plants.
The till or boulder clay has been called a "ground moraine," but there
are really no Alpine moraines at all corresponding to it. On the other
hand, it is more or less stratified, often rests on soft materials
which glaciers would have swept away, sometimes contains marine
shells, or passes into marine clays in its horizontal extension, and
invariably in its embedded boulders and its paste, shows an unoxidized
condition, which could not have existed if it had been a subaërial
deposit. When the Canadian till is excavated and exposed to the air, it
assumes a brown colour, owing to oxidation of its iron, and many of its
stones and boulders break up and disintegrate under the action of air
and frost. These are unequivocal signs of a subaqueous deposit. Here
and there we find associated with it, and especially near the bottom
and at the top, indications of powerful water action, as if of land
torrents acting at particular elevations of the land, or heavy surf
and ice action on coasts, and the attempts to explain these by glacial
streams have been far from successful. A singular objection sometimes
raised against the subaqueous origin of the till is its general want
of marine remains; but this is by no means universal, and it is well
known that coarse conglomerates of all ages are generally destitute of
fossils, except in their pebbles, and it is further to be observed that
the conditions of an ice-laden sea are not those most favourable for
the extension of marine life, and that the period of time covered by
the glacial age must have been short, compared with that represented by
some of the older formations.
It follows from all this that the great "continental moraine," which
the United States Geological Survey has now "delineated for several
thousand miles extending from the Atlantic to the Pacific," cannot be
a glacier moraine, but must be, like its great continuation northward,
the Missouri coteau, a margin of sea drift, and that we must explain
the whole of the drift of the American continent by the supposition,
first, of a period of elevation of the hills and subsidence of the
valleys in which there were great accumulations of snow on the Western
Cordillera; the Laurentian axis, and the Appalachians and Adirondacks
radiating in every direction from these points, while minor areas of
radiation may have temporarily existed on smaller elevations: that
this was followed by a period of more equal level, in which parts of
the low grounds were clothed with a temperate flora, the "Interglacial
period" so called, succeeded by a second great depression, in which the
high level boulders of the second boulder drift were wafted to great
distances by floating ice.
The late Prof. Alexander Winchell, a man who did not hesitate to
express his convictions, thus bears similar testimony:--"There has been
no continental glacier. There has been no uniform southerly movement of
glacier masses. There has been no persistent declivity as a _sine qua
non_, down which glacier movements have taken place. The continuity of
the supposed continental glacier was interrupted in the regions of the
dry and treeless plains of the west; and in the interior and Pacific
belts of the continent within the United States, ancient glaciation was
restricted to the elevated slopes."[170] He might have added that the
St. Lawrence valley was submerged and received the ends of Appalachian
and Adirondack glaciers on the south-east, and those of Laurentide
glaciers on the north-west.
[170] Nov., 1890.
My friend Prof. Claypole, who, however, has some hesitation, fearing,
I presume, to be cast out of the synagogue for heresy, ventures to
say,[171] "We deduce from the facts and arguments stated above, that
all the observations of glacial action in the northern hemisphere are
explicable by assuming the existence of enormous and confluent[172]
glacier-systems in and about the high lands of Europe, Asia, and
America, which high lands became, therefore, glacial radiants, and shed
their load of ice in all directions over the lower adjacent ground,
along the lines of easiest flow; that this theory does no violence
to the analogy of the existing order of things, requiring merely
an enlargement of actual glaciers by the intensification of actual
conditions : that abundant evidence can be obtained, as, for example,
from Switzerland, that the present glacier system of the earth was
once of sufficient magnitude to produce all the observed phenomena;
that the most important glacial radiants in the northern hemisphere
were, in North America, the district round Hudson Bay, New England
and the Adirondacks, with certain areas in the western Cordilleras,
and in Europe the Norwegian Dovrefelds and the Alps, Asia apparently
possessing none of commensurate importance; that it satisfactorily
explains, also, the previously puzzling absence of glacial action
over the great plain of Siberia, the coldest portion of the northern
temperate zone; that the belief in a vast polar ice cap, thousands of
feet thick, covering the whole Arctic region, and extending almost
continuously down to low latitudes, is an assumption doing violence to
observed physical facts and to probability, that it is not required to
account for the phenomena, and is, in fact, contradictory to some of
them."
[171] _American Geologist_, Feb., 1889.
[172] The term "confluent" is not necessary here. The glaciers of all
mountain chains may be said to be more or less confluent in the nevé,
from which individual glaciers radiate.
In Europe there is equally good evidence of the existence of huge
glaciers on the Scandinavian mountains and the Alps, and of lesser
accumulations of ice on the hills, as, for instance, those of the
British Islands; but the Scandinavian boulders scattered over the
plains of Great Britain must have been water-borne.[173]
[173] The reports of the Scottish boulder committee, and Lapworth's
recent careful examination of the deposits on the East of England
(_Journ. Geol. Soc._, Aug., 1891), strongly confirm me in this opinion.
In connection with these extracts I would observe that the writer,
and those with whom he has acted in this matter, have never held that
icebergs alone, or fields of ice alone, have produced the Pleistocene
deposits. Their contention has been that the period was one in which
glaciers, icebergs, and field ice acted together, and along with
aqueous agencies, in producing the complicated formations of this
remarkable age. They have, however, objected strenuously to the sole
employment of one agent to the exclusion of others, and to attributing
to that agent powers and extension which obviously could not belong to
it, under the known laws which regulate the movement of glaciers by
the force of gravity, and the precipitation of moisture in the form
of snow on mountains and plateaus. These laws show that the movement
of glaciers over level surfaces, or against the slope of the ground,
and their moving stones otherwise than down slopes, are physical
impossibilities, and that the accumulation of snow to form glaciers
can take place only on elevated and cold land, supplied with large
quantities of vapour from neighbouring water. Such accumulation can
under no imaginable conditions take place in the interior plains and
table lands of great continents.
Applying these laws and conclusions to the whole northern hemisphere,
we learn that the conditions to produce a glacial period are the
diversion of the warm currents from the northern seas, the submergence
of land in the temperate regions, and its invasion by cold Arctic
water, and great condensation of snow on the higher lands. Whether this
condensation has a tendency finally to rectify the state of affairs, by
pressing down the mountains and elevating the plains, we do not know,
but I should imagine that it has not; for the high lands will, in the
case supposed, be lightened by denudation, while the plains will be
burdened with a great weight of deposit. Perhaps we should rather look
to this as the agency for depressing and submerging the plains and
elevating the hills, and suppose some other and more general pressure
proceeding from the great sea basins, to effect the re-elevation of the
plains.
These questions suggest that of the date of the Glacial period. This
subject has recently been discussed by Prestwich and others, with
the result that there is no purely geological ground for referring
the Glacial age to a period so remote as that advocated by Croll on
astronomical grounds. Claypole has recently discussed the matter at
some length, and in a temperate spirit.[174] He takes the rate of
erosion of the Niagara gorge as a measure, and shows that the Falls
of St. Anthony, as described by Winchell, and all the other falls
and river gorges in North America, give similar estimates, which are
confirmed by the evidences of lake ridges, of the rate of erosion,
and of the conditions of animal and plant life. The whole go to show
that the culmination of the Glacial age may have occurred less than
10,000 years ago. He further shows that the differential elevation of
Lakes Erie and Ontario, the greater ease with which the river could
cut the lower part of its ravine, the probability that the part of the
gorge between the whirlpool and the fall was not cut, but only cleaned
out in modern times, and the possible greater flow of water in the
early modern period, all tend to shorten the time required, and that,
as Prestwich has inferred from other data, and the writer also in
various papers, some of them of old date, the so-called post-glacial
period, that of the melting away of the ice, may come within 8,000 to
10,000 years of our own time. Probably the first of these figures is
the nearest to the truth,[175] so that, geologically considered, the
Glacial age is very recent.
[174] _Trans. Edinburgh Geol. Soc._, vol. v., 1888.
[175] Upham, one of the ablest and most experienced of the Glacial
geologists in the United States, in a recent paper on the causes of the
glacial period, states similar conclusions, and adduces the evidence of
Gilbert, Andrews, Wright, Emerson and others in the same sense.
Still another question of great cosmic interest relates to the possible
alternation of glacial conditions in the northern and southern
hemispheres. There is evidence of drift in the southern part of South
America, similar to that in the north; but was it deposited at the same
time? If we could be sure that it was not, many difficulties would be
removed. The southern hemisphere is at present emphatically the ocean
hemisphere; the northern, the land hemisphere. Perhaps these conditions
may be capable of being reversed, in which case the periods of
depression in the south may have corresponded with those of elevation
in the north. One thing which we know is, that there is a polar ice
ring, not an ice cap, for we do not know what is within its edges at
the South Pole, about 2,000 miles in diameter, and this in the only
circumstances in which it can exist, namely, surrounded by a vast ocean
furnishing it with abundant aqueous vapour. We also know that from this
ice ring radiate glaciers, carrying _débris_, with which the sea bottom
is strown half way to the equator. If continents were elevated out of
the Southern Ocean, we should probably have on their surfaces glacial
deposits more widespread and continuous than any remaining on the
continents of the northern hemisphere, and like some of them thinning
out to a terminal edge or border, instead of a terminal moraine like
that of a glacier.[176] Thus we may say with some truth that the
southern hemisphere is now passing through one phase of the Glacial
period.
[176] This is now admitted by Chamberlain and others to be the case
with the oldest boulder clay on the American continent.
I have often thought that in the southern hemisphere the condition
of Kerguelen Island and Heard Island, as described in the reports of
the _Challenger_,[177] must very nearly represent the state of some
mountain ranges and peaks in North America in the Glacial age. Heard
Island, in S. latitude 53° 2′, is a mountain peak 6,000 feet high,
and 25 miles in length. It sends down large glaciers to the sea. In
its larger neighbour, Kerguelen, the glaciers do not reach the sea;
but there is evidence that at one time they did. It is still more
curious that, in Kerguelen the modern ice overlies late tertiary
deposits, holding remains of large trees, indicating a more continental
condition and mild climate at no very remote period. The glaciers of
Heard Island and Kerguelen have, no doubt, been carrying down moraine
material into the sea, and this is certainly done on a still greater
scale by those of the Antarctic continent. This sends off bergs which
fill the whole ocean south of 60°, and float much farther north. Some
of them have been seen 2,000 feet long and 200 high, and though most
of the boulders they contain are necessarily concealed, yet masses of
rock, supposed to weigh many tons, have been seen on them. The whole
sea bottom off this continent, as far south as 64°, consists of blue
mud, with boulders and pebbles, some of them glaciated, and farther
north there is, as far as 47 degrees of latitude, a considerable
percentage of drift material, and this sometimes in depths of 1,950
fathoms. It is evident that, if large areas of the southern hemisphere
were elevated into land, we should have phenomena to deal with not much
unlike those of North America at present.
[177] Vol. i. p. 370, etc.
Perhaps no discussion carries with it more of warning to geologists
to exercise caution in framing theories than this of the great ice
age; and if the collapse of extreme views on this subject shall
have the effect of inducing geologists to keep within the limits of
well-ascertained facts and sound induction, to adhere to the Lyellian
doctrine of modern causes to explain ancient phenomena, and to bear
in mind that most great effects involve not one cause, but many
co-operating causes, it may lead to consequences beneficial to science;
and so, emerging from the cold shadows of the continental glacier, we
may find ourselves in the sunshine of truth.
References:--"Acadian Geology," 1st ed., 1855; 4th ed., 1892.
Icebergs of Belle-Isle, and Glaciers of Mont Blanc, _Canadian
Naturalist_, 1865. "Notes on Pleistocene of Canada," Montreal,
1871. Papers at various dates in the _Canadian Naturalist_ and
_Canadian Record of Science_. "The Ice Age in Canada," Montreal,
1893. Canadian Pleistocene, _London Geological Magazine_, March,
1883. Flora of the Pleistocene, _Bulletin of Geological Society of
America_, vol. i., 1890, p. 311, Dawson and Penhallow.
_CAUSES OF CLIMATAL CHANGE._
DEDICATED TO
DR. T. STERRY HUNT, F.R.S.,
Whose Work in
the Chemical and Cosmical Relations of Geology
is beyond all Praise,
and is destined to command
in the Future
even greater Acceptance than in the Past.
Various Theories as to Changes of Climate--The Astronomical Theory
of Croll--The Geographical Theory of Lyell--Objections of a Geological
Character to the Former--Testimony of Geology and Physical Geography in
Favour of the Latter
[Illustration: North America in Periods of Warm and Cold Submergence.
(_A_) Early Cretaceous. (_B_) Glacial or Pleistocene. Shaded portion
land. Unshaded portion.--Snow-clad mountains.--Crosses.--Ice-laden sea.
These maps illustrate the probable geographical conditions of warm and
cold periods. (p. 388.)]