humid conditions having prevailed over middle and southern Europe at
the climax of the glacial period. The limestone-breccias of Gibraltar
have been described by Professor Ramsay and myself, and we have shown
that these could only have been formed under the influence of excessive
frost and melting snows. The limestone of the Rock has been broken up
along the ridge, and its fragments showered down the slopes, at a time
when these were more or less thickly covered with snow. Resting upon
and imbedded in this snow, the rock-rubbish would be carried downward
and outward during the gradual melting that took place in summer. And
in this way immense accumulations of débris were borne forwards over
the low-grounds that extended from the base of the Rock into regions
which are now partially submerged. Breccias which have probably had a
similar origin occur also in Corsica, Malta, and Cyprus, and doubtless
they will yet be recognised in many other places. Again, over wide
areas in northern France and the south of England, we meet with
extensive sheets of earthy clay and rock-rubbish, which have certainly
been heaped up under very different conditions of climate than obtain
now. This stony earth has evidently travelled down the gentle slopes
of the land, under the influence of frost and melting snow, in much
the same way as ice-driven rock-rubbish and soil move slowly down the
slopes of such dreary regions as Patagonia and certain low-lying tracts
within the Arctic Circle.
II.
_Changes of Climate in Europe during the Ice Age._
We come next to the very interesting question of alternations
of climate during the Ice Age. The evidence under this head has
accumulated to such an extent within recent years as to convince most
students of Pleistocene geology that very extensive changes of climate
characterised the glacial period. How many such changes took place we
are not yet in a position to say, but we know that the intensely arctic
condition of things which has just been described was interrupted more
than once by what have been termed "interglacial epochs," during which
a mild and genial climate prevailed over middle and northern Europe.
For some time it was believed that such "interglacial epochs" had only
a local significance, that they bespoke mere transitory retreats of the
ice-fields, such as are known to have taken place within historical
times in the glacier-valleys of the Alps. But increased observation
and reflection have shown that this explanation of the phenomena of
"interglacial beds" will not suffice. It is impossible to enter here
upon details, but I may briefly state that the evidence in question
is two-fold. _First_, we have the stratigraphical evidence. We have
ascertained the existence, over wide areas in this and other glaciated
countries, of several successive sheets of boulder-clay, which are
often separated from each other by fossiliferous aqueous strata. It has
been demonstrated that each of these sheets of sub-glacial detritus
is the accumulation of a separate and distinct ice-flow. _Second_, we
have the evidence of fossil organic remains. We find, for example, that
the flora which covered the low-grounds of middle and temperate Europe
during a certain stage of the glacial or Pleistocene period, consisted
of plants which are now restricted to the tops of our mountains and to
northern Scandinavia. The characteristic fauna associated with that
flora embraced the reindeer, glutton, mammoth, woolly rhinoceros,
Arctic fox lemming, chamois, and so forth. We know, indeed, that man
hunted the reindeer and the mammoth in the south of France. Similar
testimony to the coldness and humidity of the climate is borne by the
land- and freshwater shells which occur in certain Pleistocene deposits
in Italy, Corsica, southern France, Switzerland, Germany, etc. That
this flora and fauna were contemporaneous with the great glaciation of
our Continent has been as well ascertained as the fact of the Roman
occupation of Britain. But if the evidence of organic remains strongly
confirms and supports that supplied by the distribution of glacial
deposits in Europe, no less forcibly does it corroborate the physical
evidence as to the former existence of a warm and genial interglacial
climate. During interglacial times a most abundant mammalian fauna
roamed over all temperate Europe--a fauna comprising such animals as
Irish deer, urus, bison, horse, stag, saiga, brown bear, grisly bear,
several species of elephant, rhinoceros, and hippopotamus, hyæna,
lion, leopard, etc. A like tale of genial conditions is told by the
land- and freshwater shells, which occur in some of the Pleistocene
deposits of England, France, Belgium, Germany, Switzerland, and
Italy. The testimony of the associated flora is just as striking. How
genial and equable must have been the climate which permitted plants
like the Canary laurel, the Judas-tree, the fig-tree, and others to
flourish side by side in the north of France, with such forms as the
hazel, willow, ash, and sycamore! The most noteworthy additions to our
knowledge of interglacial conditions which have recently been made
are the results obtained by M. Gaudry in the valley of the Seine, and
by Dr. Penck in Bavarian Tyrol, the latter of whom has shown that
there have been at least three great advances of the Alpine glaciers,
separated by long-continued mild conditions, during which the glaciers
receded far into the mountains.
It is interesting to observe that we have, especially in our own
islands, good evidence to show that during the glacial period
considerable oscillations of the relative level of land and sea took
place. Thus, it has been ascertained, that just before the latest epoch
of extensive glaciation, the British Islands were largely submerged
in the sea. To what depth this remarkable submergence was carried
we do not know, because any marine deposits which may have been
accumulated at that time over the drowned country were for the most
part obliterated by the action of the ice-sheet which subsequently
covered and reglaciated our lands.[L] But the few fragments of such
marine deposits as have been preserved show us that the depression
reached more than 500 feet in Scotland (_i.e._, measured from the
present sea-level), and exceeding 1000 feet in Wales and Ireland. We
note, then, in passing, that the only great Pleistocene submergence of
these lands of which geologists have any knowledge took place before
the appearance of the last general ice-sheet that overflowed our
low-grounds. The submergences of a later date were of inconsiderable
importance, hardly exceeding 100 feet or thereabouts below the present
sea-level. The latest occupant of our islands and of northern Europe
was not the sea, but ice. The "Palæocrystic Sea," which we have been
recently assured would account for our glacial phenomena, is of
"the stuff that dreams are made of." There is not a jot or tittle
of evidence for the former existence of such a sea over any part of
Britain or the continent of Europe.
[L] I no longer believe in this "great submergence." The marine shells
in the high-level drift-deposits of our islands are "erratics," carried
by the ice-sheet which occupied the basin of the Irish Sea. That the
low-grounds were submerged but the amount of the submergence has not
been ascertained; probably it did not exceed a few hundred feet.
It is not necessary for my present purpose to enter further into the
evidence of interglacial conditions. The latest northern ice-sheet was
preceded by a long epoch of mild and genial conditions, during which
elephants and hippopotami ranged north as far at least as Yorkshire;
while middle Germany, as we know from the testimony of its interglacial
deposits, enjoyed a similar delightful climate. And yet the immediately
preceding glacial epoch had seen all those fertile regions covered
with an ice-sheet that extended south as far as the fiftieth parallel
of latitude. Now the question with which I am at present concerned
is the extent of the latest general glaciation. Did the last great
ice-sheet reach as far south as its predecessor? It certainly did not.
Its bottom-moraine has now been mapped out and distinguished from that
of the older ice-sheet, and we know that it does not extend so far
south as the latter. It is entirely absent over all the region to the
west of the River Elbe, from near Dresden to Hamburg and the coast of
Holland.[M] So that western Germany and Holland, which were covered
by ice during the epoch of greatest glaciation, were not invaded by
the ice-sheet underneath which the upper boulder-clay was accumulated.
This latest ice-sheet, however, overwhelmed all Mecklenburg and Mark
Brandenburg, and streamed south nearly as far as Saxony; its southern
margin extended east through Silesia, by Liegnitz and Breslau, into
Poland and Russia. But the precise line it followed in the latter
country has yet to be ascertained. We may surmise, however, that it
nowhere reached so far south or east as the ice-flow of the earlier
epoch. I may add that the southern termination of the latest ice-sheet
is in many places marked out by heaps, mounds, and ridges of earthy
sand, gravel, rolled stones, and erratics; in short, by terminal
moraines. These, however, are frequently highly degraded and washed
down.
[M] Klockmann, _Jahrb. der k. preuss. geol. Landesanstalt für 1883_, p.
330.
Of the extension of glacier-ice in the British Islands at the epoch
in question I shall only say that the glaciation of Scotland was
hardly, if at all, less extensive than during the climax of the Ice
Age. Ireland, too, appears to have been almost as thickly mantled;
but the ice-sheet that covered England and Wales did not extend so
far south as that of the penultimate glacial epoch, a considerable
area in East Anglia and the midland counties remaining apparently free
from invasion. The Scandinavian and British ice-sheets, however, again
coalesced upon the floor of the North Sea.
III.
_The Results of Fluvio-glacial Action in Europe._
The third question which I now proceed to consider is the result
produced by the rivers and torrents of the Ice Age. This, I am aware,
is a wide subject, and one upon which much has been written. But there
are a few points which may be advantageously discussed for the purpose
of bringing into prominent view the conditions which obtained in the
river-valleys of Europe during the last great extension of glacier-ice.
A little consideration will serve to convince one that the intense
glacial conditions that obtained in our Continent during the cold
epochs of the glacial period were due to a low temperature, combined
with excessive snow-fall. The winters, we can have no doubt, must have
been prolonged and severe. But mere low temperature will not account
for the enormous precipitation of snow. For this, great evaporation
was required. And we are therefore forced to admit that the direct
heat of the sun in summer must have been greater than it is in the
same regions at the present day. Now, if this were really the case
(and I do not see how otherwise the facts can be explained), then we
ought to meet with evidence of swollen rivers, torrents, and widespread
inundations everywhere outside of the glaciated areas. And this is
precisely what we do find. Immense accumulations of coarse gravels
are widely spread over all the valleys that head in regions which
were formerly the sites of snow-fields and glaciers. These gravels
are of such a character and are so distributed as to make it certain
that they could not have been transported to and deposited in their
present positions by rivers like those which now wind their way down
the valleys of middle Europe. Still more remarkable are the enormous
sheets of loam which are spread over much wider areas and reach to more
considerable heights than the gravels. The origin of the gravels is
sufficiently evident; they are simply the coarser detritus, swept along
by the enormously flooded rivers of the glacial period, and meet with
their analogues in the torrential gravels of modern glacier-valleys
in the Alps and other elevated regions. The more widely-spread loams,
according to the opinion of most glacialists, represent the finer mud
and silt deposited from the muddy waters of the same period. But the
height to which such gravels and loams ascend is so great that those
who hold them to be of fluvio-glacial origin have found it difficult
to maintain this view. Some writers, indeed, who have not sufficiently
considered the weight of the evidence in its favour, have set it aside,
and boldly suggested all kinds of wonderful hypotheses in its place.
One imaginative author, for example, believes the wide-spread loams to
be of volcanic origin, while another finds in the same deposits strong
evidence of the Deluge. By a well-known and experienced observer,
the famous löss of middle Europe is considered to be an Æolian
accumulation--that is to say, a wind-blown deposit--the result of
long-continued or frequently-repeated dust-storms. This is the opinion
of Baron Richthofen, whose great work on China is so justly esteemed.
He infers that at the time of the formation of our löss central Europe
was a dry desiccated region, just as wide areas in central Asia are
in our own day. He does not attempt to show us, however, how such
climatic conditions could ever obtain in Europe. In point of fact, the
geographical conditions of our Continent have not changed materially
since Pleistocene times, and the presence of the wide Atlantic Ocean,
that laves all our western shores, is of itself sufficient to preclude
the possibility of such a climate having obtained in middle Europe.
Richthofen's theory likewise fails to account for the geographical
distribution of the löss, and for many facts relating to its geology.
Only one of these last shall I mention. The löss is intimately
associated with accumulations, the glacial and fluvio-glacial origin of
which cannot be doubted. It belongs, in fact, to the glacial series,
and was laid down at a time when vast snow-fields and ice-sheets
existed, and when it is quite impossible that a dry climate could
have characterised any part of our Continent. In common with most
geologists, I believe that the löss is simply an inundation-mud,
deposited in temporary lakes and over flooded areas during the summer
meltings of the snow- and ice-fields; and I shall now try to show how
the occurrence at high levels of gravels and such loams as the löss
may be accounted for without having recourse to volcanic action or to
winds, or even to the Deluge. I shall invoke no agencies other than
those which we are perfectly well assured were in full operation during
the Ice Age.
Now, I ask you, in the first place, to bear in mind that while a
glacial epoch continued, extreme conditions could not have been
restricted to the areas undergoing glaciation. There is abundant
evidence, indeed, to show that heavy, snows occasionally covered other
regions, and that in such places severe frosts acted upon the rocks
and soils even of the low-grounds. Need we wonder if at a time when
the northern ice-sheet approached the fiftieth parallel of latitude
in middle Europe, when almost every mountain-group of central and
southern Europe had its snow-fields and glaciers--need we wonder if at
such a time the climate of wide areas outside of the glaciated tracts
was extremely ungenial? The more closely the superficial accumulations
of such areas are studied, the more clearly do we perceive in them
the evidence of cold and humid conditions. Try, then, to picture to
yourselves the probable aspect of those regions during a glacial epoch.
Immediately south of the northern ice-sheet deep snows must have buried
large tracts of country, and such snows may have endured often for long
years, notwithstanding the great melting that took place in summer.
Even much further south, as in Spain and Italy, deep snows would cover
the lesser hills and hill-ranges, while frost would act energetically
in many a district where such action is now either inconsiderable or
unknown. Such being the general conditions that must have obtained in
the non-glaciated areas, let us very briefly consider what the results
of such conditions must necessarily have been. Every one has noticed,
during the more or less rapid melting of snow in winter and early
spring, that our streams and rivers are then much muddier than when in
summer and autumn they are swollen by heavy rains. This of course is
due to the action of frost, by means of which rocks are disintegrated
and soils are broken up and pulverised, so that when thaw supervenes,
the superficial covering becomes soaked with moisture like a sponge.
To such an extent does this take place, that one may often see the
saturated soil creeping, slipping, and even flowing down the slopes.
The effect of mere thaw is of course much intensified when the water
derived from melting snows is present. Rills and tiny brooks then
become converted into dark muddy torrents, and enormous quantities of
fine-grained detritus are eventually swept into the rivers. The rivers
rise in flood and inundate their plains, over the surface of which
considerable deposits of loam and silt often accumulate. We cannot
doubt that similar but much more intense action must have taken place
over very wide regions in Europe during a glacial epoch. Such having
necessarily been the case, we are not required to suppose that the
löss and similar loams have been deposited entirely by rivers flowing
from glaciers. It is doubtless true that most of the rivers headed in
those days in glacier regions, and must in consequence have been highly
discoloured with glacial mud, and probably a very large proportion of
the loams in question consists of the fine flour of rocks--the result
of glacial grinding. But the action of frost and thaw and melting snow
upon the low-grounds, such as I have described, cannot be ignored,
and seems to have played a more important _rôle_ than has yet been
recognised. I think it helps us better to explain the well-known fact
that land-shells are more or less commonly distributed through the
löss. One can readily understand, at all events, how snail-shells might
be swept down the slopes of the land at the time of the spring thaws,
and how large numbers might find their way eventually into the swollen
glacial rivers. I have often observed, during the melting of snow and
the thawing of soils, quantities of snail-shells in the very act of
being swept into our brooks and rills. And we are all familiar with the
fact that, after a spring-flood has subsided, snail-shells, along with
vegetable débris, are often plentifully stranded upon the valley-slopes
and flood-plains of our rivers.
Admitting, then, that the löss and similar accumulations are simply
inundation-loams formed at a time when glaciers were discharging
immense volumes of muddy water, and when the low-grounds were liable
every summer to the denuding action of melting snows, and so forth, I
have yet to account for the fact that these supposed inundation-loams
sometimes occur at a height of 100 feet, or even of 300 feet, above
the present levels of the rivers. Two theories have been advanced in
explanation, each of which seems to me to contain an element of truth.
It has, in the first place, been maintained, as by Prestwich, that the
löss at the higher levels was probably deposited long before the rivers
had excavated their channels to their present depths. Thus, during
flood, they would be enabled to overflow tracts which they could not
possibly have reached when they had deepened their valleys to a much
greater degree. But while we must fully admit that the erosion effected
by the rivers of the Pleistocene or Glacial period was excessive, yet
we find it difficult or impossible to believe that great valleys,
several miles in width, and two or three hundred feet in depth, were
excavated in hard Devonian and other equally durable rocks by the
swollen and active rivers of the Ice Age. And although it is extremely
probable that the löss at the highest levels is older than the similar
deposit at the lowest levels of such a valley as the Rhine, yet this
does not get us out of our difficulty.
The other view to which I have alluded takes little or no account
of river-erosion, but maintains that the floods of the Ice Age were
sufficiently great to reach the highest levels at which river-gravels
and loams occur. It is likely enough that, under present conditions, we
can form but a very inadequate idea of the vast bodies of freshwater
which formerly swept down our valleys, but we may be pardoned if we
express our inability to conceive of our European rivers flowing with a
breadth of many miles, and a depth of two or three hundred feet.
A few years before his death, Mr. Darwin made a suggestion to me, which
I think gives us the true solution of the problem. He thought that
during an Ice Age great beds of frozen snow might have accumulated
over the low-grounds outside of the glaciated areas (in the manner I
have already described), and that many valleys might have been filled
to a considerable depth during a large part of the year with blown
snow, afterwards congealed. In autumn, when the running water failed,
the lines of drainage might in many cases be more or less choked, and
it would be a mere chance whether the drainage, together with gravel,
sand, and mud, would follow precisely the same lines during the next
summer. Such action being repeated year after year, it might well
happen that many river-valleys might become largely filled with rudely
alternating layers of frozen snow and fluviatile detritus. And if this
were so, the flooded rivers in summer would be enabled to overflow much
wider and more elevated tracts than they could otherwise have reached.
As the climate became less excessive, we can conceive of the frozen
snows gradually melting, and of river-detritus being deposited at lower
and lower levels in the valleys.
The probability of such frozen masses having choked up valleys and
impeded the drainage during the Ice Age is not a mere plausible
conjecture. In the far north of Alaska--in a region which was certainly
not overflowed by the North American ice-cap--extensive sheets of ice
occur, more or less deeply buried under thick soil. Nor can there
be much doubt that these ice-masses date back to the Glacial period
itself, seeing that in the soils which overlie them we meet with
remains of the mammoth and other contemporaneous mammalian forms.
Here, then, we have direct proof of the fact of frozen snow and ice
having accumulated in the hollows of the land outside of the glaciated
areas.[N]
[N] I have given Mr. Darwin's views, and discussed the origin of the
Pleistocene fluvio-glacial deposits at some length in _Prehistoric
Europe_, chaps, viii. and ix. To this work I refer for detailed
geological evidence in support of the view advocated above.