Sea opposite the south of Scotland and the west of England, while
preserving a general southerly trend, would vary at different periods.
Before the _mer de glace_ in that basin had attained its climax there
must have been a time when the ice, streaming outwards from the
high-grounds of Cumberland, was enabled to push its way far westward
out into the basin of the Irish Sea. At that time it was still able
to hold its own against the pressure exerted by the Scottish ice. But
as the general _mer de glace_ increased in thickness, the course of
the Cumberland ice would be diverted ever further and further to the
south-east, until, eventually, the Scottish ice came to hug the coast
of Cumberland, and to overflow Lancashire in its progress towards the
south-east. So gorged with ice did the basin of the Irish Sea become,
that a portion of the Scottish ice was forced over the plain of
Cumberland and up the valley of the Eden, where it coalesced with the
ice coming north from the Shap district, and thereafter flowed in an
easterly direction to join the great _mer de glace_ of the North Sea
basin.
Thus the intercrossings of the Criffel and Cumberland erratics
described by Mr. Mackintosh receive a ready explanation by the land-ice
theory. Nor do the intercrossings of the Welsh erratics with those
derived from Scotland and Cumberland offer any difficulty. The ice
coming from the Welsh mountains would naturally be deflected towards
south-east by the _mer de glace_ that streamed in that direction, and
might quite well have carried its characteristic boulders as far as
Birmingham before the general _mer de glace_ had attained its greatest
dimensions. But when that period of maximum glaciation arrived, the
Welsh boulders would be unable to travel so far towards the east, and
the Scottish and Cumberland boulders would then cross the path formerly
followed by the felstone erratics from Great Arenig.
Again, it is evident that when the _mer de glace_ was gradually
decreasing similar oscillations of the ice-flow would take place,
but in reverse order, and thus would give rise to a second series of
intercrossings. Moreover, we must remember that the Glacial period
was characterised by several great changes of climate. It was not one
continuous and prolonged period of cold conditions, but consisted
rather of a succession of arctic and genial climates; so that the same
countries were overrun at different epochs by successive _mers de
glace_, each of which would rework, denude, and redistribute to a large
extent the morainic materials of its predecessor, and thus might well
cause even greater complexity in the dispersion of erratics than has
yet been recognised anywhere in these islands.
Mr. Mackintosh refers to the occurrence of chalk-flints and Lias
fossils associated with northern erratics in the drift-deposits of
the west of England, the presence of which, he thinks, is fatal to
the theory of transport by land-ice. Thus, he says, chalk-flints,
etc., have been met with at Lillieshall (east of Wellington), at
Strethill (near Ironbridge), at Seisdon (between Wolverhampton and
Bridgenorth), at Wolverhampton, near Stafford, and near Bushbury.
Chalk-flints have also been found as far west as Malvern and Hatfield
Camp, south of Ledbury. All these erratics have crossed England from
the east, according to Mr. Mackintosh and other observers. Not only
so, but, as Mr. Mackintosh remarks, those found at Wolverhampton,
Birmingham, etc., "must have _crossed the course_ of the northern
boulders near its southerly termination." And since both northern and
eastern erratics are found associated in the same drift-deposit, it
seems to him "impossible to explain the intercrossing by land-ice or
glaciers." Now, on the contrary, those eastern erratics are scattered
over the very districts where I should have expected to find them. The
observations of geologists in East Anglia have shown that that region
has been invaded by the _mer de glace_ of the North Sea basin.[W]
This remarkable glacial invasion is proved not only by the direction
followed by stones of local derivation, and by boulders which have come
south from Scotland and the northern counties, but by the occurrence
in the boulder-clay at Carnelian Bay and Holderness of erratics of
certain well-known Norwegian rocks, which have been recognised by
Mr. Amund Helland. The occurrence of chalk-flints and fragments of
Oolitic rocks in the neighbourhoods mentioned by Mr. Mackintosh thus
only affords additional evidence in favour of the land-ice origin of
the drift-deposits described by him. The _mer de glace_ that flowed
down the east coast of England seems to have encroached more and more
upon the land, until eventually it swept over the low-lying Midlands
in a south-westerly direction, and coalesced with the _mer de glace_
that streamed inland from the basin of the Irish Sea, and the ice that
flowed outwards from the high-grounds of Wales. The united ice-stream
would thereafter continue on its south-westerly course down the Severn
valley to the Bristol Channel. I have no doubt that Mr. Mackintosh will
yet chronicle the occurrence of chalk-flints and other eastern erratics
from localities much further to the south than Ledbury.
[W] See Mr. Skertchly's description of East Anglian deposits in _Great
Ice Age_, 2nd edit., p. 358.
Again, considerable stress has been laid by Mr. Mackintosh upon the
occurrence of chalk-flints in the drift-deposits of Blackpool, Dawpool,
Parkgate, Halkin Mountain, Wrexham, the peninsula of Wirral, Runcorn,
Delamere, Crewe, Leylands, Piethorne (near Rochdale), and other places.
"All these flints," Mr. Mackintosh remarks, "belong to the basin of the
Irish Sea, and have almost certainly crossed the general course of the
northern boulders on their way from Ireland." Here, unfortunately, the
Irish Sea intervenes to conceal the evidence that is needed to enable
us to track the exact path followed by the erratics in question. I am
not so certain as Mr. Mackintosh that the chalk-flints he refers to
came from the north of Ireland. Chalk-flints occur pretty numerously in
the drift-deposits in the maritime districts of north-eastern Scotland,
which we have every reason to believe have been derived from an area
of Cretaceous rocks covering the bottom of the adjacent sea; and for
aught one can say to the contrary, patches of chalk-with-flints may
occur in like manner in the bed of the Irish Sea. I cannot at present
remember whether any boulders of the basalt-rocks, which are associated
with the Chalk in the north of Ireland, have been recognised in the
drifts of the west of England; but if the chalk-flints really came from
Antrim, it is more than probable that they would be accompanied by
fragments of the hard igneous rocks which overlie the Cretaceous strata
of north Ireland. Chalk and chalk-flints occur in the boulder-clay of
the Isle of Man, where they are associated, Mr. Horne tells us, with
Criffel granite and fragments of a dark trap-rock.[X] Possibly these
last are basalt-rocks from Antrim. It seems reasonable, therefore,
to believe that erratics of Irish origin have found their way to the
Isle of Man; and if this be so, it may be permissible to assume that
the chalk-flints of Blackpool, etc. (and perhaps also some of the
basalt-rocks), have come from the same quarter. Mr. Horne has no doubt
that the Irish erratics were brought to the Isle of Man by land-ice.
Referring to the conclusion arrived at by Mr. Close that the Irish _mer
de glace_ "was probably not less than 3000 feet in depth," he remarks:
"It is highly probable that this great mass of Irish ice succeeded,
after a hard battle (_i.e._, with the Scottish ice-sheet), in reaching
the Manx coast-line. It is not to be supposed that the normal momentum
of the respective ice-sheets remained constant. The moving force must
have varied with changing conditions. On the other hand, it is quite
possible that there may have been an 'under-tow' of the ice from the
north-east coast of Ireland, which would easily account for Antrim
chalk and chalk-flints in the Manx till." I would go further, and state
my conviction that before the united ice-sheets had attained their
maximum development, it is almost certain that the ice flowing into the
Irish Sea basin by the North Channel would for a long time exceed in
mass the coalescent glaciers that descended from the Southern Uplands
of Scotland, and would therefore be enabled to extend much further to
the east than it could at a later date, when the general _mer de glace_
had reached its climax. It might thus have advanced as far as and even
beyond the Isle of Man. This inference is based upon the simple fact
that the area drained by the _mer de glace_ of the North Channel was
very much greater than the area extending from the watershed of the
Southern Uplands of Scotland to the Isle of Man. Erratics from the
north of Ireland would thus travel down the bed of the North Channel,
and eventually be distributed over a wide area up to and possibly even
some distance beyond the Isle of Man. But as the Scottish and Cumbrian
ice-flows gradually increased in importance, the _mer de glace_ coming
from the North Channel would be forced further and further to the west,
until the ice-flow issuing from the high-grounds of Kirkcudbright at
last succeeded in reaching the middle of the Irish Sea basin. This
gradual modification of the general ice-flow in that basin would of
course give rise to a redistribution of the ground-moraine, and the
Irish erratics would then travel onwards underneath the Scottish ice,
and eventually reach the low-grounds of Lancashire and Cheshire,
along with erratics from Criffel and the Cumbrian mountains. It is,
therefore, quite unnecessary to suppose that the _mer de glace_ of
the North Channel actually crossed the whole breadth of the basin of
the Irish Sea to invade Lancashire, Cheshire, and north Wales. Had
this been the case, chalk-flints, chalk, and many other kinds of rock
derived from the north of Ireland, and even from Arran and Argyll,
would have abounded in the drifts of the west of England. Erratics
coming from Ireland could not possibly have travelled underneath
Irish ice further east than the Isle of Man. There or thereabouts, as
I have said, the _mer de glace_ of the North Channel would begin to
encounter the ice streaming down from the uplands of Galloway and the
mountains of Cumberland: and as the ice from these quarters increased
in thickness, it would gradually override what had formerly been the
bottom-moraine or till of the North Channel _mer de glace_. Thus Irish
erratics would become commingled with erratics from Criffel, etc., and
be carried forward in a southerly and south-easterly direction. The
chalk-flints in the drifts of Lancashire, Cheshire, etc., are probably
therefore _remaniés_--the relics of the bottom-moraine of the North
Channel _mer de glace_ rearranged and redistributed. And this is why
they and other Irish rocks are so comparatively rare in the glacial
accumulations of the west of England.
[X] _Trans. Edin. Geol. Soc._, vol. ii., 1874.
Thus all the instances of intercrossings adduced by Mr. Mackintosh
as favouring the iceberg theory, and condemning its rival, I would
cite as proving exactly the opposite. So far from presenting any real
difficulty to an upholder of the land-ice theory, they, in point of
fact, as I have already remarked, lend that view additional support.
It is not my purpose to criticise all the arguments and reasons
advanced by Mr. Mackintosh in favour of his special views, but I may be
allowed a few remarks on the somewhat extraordinary character of the
agents which, according to him, were mainly instrumental in producing
the drift-phenomena of western England. Before doing so, however, I
may point out that, in ascribing the transport of erratics in that
region (and, by implication, the formation of the boulder-clays, etc.,
with which most of these erratics are associated) to floating-ice and
sea-currents, Mr. Mackintosh has failed to furnish us with any "fossil
evidence" to show that western England was under water at the time
the boulder-clays and erratics were being accumulated. He speaks of
cold and warm currents, but where do we find any traces of the marine
organisms which must have abounded in those waters? Where are the
raised sea-beaches which must have marked the retreat of the sea? Where
do we encounter any organic relics that might help us to map out the
zones of shallow and deep water? The sea-shells, etc., which occur in
the boulder-clays are undeniably _remaniés_; they are erratics just
as much as the rock-fragments with which they are associated. Similar
assemblages of organic remains are met with in the till of Caithness,
where shallow-water and deep-sea shells, and shells indicative of
genial and again of cold conditions, are all confusedly distributed
throughout one and the same deposit. The same or analogous facts are
encountered in the _Blocklehm_ of some parts of Prussia, marine and
freshwater shells occurring commingled in the boulder-clay. Nay,
even in the _moraine profonde_ of the ancient Rhone glacier, broken
and well-preserved shells of Miocene and Pliocene species appear
enclosed in the tumultuous accumulation of clay, sand, and erratics.
And precisely similar phenomena confront us in the glacial deposits
of the neighbourhood of Lago Lugano. Mr. Mackintosh refers to the
so-called "stratification" of the boulder-clay, as if that were a proof
of accumulation in water. But a rude kind of bedding, generally marked
by differences of colour, and sometimes by lines of stones, was the
inevitable result of the sub-glacial formation of the boulder-clay.
The "lines of bedding" are due to the shearing of the clay under great
pressure, and may be studied in the boulder-clay of Switzerland and
Italy, and in the till not only of the Lowlands but of the Highlands
of Scotland. Occasionally the "lines" are so close that the clay
sometimes presents the appearance of rude and often wavy and irregular
lamination--a section of such a boulder-clay reminding one sometimes
of that of a gnarled gneiss or crumpled schist. And these appearances
may be noted in boulder-clays which occupy positions that preclude
the possibility of their being marine--as in certain valleys of the
Highlands, such as Strathbraan, and in the neighbourhood of Como, in
Italy. This "lamination" is merely indicative of the intense pressure
to which the till was subjected during its gradual accumulation under
the ice. It is assuredly not the result of aqueous action. Aqueous
lamination is due to sifting and winnowing--the coarser or heavier
and finer or lighter particles being separated in obedience to their
different specific gravity, and arranged in layers of more or less
regularity according to circumstances. There is nothing of this kind of
arrangement, however, in the so-called stratified boulder-clay. If the
clay of an individual lamina be washed and carefully sifted, it will be
found to be composed of grains of all shapes, sizes, and weights, down
to the finest and most impalpable flour. It is impossible to believe
that such a heterogeneous assemblage of grains could have been dropt
into water without the particles being separated and sifted in their
progress to the bottom. Of course, every one knows that patches and
beds of laminated clay and sand of veritable aqueous origin occur now
and again in boulder-clay. I suppose there is no boulder-clay without
them. I have seen them in the till of Italy and Switzerland, where
they show precisely the same features as the similar laminated clays in
the till of our own islands. But these included patches and beds point
merely to the action of sub-glacial waters, such as we know circulate
under the glaciers of the Alps, of Norway, and of Greenland.
Again, I would remark that Mr. Mackintosh has ignored all the evidence
which has been brought forward from time to time to demonstrate
the sub-glacial origin of boulder-clay, and to prove the utter
insufficiency of floating-ice to account for the phenomena. And he
adduces no new facts in support of the now discredited iceberg theory,
unless it be his statement that _flat_ striated rock-surfaces (such
as those near Birkenhead) have been caused by floating-ice--the
dome-shaped _roches moutonnées_ being, on the other hand the work
of land-ice. As a matter of personal observation, I can assure Mr.
Mackintosh that _flat_ striated surfaces are by no means uncommonly
associated in one and the same region with _roches moutonnées_.
What are _roches moutonnées_ but the rounded relics of what were
formerly rough uneven tors, projecting bosses, and prominent rocks?
The general tendency of glacial action is to reduce the asperities of
a land-surface; hence projecting points are rounded off, while flat
surfaces are simply, as a rule, planed smoother. Mr. Mackintosh might
traverse acres of such smoothed rock-surfaces in regions where the
strata are comparatively horizontal--for example, in the case of the
basaltic plateaux of the Faröes and of Iceland, which have certainly
been glaciated by land-ice. Similar flat glaciated surfaces are met
with again and again both in the Highlands and Lowlands of Scotland,
occupying positions and associated with _roches moutonnées_ and till
of such a character as to prove beyond any doubt that they no less
certainly are the result of the action of land-ice. But it is needless
to discuss the probability or possibility of glaciation of any kind
being due to floating-ice. We know that glaciers can and do polish and
striate rock-surfaces; no one, however, can say the same of icebergs:
and until some one can prove to us that icebergs have performed this
feat, or can furnish us with well-considered reasons for believing
them to be capable of it, glacialists will continue sceptical.
But leaving these and other points which serve to show the
weakness of the cause which Mr. Mackintosh supports with such keen
enthusiasm, I may, in conclusion, draw attention to certain very
remarkable theoretical views of his which seem to me to be not
only self-contradictory, but opposed to well-known natural laws.
Briefly stated, his general view is that the erratics of the west
of England have been distributed by floating-ice during a period of
submergence--the scattering of erratics and the accumulation of the
associated glacial deposits having commenced at or about the time
when the land began to sink, and continued until the submergence
reached some 2000 feet below the present sea-level. In applying this
hypothesis to explain the phenomena, Mr. Mackintosh makes rather free
use of sea-currents and winds. For example, he holds that a current
coming from Criffel carried with it boulder-laden ice which flowed
south-west to the Isle of Man, south to north Wales, and south-east in
the direction of Blackpool and Manchester, Liverpool and Wolverhampton,
Dawpool and Church Stretton. Now, in the first place, it is very
strange that there is not a vestige or trace of any such submergence,
either in the neighbourhood of Criffel itself or in the region to the
north of it. The whole of that region has been striated and rubbed by
land-ice coming down from the watershed of the Galloway mountains, to
the north of which the striæ, _roches moutonnées_, and tracks followed
by erratics, indicate an ice-flow _towards_ the north-west, north,
and north-east. It is, therefore, absolutely certain that at the time
the granite erratics are supposed to have sailed away from Criffel
on floating-ice, the whole of the Southern Uplands of Scotland were
covered with a great ice-field extending from Wigtown to Berwickshire;
so that, according to Mr. Mackintosh's hypothesis, we should be forced
to believe that an ocean-current originated in Criffel itself! But
waiving this and other insuperable objections which will occur to any
geologist who is familiar with the glacial phenomena of the south
of Scotland, and confining myself to the evidence supplied by the
English drifts, I would remark that Mr. Mackintosh's hypothesis is not
consistent with itself. A current flowing in the direction supposed
could not possibly have permitted floating-ice to sail from Cumbria
to the Isle of Man, to Moel-y-Tryfane and Colwyn Bay. Mr. Mackintosh
admits this himself, but infers that the transport of the Cumbrian
erratics may have taken place at a different time. But how could this
be, seeing that the Criffel and Cumbrian erratics occur side by side
in one and the same deposit? Again, the hypothesis of an ocean-current
coming from Criffel is inconsistent with the presence of the Irish
chalk-flints in the drifts of the west of England. Did these also
come at a different time? And what about the dispersion of erratics
from Great Arenig, which have gone north-east and north-north-east,
almost exactly in the face of the supposed Criffel current? Here an
ocean-current is obviously out of the question; and accordingly we are
told that this dispersion of Welsh boulders was probably the result
of wind. But why should this wind have propelled the floating-ice so
far and no further in an easterly direction? Surely if floating-ice
was swept outwards from Great Arenig as far as Eryrys, bergs must
have been carried now and again much further to the east. And if they
did not sail eastwards, what became of them? Did they all melt away
immediately when they came into the ice-laden current that flowed
towards the south-east?[Y] A still greater difficulty remains. The
Criffel and Cumbrian erratics suddenly cease when they are followed
to the south, great quantities of them being accumulated over a belt
of country extending from beyond Wolverhampton to Bridgenorth. What
was it that defined the southern limits of these northern boulders?
It is clear that it could not have been high-ground, for the Severn
valley, not to speak of low-lying regions further to the north-east,
must have been submerged according to Mr. Mackintosh's hypothesis.
There was therefore plenty of sea-room for the floating-ice to escape
southwards. And yet, notwithstanding this, vast multitudes of bergs and
floes, as soon as they arrived at certain points, suddenly melted away
and dropt their burdens! In what region under the sun does anything
like that happen at the present day? Mr. Mackintosh thinks that the
more or less sharply-defined boundary-line reached by the erratics
"could only have resulted from close proximity to a persistent current
of water (or air?) sufficiently warm to melt the boulder-laden ice."
He does not tell us, however, where this warm current of water or
air came from, or in what direction it travelled. He forgets some of
his own facts connected with the appearance of erratics of eastern
derivation, and which, according to him, point to an ocean-current
that flowed across from Lincolnshire into the very sea in which the
Criffel granite and Cumbrian boulders were being dropt. The supposed
warm ocean-current, then, if such it was rather than air, could hardly
have come from the east. Neither is it at all likely that it could
have come from the west, sheltered as the region of the Severn valley
must have been by the ice-laden mountains of Wales. Again, the south
is shut to us; for there are no erratics in the south of England from
which to infer a submergence of that district. If it be true that
all the northern erratics which are scattered over the low-grounds
of England, Denmark, Holland, Germany, Poland, and Russia, owe their
origin to boulder-laden ice carried by ocean-currents, no such warm
water as Mr. Mackintosh desiderates could possibly have come from the
east or south-east. We are left, then, to infer that the supposed warm
current[Z] must have flowed up the Severn valley directly in the face
of the Criffel current, underneath which it suddenly plunged at a high
temperature, the line of junction between it and the cold water being
sharply defined, and retaining its position unchanged for a long period
of time! However absurd this conclusion may be, it is forced upon us if
we admit the hypothesis at present under review. For we must remember
that the floating-ice is supposed to have melted whenever it came into
contact with the warm current. The erratics occur up to a certain
boundary-line, where they are concentrated in enormous numbers, and
south of which they do not appear. Here, then, large and small floes
alike must have vanished at once! Certainly a very extraordinary case
of dissolution.