erratics in west and south Wales. Were the paths of these erratics
delineated upon a map, we should find it necessary to suppose that the
wind- or sea-current by which the floating-ice was propelled had flowed
outwards in all directions from the dominant heights!
[Z] It must have likewise flowed in more or less direct opposition
to the current which, in accordance with the iceberg hypothesis,
transported boulders southwards from the high-grounds of south Wales!
If we dismiss the notion of a warm ocean-current for that of a warm
wind, we do not improve our position a whit. Where did the warm wind
come from? Not, certainly, from the ice-laden seas to the east. Are
we to suppose, then, that it flowed in from the south or south-west?
If so, we might well ask how it came to pass that in the immediate
proximity of such a very warm wind as the hypothesis demands, great
snow-fields and glaciers were allowed to exist in Wales? Passing that
objection, we have still to ask how this wind succeeded in melting
large and small masses of floating-ice with such rapidity that it
prevented any of them ever trespassing south of a certain line? It is
obvious that it must have been an exceedingly hot wind; and that, just
as the hypothetical warm ocean-current must have suddenly dived under
the cold water coming from the north, so the hot wind, after passing
over the surface of the sea until it reached a certain more or less
well-defined line, must have risen all at once and flowed vertically
upwards into the cold regions above.
Thus, in seeking to escape from what he doubtless considers the
erroneous and extravagant views of "land-glacialists," Mr. Mackintosh
adopts a hypothesis which lands him in self-contradictions and a
perfect "sea of troubles"--a kind of chaos, in fact. In attempting to
explain the drifts of western England and east Wales he has ignored
the conditions that must have obtained in contiguous regions--thus
forgetting that "nothing in the world is single," and that one ought
not to infer physical conditions for one limited area without stopping
to inquire whether these are in consonance with what is known of
adjacent districts, or in harmony with the existing phenomena of nature.
I have so strongly opposed Mr. Mackintosh's explanation of the
sudden termination of the northern erratics in the neighbourhood
of Wolverhampton and elsewhere, that perhaps I ought to offer an
explanation of my own, that it may, in its turn, undergo examination.
I labour under the disadvantage, however, of not having studied the
drifts in and around Wolverhampton, etc., and the suggestion which I
shall throw out must therefore be taken for what it is worth. It seems
to me, then, that the concentration of boulders in the neighbourhood
of Wolverhampton, and the limits reached by the northern erratics
generally, mark out, in all probability, the line of junction between
the _mer de glace_ coming from the basin of the Irish Sea and that
flowing across the country from the vast _mer de glace_ that occupied
the basin of the German Ocean. Along this line the southerly transport
of the northern boulders would cease, and here they would therefore
tend to become concentrated. But it is most likely that now and again
they would get underneath the ice-flow that set down the Severn valley,
and I should anticipate that they will yet be detected, along with
erratics of eastern origin, as far south even as the Bristol Channel.
If it be objected to this view that erratics from Great Arenig have
been met with south of Wolverhampton, at Birmingham and Bromsgrove,
I would reply that these erratics were probably carried south either
before or after the general _mer de glace_ had attained its climax--at
a period when the Welsh ice was able to creep out further to the east
than it could when the invasion of the North Sea ice was at its height.
I cannot conclude this paper without expressing my admiration for the
long-continued and successful labours of the well-known geologist
whose views I have been controverting. Although I have entered my
protest against his iceberg hypothesis, and have freely criticised his
theoretical opinions, I most willingly admit that the practical results
of his unwearied devotion to the study of those interesting phenomena
with which he is so familiar have laid all his fellow-workers under a
debt of gratitude.
VIII.
Recent Researches in the Glacial Geology of the Continent.[AA]
[AA] Presidential Address to the Geological Section of the British
Association, Newcastle, 1889.
THE President of this section must often have some difficulty in
selecting a subject for his address. It is no longer possible to give
an interesting and instructive summary of the work done by the devotees
of our science during even one year. So numerous have the students of
geological science become--so fertile are the fields they cultivate--so
abundant the harvests they reap, that one in my present position may
well despair of being able to take stock of the numerous additions to
our knowledge which have accumulated within the last twelve months.
Neither is there any burning question which at this time your President
need feel called upon to discuss. True, there are controversies that
are likely to remain unsettled for years to come--there are still not a
few matters upon which we must agree to differ--we do not yet see eye
to eye in all things geological. But experience has shown that as years
advance truth is gradually evolved, and old controversies die out, and
so doubtless it will continue to be. The day when controversies shall
cease, however, is yet, I hope, far in the future; for should that dull
and unhappy time ever arrive, it is quite certain that mineralogists,
petrologists, palæontologists, and geologists shall have died out of
the world. Following the example of many of my predecessors, I shall
confine my remarks to certain questions in which I have been specially
interested; and in doing so I shall endeavour to steer clear, as far
as I can, of controversial matters. My purpose, then, is to give an
outline of some of the results obtained during the last few years by
Continental workers in the domain of glacial geology.
Those who are not geologists will probably smile when they hear one
declare that wielders of the hammer are extremely conservative--that
they are slow to accept novel views, and very tenacious of opinions
which have once found favour in their eyes. Nevertheless, such is the
case, and well for us that it is so. However captivating, however
imposing, however strongly supported by evidence a new view may appear
to be, we do well to criticise, to sift the evidence, and to call for
more facts and experiments, if such are possible, until the proofs
become so strong as to approach as near a demonstration as geologists
can in most cases expect such proofs to go. The history of our science,
and indeed of most sciences, affords abundant illustration of what
I say. How many long years were the views of sub-aërial erosion, as
taught by Hutton and Playfair, canvassed and controverted before
they became accepted! And even after their general soundness had
been established, how often have we heard nominal disciples of these
fathers of physical geology refuse to go so far as to admit that the
river-valleys of our islands have been excavated by epigene agents!
If, as a rule, it takes some time for a novel view to gain acceptance,
it is equally true that views which have long been held are only with
difficulty discarded. Between the new and the old there is a constant
struggle for existence, and if the latter should happen to survive,
it is only in a modified form. I have often thought that a history of
the evolution of geological theories would make a very entertaining
and instructive work. We should learn from it, amongst other things,
that the advance of our science has not always been continuous--now
and again, indeed, it has almost seemed as if the movement had been
retrograde. Knowledge has not come in like an overwhelming flood--as
a broad majestic river--but rather like a gently-flowing tide, now
advancing, now retiring, but ever, upon the whole, steadily gaining
ground. The history I speak of would also teach us that many of
the general views and hypotheses which have been from time to time
abandoned as unworkable, are hardly deserving of the reproach and
ridicule which we in these latter days may be inclined to cast upon
them. As the Scots proverb says: "It is easy to be wise behindhand."
It could be readily shown that not a few discarded notions and
opinions have frequently worked for good, and have rather stimulated
than checked inquiry. Such reflections should be encouraging to every
investigator, whether he be a defender of the old or an advocate of the
new. Time tries all, and each worker may claim a share in the final
establishment of the truth.
Perhaps there is no department of geological inquiry that has given
rise to more controversy than that which I have selected for the
subject of this address. Hardly a single step in advance has been
taken without vehement opposition. But the din of contending sides is
not so loud now--the dust of the conflict has to some extent cleared
away, and the positions which have been lost or maintained, as the case
may be, can be readily discerned. The glacialist who can look back
over the last twenty-five years of wordy conflict has every reason to
be jubilant and hopeful. Many of those who formerly opposed him have
come over to his side. It is true he has not had everything his own
way. Some extreme views have been abandoned in the struggle; that of a
great Polar ice-sheet, for example, as conceived of by Agassiz. I am
not aware, however, that many serious students of glacial geology ever
adopted that view. But it was quite an excusable hypothesis, and has
been abundantly suggestive. Had Agassiz lived to see the detailed work
of these later days, he would doubtless have modified his notion and
come to accept the view of large continental glaciers which has taken
its place.
The results obtained by geologists who have been studying the
peripheral areas of the drift-covered regions of our Continent,
are such as to satisfy us that the drifts of those regions are not
iceberg-droppings, as we used to suppose, but true morainic matter
and fluvio-glacial detritus. Geologists have not jumped to this
conclusion--they have only accepted it after laborious investigation
of the evidence. Since Dr. Otto Torell, in 1875, first stated his
belief that the Diluvium of north Germany was of glacial origin a
great literature on the subject has sprung up, a perusal of which will
show that with our German friends glacial geology has passed through
much the same succession of controversial phases as with us. At first
icebergs are appealed to as explaining everything--next we meet with
sundry ingenious attempts at a compromise between floating-ice and a
continuous ice-sheet. As observations multiply, however, the element
of floating-ice is gradually eliminated, and all the phenomena are
explained by means of land-ice and "Schmelz-wasser" alone. It is a
remarkable fact that the iceberg hypothesis has always been most
strenuously upheld by geologists whose labours have been largely
confined to the peripheral areas of drift-covered countries. In the
upland and mountainous tracts, on the other hand, that hypothesis has
never been able to survive a moderate amount of accurate observation.
Even in Switzerland--the land of glaciers--geologists at one time
were of opinion that the boulder-clays of the low-grounds had a
different origin from those which occur in the mountain-valleys.
Thus, it was supposed that at the close of the Pleistocene period the
Alps were surrounded by great lakes or by gulfs of some inland sea,
into which the glaciers of the high valleys flowed and calved their
icebergs--these latter scattering erratics and earthy débris over
the drowned areas. Sartorius von Waltershausen[AB] set forth this
view in an elaborate and well-illustrated paper. Unfortunately for
his hypothesis no trace of the supposed great lakes or the inland
sea has ever been detected: on the contrary, the character of the
morainic accumulations, and the symmetrical grouping and radiation of
the erratics and perched blocks over the foot-hills and low-grounds,
show that these last have been invaded and overflowed by the glaciers
themselves. Even the most strenuous upholders of the efficacy of
icebergs as originators of some boulder-clays, admit that the
boulder-clay or till, of what we may call the inner or central region
of a glaciated tract is the product of land-ice. Under this category
comes the boulder-clay of Norway, Sweden, and Finland, and of the
Alpine Lands of central Europe, not to speak of the hilly parts of our
own islands.
[AB] "Untersuchungen über die Klimate der Gegenwart und der Vorwelt,"
etc.--_Natuurkundige Verhandelingen v. d. Holland. Maatsch. d.
Wetensch. te Haarlem_, 1865.
When we come to study the drifts of the peripheral areas, it is
not difficult to see why these should be considered to have had a
different origin. They present certain features which, although not
absent from the glacial deposits of the inner region, are not nearly
so characteristic of such upland tracts. I refer especially to the
frequent interstratification of boulder-clays with well-bedded deposits
of clay, sand, and gravel; and to the fact that these boulder-clays are
often less compressed than those of the inner region, and have even
occasionally a silt-like character. Such appearances do seem at first
to be readily explained on the assumption that the deposits have been
accumulated in water opposite the margin of a continental glacier or
ice-sheet--and this was the view which several able investigators in
Germany were for some time inclined to adopt.
But when the phenomena came to be studied in greater detail, and over
a wider area, this preliminary hypothesis did not prove satisfactory.
It was discovered, for example, that "giants' kettles"[AC] were more
or less commonly distributed under the glacial deposits, and such
"kettles" could only have originated at the bottom of a glacier. Again,
it was found that pre-glacial accumulations were plentifully developed
in certain places below the drift, and were often involved with the
latter in a remarkable way. The "brown-coal formation" in like manner
was violently disturbed and displaced, to such a degree that frequently
the boulder-clay is found to underlie it. Similar phenomena were
encountered in regions where the drift overlies the Chalk--the latter
presenting the appearance of having been smashed and shattered--the
fragments having often been dragged some distance, so as to form a kind
of friction-breccia underlying the drift, while large masses are often
included in the clay itself. All the facts pointed to the conclusion
that these disturbances were due to tangential thrusting or crushing,
and were not the result of vertical displacements, such as are produced
by normal faulting, for the disturbances in question die out from above
downwards. Evidence of similar thrusting or crushing is seen in the
remarkable faults and contortions that so often characterise the clays
and sands that occur in the boulder-clay itself. The only agent that
could produce the appearances, now briefly referred to, is land-ice,
and we must therefore agree with German geologists that glacier-ice
has overflowed all the drift-covered regions of the peripheral area.
No evidence of marine action in the formation of the stony clays is
forthcoming--not a trace of any sea-beach has been detected. And yet,
if these clays had been laid down in the sea during the retreat of
the ice-sheet from Germany, surely such evidence as I have indicated
ought to be met with. To the best of my knowledge the only particular
facts which have been appealed to, as proofs of marine action, are the
appearance of bedded deposits in the boulder-clays, and the occasional
occurrence in the clays themselves of a sea-shell. But other organic
remains are also met with now and again in similar positions, such as
mammalian bones and freshwater shells. All these, however, have been
shown to be derivative in their origin--they are just as much erratics
as the stones and boulders with which they are associated. The only
phenomena, therefore, that the glacialist has to account for are the
bedded deposits which occur so frequently in the boulder-clays of the
peripheral regions, and the occasional silty and uncompressed character
of the clays themselves.
[AC] These appear to have been first detected by Professor Berendt and
Professor E. Geinitz.
The intercalated beds are, after all, not hard to explain. If
we consider for a moment the geographical distribution of the
boulder-clays, and their associated aqueous deposits, we shall find
a clue to their origin. Speaking in general terms, the stony clays
thicken out as they are followed from the mountainous and high-lying
tracts to the low-grounds. Thus they are of inconsiderable thickness
in Norway, the higher parts of Sweden, and in Finland, just as we find
is the case in Scotland, northern England, Wales, and the hilly parts
of Ireland. Traced south from the uplands of Scandinavia and Finland,
they gradually thicken out as the low-grounds are approached. Thus in
southern Sweden they reach a thickness of 43 metres or thereabout,
and of 80 metres in the northern parts of Prussia, while over the
wide low-lying regions to the south they attain a much greater
thickness--reaching in Holstein, Mecklenburg, Pomerania, and west
Prussia, a depth of 120 to 140 metres, and still greater depths in
Hanover, Mark Brandenburg, and Saxony. In those regions, however,
a considerable portion of the diluvium consists, as we shall see
presently, of water-formed beds.
The geographical distribution of the aqueous deposits, which are
associated with the stony clays, is somewhat similar. They are very
sparingly developed in districts where the boulder-clays are thin. Thus
they are either wanting, or only occur sporadically in thin irregular
beds, in the high-grounds of northern Europe generally. Further
south, however, they gradually acquire more importance, until in the
peripheral regions of the drift-covered tracts they come to equal and
eventually to surpass the boulder-clays in prominence. These latter,
in fact, at last cease to appear, and the whole bulk of the diluvium,
along the southern margin of the drift area, appears to consist of
aqueous accumulations alone.
The explanations of these facts advanced by German geologists
are quite in accordance with the views which have long been held
by glacialists elsewhere, and have been tersely summed up by Dr.
Jentzsch.[AD] The northern regions, he says, were the feeding-grounds
of the inland-ice. In those regions melting was at a minimum, while the
grinding action of the ice was most effective. Here, therefore, erosion
reached its maximum--ground-moraine or boulder-clay being unable to
accumulate to any thickness. Further south melting greatly increased,
while ground-moraine at the same time tended to accumulate--the
conjoint action of glacier-ice and sub-glacial water resulting in
the complex drifts of the peripheral area. In the disposition and
appearance of the aqueous deposits of the diluvium we have evidence
of an extensive sub-glacial water-circulation--glacier-mills that
gave rise to "giants' kettles"--chains of sub-glacial lakes in which
fine clays gathered--streams and rivers that flowed in tunnels under
the ice, and whose courses were paved with sand and gravel. Nowhere
do German geologists find any evidence of marine action. On the
contrary, the dovetailing and interosculation of boulder-clay with
aqueous deposits are explained by the relation of the ice to the
surface over which it flowed. Throughout the peripheral area it did
not rest so continuously upon the ground as was the case in the inner
region of maximum erosion. In many places it was tunnelled by rapid
streams and rivers, and here and there it arched over sub-glacial
lakes, so that accumulation of ground-moraine proceeded side by side
with the formation of aqueous sediments. Much of that ground-moraine
is of the usual tough and hard-pressed character, but here and there
it is somewhat less coherent and even silt-like. Now a study of the
ground-moraines of modern glaciers affords us a reasonable explanation
of such differences. Dr. Brückner[AE] has shown that in many places the
ground-moraine of the Alpine glaciers is included in the bottom of
the ice itself. The ground-moraine, he says, frequently appears as an
ice-stratum abundantly impregnated with silt and rock-fragments--it is
like a conglomerate or breccia which has ice for its binding material.
When this ground-moraine melts out of the ice--no running water being
present--it forms a layer of unstratified silt or clay, with stones
scattered irregularly through it. Such being the case in modern
glaciers, we can hardly doubt that over the peripheral areas occupied
by the old northern ice-sheet boulder-clay must frequently have been
accumulated in the same way. Nay, when the ground-moraine melted out
and dropt here and there into quietly-flowing water it might even
acquire in part a bedded character.
[AD] _Jahrb. d. königl. preuss. geologischen Landesanstalt für 1884_,
p. 438.
[AE] "Die Vergletscherung des Salzachgebietes, etc.": _Geographische
Abhandlungen herausgegeben v. A. Penck_, Band i. Heft 1.
The limits reached by the inland-ice during its greatest extension are
becoming more and more clearly defined, although its southern margin
will probably never be so accurately determined as that of the latest
epoch of general glaciation. The reasons for this are obvious. When
the inland-ice flowed south to the Harz and the hills of Saxony it
formed no great terminal moraines. Doubtless many erratics and much
rock-rubbish were showered upon the surface of the ice from the higher
mountains of Scandinavia, but owing to the fanning-out of the ice on
its southward march, such superficial débris was necessarily spread
over a constantly-widening area. It may well be doubted, therefore,
whether it ever reached the terminal front of the ice-sheet in
sufficient bulk to form conspicuous moraines. It seems most probable
that the terminal moraines of the great inland-ice would consist of
low banks of boulder-clay and aqueous materials-the latter, perhaps,
strongly predominating, and containing here and there larger and
smaller angular erratics which had travelled on the surface of the
ice. However that may be, it is certain that the whole region in
question has been considerably modified by subsequent denudation, and
to a large extent is now concealed under deposits belonging to later
stages of the Pleistocene period. The extreme limits reached by the ice
are determined rather by the occasional presence of rock-striæ and
_roches moutonnées_, of boulder-clay and northern erratics, than by
recognisable terminal moraines. The southern limits reached by the old
inland-ice appear in this way to have been tolerably well ascertained
over a considerable portion of central Europe. Some years ago I
published a small sketch-map[AF] showing the extent of surface formerly
covered by ice. On this map I did not venture to draw the southern
margin of the ice-sheet in Belgium further south than Antwerp, where
northern erratics were known to occur, but the more recent researches
of Belgian geologists show that the ice probably flowed south for some
little distance beyond Brussels.[AG] Here and there in other parts of
the Continent the southern limits reached by the northern drift have
also been more accurately determined, but, so far as I know, none of
these later observations involves any serious modification of the
sketch-map referred to.
[AF] _Prehistoric Europe_, 1881.
[AG] See a paper by M. E. Delvaux: _Ann. de la. Soc. géol. de Belg._,
t. xiii. p. 158.