possibility of deformation of the crust having been induced by the
old ice-sheets. The geological evidence is certainly suggestive of
such having been the case. But I much doubt whether the sinking of the
surface was brought about by the mere weight of the ice pressing the
crust down into a subjacent liquid layer. Dr. Drygalski's explanation
would better account for the geological phenomena, but, according to
Rev. Osmond Fisher, it cannot be maintained.
If it be difficult to understand how the "earth-movement hypothesis"
can account for the origin of one glacial epoch, the difficulty is
not lessened when we remember that there are two or more such epochs
to account for. And until the advocates of that hypothesis can
furnish us with some reliable evidence, they can hardly expect us to
believe in their mysterious upheavals and depressions of northern
and temperate regions, and in the no less wonderfully rhythmic
movements of the Isthmus of Panama. In fine, the views which I have
been controverting seem to me to be untenable, inasmuch as they are
founded on mere assumptions, and do not even give a reasonable and
intelligible explanation of the phenomena of glaciated regions, while
they practically ignore or leave unsolved the problem of interglacial
conditions.
Some five-and-twenty years have now elapsed since my lamented friend
and colleague, James Croll, published his well-known physical theory of
the Glacial period. That theory, as you all know, has been frequently
criticised by physicists and others, to whose objections Croll made
a final reply in his _Climate and Cosmology_. In that work he has
successfully defended his views, and even added considerably to the
strength of his general argument. I am not aware that since then any
serious objections to Croll's theory have appeared. The only one indeed
that seems to have attracted attention is that which has been urged
especially by certain American geologists. Their belief is that the
close of the Glacial period must have taken place at a much more recent
date than Croll has inferred. And this belief of theirs is based upon
various estimates which have been made as to the time required for the
erosion of valleys and the accumulation of alluvial deposits since the
Glacial period. Thus, according to Mr. Gilbert, the post-glacial gorge
of Niagara, at the present rate of erosion, must have been excavated
within 7000 years; while Mr. Winchell, from similar measurements of
the post-glacial erosion of the Falls of St. Anthony, concludes that
8000 years have elapsed since the close of the Ice Age. I might cite
a number of similar estimates that tend to show that since the close
of the Glacial period only 7000 or 10,000 years have elapsed. What
will archæologists say to this conclusion? We know that Egypt was
already occupied by a civilised people nearly 6000 years ago, and
their marvellously advanced civilisation at that time presupposes,
according to Egyptologists, many thousands of years of development. Are
we, then, prepared to admit that the close of the Ice Age coincided
with the dawn of Egyptian civilisation? But all American observers are
not so parsimonious with regard to post-glacial time. Thus Professor
Spencer has given the age of the Falls of Niagara as 24,000 years, and
he informed me recently that this does not represent half of the time
since the formation of the third great series of glacial deposits of
the Canadian uplands. In our own Continent similar estimates have been
based on the rate of erosion of river-valleys, the rate of accumulation
of alluvial deposits, of peat-bogs, of stalagmite in caves, and what
not, with results that, to say the least, are rather discordant. The
fact is that all such measurements and estimates, however carefully
conducted and cautiously made, are in the nature of things unreliable.
We are insufficiently acquainted with all the factors of the problem
to be solved, and I cannot therefore agree with those who attribute
much weight to conclusions based on such uncertain data. Dr. Croll's
theory may eventually be modified, but I feel sure that it will not be
overturned by the inconclusive and unsatisfactory estimates to which I
have referred. Moreover, opponents of that theory may be reminded that
its truth does not rest on the accuracy of its author's conclusion as
to the date of the last Ice Age. That periods of high eccentricity of
the earth's orbit have occurred is beyond all doubt, but whether the
formulæ employed by Croll in calculating the date of the last great
cycle can be relied upon for that purpose is quite another question.
At present, so far as I understand the facts, the glacial and the
interglacial phenomena are explained by the astronomical theory, and by
no other. It gives a simple, coherent, and consistent interpretation of
the climatic vicissitudes of the Pleistocene and post-glacial periods,
and in especial it is the only theory that throws any light on the very
remarkable climates of interglacial times.
X.
The Glacial Succession in Europe.[BZ]
[BZ] _Trans. Royal Soc. Edinburgh_, vol. xxxvii. (1892).
For many years geologists have recognised the occurrence of at least
two boulder-clays in the British Islands and the corresponding
latitudes of the Continent. It is no longer doubted that these are the
products of two separate and distinct glacial epochs. This has been
demonstrated by the appearance of intercalated deposits of terrestrial,
freshwater, or, as the case may be, marine origin. Such interglacial
accumulations have been met with again and again in Britain, and they
have likewise been detected at many places on the Continent, between
the border of the North Sea and the heart of Russia. Their organic
contents indicate in some cases cold climatic conditions; in others,
they imply a climate not less temperate or even more genial than
that which now obtains in the regions where they occur. Nor are such
interglacial beds confined to northern and north-western Europe. In
the Alpine Lands of the central and southern regions of our Continent
they are equally well developed. Impressed by the growing strength of
the evidence, it is no wonder that geologists, after a season of doubt,
should at last agree in the conclusion that the glacial conditions of
the Pleistocene period were interrupted by at least one protracted
interglacial epoch. Not a few observers go further, and maintain that
the evidence indicates more than this. They hold that three or even
more glacial epochs supervened in Pleistocene times. This is the
conclusion I reached many years ago, and I now purpose reviewing the
evidence which has accumulated since then, in order to show how far it
goes to support that conclusion.
In our islands we have, as already remarked, two boulder-clays, of
which the lower or older has the wider extension southwards, for it has
been traced as far as the valley of the Thames. The upper boulder-clay,
on the other hand, does not extend south of the midlands of England.
In the north of England, and throughout Scotland and the major portion
of Ireland, it is this upper boulder-clay which usually shows at the
surface. The two clays, however, frequently occur together, and are
exposed again and again in deep artificial and natural sections, as
in pits, railway-cuttings, quarries, river-banks, and sea-cliffs.
Sometimes the upper clay rests directly upon the lower; at other times
they are separated by alluvial and peaty accumulations or by marine
deposits. The wider distribution of the lower till, the direction of
transport of its included erratics, and the trend of the underlying
_roches moutonnées_ and rock-striæ, clearly show that the earlier _mer
de glace_ covered a wider area than its successor, and was confluent
on the floor of the North Sea with the Scandinavian ice-sheet. It was
during the formation of the lower till, in short, that glaciation in
these islands attained its maximum development.
The interglacial beds, which in many places separate the lower from the
upper till, show that after the retreat of the earlier _mer de glace_
the climate became progressively more temperate, until eventually the
country was clothed with a flora essentially the same as the present.
Wild oxen, the great Irish deer, and the horse, elephant, rhinoceros,
and other mammals then lived in Britain. From the presence of such a
flora and fauna we may reasonably infer that the climate during the
climax of interglacial times was as genial as now. The occurrence of
marine deposits associated with some of the interglacial peaty beds
shows that eventually submergence ensued; and as the shells in some
of the marine beds are boreal and arctic forms, they prove that cold
climatic conditions accompanied the depression of the land. To what
extent the land sank under water we cannot tell. It may have been 500
feet or not so much, for the evidence is somewhat unsatisfactory.
The upper boulder-clay of our islands is the product of another _mer de
glace_, which in Scotland would seem to have been hardly less thick and
extensive than its predecessor. Like the latter, it covered the whole
country, overflowed the Outer Hebrides, and became confluent with the
Scandinavian inland-ice on the bed of the North Sea. But it did not
flow so far to the south as the earlier ice-sheet.
It is well known that this later _mer de glace_ was succeeded in our
mountain-regions by a series of large local glaciers, which geologists
generally believe were its direct descendants. It is supposed, in
short, that the inland-ice, after retreating from the low-grounds,
persisted for a time in the form of local glaciers in mountain-valleys.
This view I also formerly held, although there were certain appearances
which seemed to indicate that, after the ice-sheet had melted away from
the Lowlands and shrunk far into the mountains, a general advance of
great valley-glaciers had taken place. I had observed, for example,
that the upper boulder-clay is often well developed in the lower
reaches of our mountain-valleys--that, in fact, it may be met with more
or less abundantly up to the point at which large terminal moraines
are encountered. More than this, I had noticed that upland valleys,
in which no local or terminal moraines occur, are usually clothed and
paved with boulder-clay throughout. Again, the aspect of valleys which
have been occupied by large local glaciers is very suggestive. Above
the point at which terminal moraines occur only meagre patches of till
are met with on the bottoms of the valleys. The adjacent hill-slopes
up to a certain line may show bare rock, sprinkled perchance with
erratics and superficial morainic detritus; but above this line, if
the acclivity be not too great, boulder-clay often comes on again.
These appearances are most conspicuously displayed in the southern
Uplands of Scotland, particularly in south Ayrshire and Galloway, and
long ago they led me to suspect that the local glaciers into which
our latest _mer de glace_ was resolved, after retreating continuously
towards the heads of their valleys, so as to leave the boulder-clay in
a comparatively unmodified condition, had again advanced and ploughed
this out, down to the point at which they dropped their terminal
moraines. Subsequent observations in the Highlands and the Inner and
Outer Hebrides confirmed me in my suspicion, for in all those regions
we meet with phenomena of precisely the same kind. My friends and
colleagues, Messrs. Peach and Horne, had independently come to a
similar conclusion; and the more recent work of the Geological Survey
in the north-west Highlands, as they inform me, has demonstrated that
after the dissolution of the general ice-sheet underneath which the
upper boulder-clay was accumulated, a strong recrudescence of glacial
conditions supervened, and a general advance of great valley-glaciers
took place--the glaciers in many places coalescing upon the low-grounds
to form united _mers de glace_ of considerable extent.
The development of these large glaciers, therefore, forms a distinct
stage in the history of the Glacial period. They were of sufficient
extent to occupy all the fiords of the northern and western Highlands,
at the mouths of which they calved their icebergs, and they descended
the valleys on the eastern slopes of the land into the region of the
great lakes, at the lower ends of which we encounter their outermost
terminal moraines. The Shetland and Orkney Islands and the Inner and
Outer Hebrides at the same time nourished local glaciers, not a few
of which flowed into the sea. Such, for example, was the case in
Skye, Harris, South Uist, and Arran. The broad Uplands of the south
were likewise clothed with snow-fields that fed numerous glaciers.
These were especially conspicuous in the wilds of Galloway, but they
appeared likewise in the Peeblesshire hills; and even in less elevated
tracts they have left more or less well-marked traces of their former
presence.
It is to this third epoch of glaciation that I would assign the
final scooping out of our lake-basins and the completion of the deep
depressions in the beds of our Highland fiords. All the evidence,
indeed, leads to the conviction that the epoch was one of long duration.
It goes without saying that what holds good for Scotland must, within
certain limits, hold good also for Ireland and England. In Wales and
the Cumberland lake district, and in the mountain-regions of the sister
island, we meet with evidence of similar conditions. Each of those
areas has obviously experienced intense local glaciation subsequent to
the disappearance of the last big ice-sheet.
Attention must now be directed to another series of facts which help
us to realise the general conditions that obtained during the epoch
of local glaciation. In the basin of the estuary of the Clyde, and at
various other places both on the west and east coasts of Scotland,
occur certain clays and sands, which overlie the upper boulder-clay,
and in some places are found wrapping round the kames and osar of
the last great ice-sheet. These beds are charged with the relics of
a boreal and arctic fauna, and indicate a submergence of rather more
than 100 feet. In the lower reaches of the rivers Clyde, Forth, and
Tay the clays and sands form a well-marked terrace, and a raised
sea-beach, containing similar organisms, occurs here and there on the
sea-coast, as between Dundee and Arbroath, on the southern shores of
the Moray Firth, and elsewhere. When the terraces are traced inland
they are found to pass into high-level fluviatile gravels, which may
be followed into the mountain-valleys, until eventually they shade off
into fluvio-glacial detritus associated with the terminal moraines of
the great local glaciers. It is obvious, in short, that the epoch of
local ice-sheets and large valley-glaciers was one also of partial
submergence. This is further shown by the fact that in some places the
glaciers that reached the sea threw down their moraines on the 100-feet
beach. It must have been an epoch of much floating ice, as the marine
deposits contain now and again many erratics, large and small, and are,
moreover, frequently disturbed and contorted as if from the grounding
of pack-ice.
The phenomena which I have thus briefly sketched suffice to show that
the epoch of local glaciation is to be clearly distinguished from that
of the latest general _mer de glace_. I have long suspected, indeed,
that the two may have been separated by as wide an interval of time
as that which divided the earlier from the later epoch of general
glaciation. Again and again I have searched underneath the terminal
moraines, in the faint hope of detecting interglacial accumulations. My
failure to discover these, however, did not weaken my conviction, for
it was only by the merest chance that interglacial beds could ever have
been preserved in such places. I feel sure, however, that they must
occur among the older alluvia of our Lowlands. Indeed, as I shall point
out in the sequel, it is highly probable that they are already known,
and that we have hitherto failed to recognise their true position in
the glacial series.
Although we have no direct evidence to prove that a long interglacial
epoch of mild conditions immediately preceded the advent of our local
ice-sheets and large valley-glaciers, yet the indirect evidence is so
strong that we seem driven to admit that such must have been the case.
To show this I must briefly recapitulate what is now known as to the
glacial succession on the Continent. It has been ascertained, then,
that the Scandinavian ice has invaded the low-grounds of Germany on
two separate occasions, which are spoken of by Continental geologists
as the "first" and "second" glacial epochs. The earlier of these was
the epoch of maximum glaciation, when the inland ice flowed south into
Saxony, and overspread a vast area between the borders of the North
Sea and the base of the Ural Mountains. This ice-sheet unquestionably
coalesced with the _mer de glace_ of the British Islands. Its
bottom-moraine and the associated fluvio-glacial detritus are known
in Germany as "Lower Diluvium," and the various phenomena connected
with it clearly show that the inland-ice radiated outwards from the
high-grounds of Scandinavia. The terminal front of that vast _mer de
glace_ is roughly indicated by a line drawn from the south coast of
Belgium round the north base of the Harz, and by Leipzig and Dresden
to Krakow, thence north-east to Nijnii Novgorod, and further north to
the head-waters of the Dvina and the shores of the Arctic Sea near the
Tcheskaia Gulf.
The lower diluvium is covered in certain places by interglacial
deposits and an overlying upper diluvium--a succession clearly
indicative of climatic changes. In the interglacial beds occur remains
of _Elephas antiquus_ and other Pleistocene mammals, and a flora which
denotes a genial temperate climate. One of the latest discoveries
of interglacial remains is that of two peat-beds lying between the
lower and upper diluvium near Grünenthal in Holstein.[CA] Among the
abundant plant-relics are pines and firs (no longer indigenous to
Schleswig-Holstein), aspen, willow, white birch, hazel, hornbeam, oak,
and juniper. Associated with these are _Ilex_ and _Trapa natans_, the
presence of which, as Dr. Weber remarks, betokens a climate like that
of western middle Germany. Amongst the plants is a water-lily, which
occurs also in the interglacial beds of Switzerland, but is not now
found in Europe. The evidence furnished by this and other interglacial
deposits in north Germany shows that, after the ice-sheet of the lower
diluvium had melted away, the climate became as temperate as that now
experienced in Europe. Another recent find of the same kind[CB] is
the "diluvial" peat, etc., of Klinge, in Brandenburg, described by
Professor Nehring. These beds have yielded remains of elk (_Cervus
alces_), rhinoceros (species not determined), a small fox (?), and
Megaceros. This latter is not the typical great Irish deer, but a
variety (_C. megaceros_, var. _Ruffii_, Nehring). The plant-remains
include pine, fir (_Picea excelsa_), hornbeam, warty birch (_Betula
verrucosa_), various willows (_Salix repens_, _S. aurita_, _S. caprea_
[?], _S. cinerea_), hazel, poplar (?), common holly, etc. It is worthy
of note that here also the interglacial water-lily (_Cratopleura_) of
Schleswig-Holstein and Switzerland makes its appearance. Dr. Weber
writes me that the facies of this flora implies a well-marked temperate
insular climate (Seeklima). The occurrence of holly in the heart of the
Continent, where it no longer grows wild, is particularly noteworthy.
The evidence furnished by such a flora leads one to conclude that
at the climax of the genial interglacial epoch, the Scandinavian
snow-fields and glaciers were not more extensive than they are at
present.
[CA] _Neues Jahrbuch f. Min. Geol. u. Palæont._, 1891, ii., pp. 62,
228; _Ibid._, 1892, i., p. 114.
[CB] _Naturwissenschaftliche Wochenschrift_, Bd. vii. (1892), No. 4,
p. 31. The plants were determined by Dr. Weber, Professor Wittmack,
and Herr Warnstorf. [More recent investigations have considerably
increased our knowledge of this flora. See _Naturwissenschaftliche
Wochenschrift_, Bd. vii. (1892), Nr. 24, 25. _Ausland_, 1892, Nr. 20;
_Neues Jahrb. f. Min., etc._, 1893, Bd. i., p. 95.]
The presence of the upper diluvium, however, proves that such genial
conditions eventually passed away, and that an ice-sheet again invaded
north Germany. But this later invasion was not on the same scale as
that of the preceding one. The geographical distribution of the upper
diluvium and the position of large terminal moraines put this quite
beyond doubt. The boulder-clay in question spreads over the Baltic
provinces of Germany, extending south as far as Berlin,[CC] and west
into Schleswig-Holstein and Denmark. At the climax of this later
cold epoch glaciers occupied all the fiords of Norway, but did not
advance beyond the general coast-line. Norway at that time must have
greatly resembled Greenland--the inland-ice covering the interior of
the country, and sending seawards large glaciers that calved their
icebergs at the mouths of the great fiords. In the extreme south,
however, the glaciers did not quite reach the sea, but piled up large
terminal moraines on the coast-lands, which may be followed thence
into Sweden in an easterly direction by the lower end of Lake Wener
and through Lake Wetter. A similar belt of moraines marks out the
southern termination of the ice-sheet in Finland. Between Sweden and
Finland lies the basin of the Baltic, which, at the epoch in question
was filled with ice, forming a great Baltic glacier. This glacier
overflowed the Öland Islands, Gottland, and Öland, fanning out as it
passed towards the south-west and west, so as to invade on the south
the Baltic provinces of Germany, while in the north it traversed the
southern part of Scania, and overwhelmed the Danish islands as it
spread into Jutland and Schleswig-Holstein. The course of this second
ice-sheet is indicated by the direction of transport of erratics, etc.,
and by the trend of rock-striæ and _roches moutonnées_, as well as by
the position of its terminal and lateral moraines.
[CC] Not quite so far south. There is no reason to believe that the
ice-sheet of the so-called Great Baltic Glacier advanced beyond the
Baltic ridge. The upper boulder-clay south of that ridge is the
ground-moraine of an earlier glaciation--the equivalent of our upper
boulder-clay. See note, page 324. Nov. 1, 1892.