geologists in Scandinavia, north Germany, and Finland. The occurrence
of two glacial epochs, separated by a long interval of temperate
conditions, has been proved. The evidence, however, does not show
that there may not have been more than two glacial epochs. There are
certain phenomena, indeed, connected with the glacial accumulations
of the regions in question which strongly suggest that the succession
of changes was more complex than is generally understood. Several
years ago Dr. A. G. Nathorst adduced evidence to show that a great
Baltic glacier, similar to that underneath which the upper diluvium
was amassed, existed before the advent of the vast _mer de glace_ of
the so-called "first glacial epoch,"[CD] and his observations have
been confirmed and extended by H. Lundbohm.[CE] The facts set forth by
them prove beyond doubt that this early Baltic glacier smoothed and
glaciated the rocks in southern Sweden in a direction from south-east
to north-west, and accumulated a bottom-moraine whose included
erratics are equally cogent evidence as to the trend of glaciation.
That old moraine is overlaid by the lower diluvium--_i.e._, the
boulder-clay, etc., of the succeeding vast _mer de glace_ that flowed
south to the foot of the Harz--the transport of the stones in the
superjacent clay indicating a movement from NNE. to SSW., or nearly
at right angles to the trend of the earlier Baltic glacier. It is
difficult to avoid the conclusion that we have here to do with the
products of two distinct ice-epochs. But hitherto no interglacial
deposits have been detected between the boulder-clays in question. It
might, therefore, be held that the early Baltic glacier was separated
by no long interval of time from the succeeding great _mer de glace_,
but may have been merely a stage in the development of the latter.
It is at all events conceivable that before the great _mer de glace_
attained its maximum extension, it might have existed for a time
as a large Baltic glacier. I would point out, however, that if no
interglacial beds had been recognised between the lower and the upper
diluvium, geologists would probably have considered that the last great
Baltic glacier was simply the attenuated successor of the preceding
continental _mer de glace_. But we know this was not so; the two were
actually separated by a long epoch of genial temperate conditions.
[CD] "Beskrifning. till geol. Kartbl. Trolleholm": _Sveriges Geologiska
Undersökning_, Ser. Aa., Nr. 87.
[CE] "Om de äldre baltiska isströmmen i södra Sverige": _Geolog.
Förening. i Stockholm Förhandl._, Bd. x., p. 157.
There are certain other facts that may lead us to doubt whether in the
glacial phenomena of the Baltic coast-lands we have not the evidence
of more than two glacial epochs. Three, and even four, boulder-clays
have been observed in east and west Prussia. They are separated, the
one from the other, by extensive aqueous deposits, which are sometimes
fossiliferous. Moreover, the boulder-clays in question have been
followed continuously over considerable areas. It is quite possible,
of course, that all those boulder-clays may be the product of one
epoch, laid down during more or less considerable oscillations of an
ice-sheet. In this view of the case the intercalated aqueous deposits
would indicate temporary retreats, while the boulder-clays would
represent successive re-advances of one and the same _mer de glace_. On
the other hand, it is equally possible, if not more probable, that the
boulder-clays and intercalated beds are evidence of so many separate
glacial and interglacial epochs. We cannot yet say which is the true
explanation of the facts. But these being as they are, we may doubt if
German glacialists are justified in so confidently maintaining that
their lower and upper diluvial accumulations are the products of the
"first" and "second" glacial epochs. Indeed, as I shall show presently,
the upper diluvium of north Germany and Finland cannot represent the
second glacial epoch of other parts of Europe.
For a long time it has been supposed that the glacial deposits of the
central regions of Russia were accumulated during the advance and
retreat of one and the same ice-sheet. In 1888, however, Professor
Pavlow brought forward evidence to show that the province of Nijnii
Novgorod had been twice invaded by a general _mer de glace_. During the
first epoch of glaciation the ice-sheet overflowed the whole province,
while only the northern half of the same region was covered by the
_mer de glace_ of the second invasion. Again, Professor Armachevsky
has pointed out that in the province of Tchernigow two types of
glacial deposits appear, so unlike in character and so differently
distributed that they can hardly be the products of one and the same
ice-sheet. But until recently no interglacial deposits had been
detected, and the observations just referred to failed, therefore, to
make much impression. The missing link in the material evidence has
now happily been supplied by M. Krischtafowitsch.[CF] At Troïzkoje, in
the neighbourhood of Moscow, occur certain lacustrine formations which
have been long known to Russian geologists. These have been variously
assigned to Tertiary, lower glacial, post-glacial, and pre-glacial
horizons. They are now proved, however, to be of interglacial age,
for they rest upon and are covered by glacial accumulations. Amongst
their organic remains are oak (_Quercus pedunculata_), alder (_Alnus
glutinosa_, _A. incana_), white birch, hazel, Norway maple (_Acer
platanoides_), Scots fir, willow, water-lilies (_Nuphar_, _Nymphæa_),
mammoth, pike, perch, _Anadonta_, wing-cases of beetles, etc. The
character of the plants shows that the climate of central Russia was
milder and more humid than it is to-day.
[CF] _Bull. de la Soc. Impér. des Naturalistes de Moskau_, No. 4, 1890.
It is obvious that the upper and lower glacial deposits of central
Russia cannot be the equivalents of the upper and lower diluvium of
the Baltic coast-lands. The upper diluvium of those regions is the
bottom-moraine of the so-called great Baltic glacier. At the time that
glacier invaded north Germany, Finland was likewise covered with an
ice-sheet, which flowed towards the south-east, but did not advance
quite so far as the northern shores of Lake Ladoga. A double line of
terminal moraines, traced from Hango Head on the Gulf of Finland,
north-east to beyond Joensuu, puts this beyond doubt.[CG] The morainic
deposits that overlie the interglacial beds of central Russia cannot,
therefore, belong to the epoch of the great Baltic glacier. They are
necessarily older. In short, it is obvious that the upper and lower
glacial accumulations near Moscow must be on the horizon of the lower
diluvium of north Germany. And if this be so, then it is clear that
the latter cannot be entirely the product of one and the same _mer de
glace_. When the several boulder-clays described by Schröder and others
as occurring in the Baltic provinces of Germany are reinvestigated,
they may prove to be the bottom-moraines of as many distinct and
separate glacial epochs.
[CG] Sederholm, _Fennia_, i., No. 7; Frosterus, _ibid._, iii., No. 8;
Ramsay, _ibid._, iv., No. 2.
It may be contended that the glacial and interglacial deposits of
central Russia are perhaps only local developments--that their evidence
may be accounted for by the oscillations of one single _mer de glace_.
This explanation, as already pointed out, has been applied to the
boulder-clays and intercalated aqueous beds of the lower diluvium of
north Germany, and the prevalent character of the associated organic
remains makes it appear plausible. It is quite inapplicable, however,
to the similar accumulations in central Russia. During the formation
of the freshwater beds of Troïzkoje, no part of Russia could have
been occupied by an ice-sheet; the climate was more genial and less
"continental" than the present. Yet that mild interglacial epoch was
preceded and succeeded by extremely arctic conditions. It is impossible
that such excessive changes could have been confined to central Russia.
Germany, and indeed all northern and north-western Europe, must have
participated in the climatic revolutions.
So far, then, as the evidence has been considered, we may conclude
that three glacial and two interglacial epochs at least have been
established for northern Europe. If this be the case, then a
similar succession ought to occur in our own islands; and a little
consideration of the evidence already adduced will suffice to show that
it does. It will be remembered that the lower and upper boulder-clays
of the British Islands are the bottom-moraines of two separate and
distinct ice-sheets, each of which in its time coalesced on the floor
of the North Sea with the inland-ice of Scandinavia. It is obvious,
therefore, that our upper boulder-clay cannot be the equivalent of
the upper diluvium of the Baltic coast-lands, of Sweden, Denmark, and
Schleswig-Holstein. De Geer and others have shown that while the great
Baltic glacier was accumulating the upper diluvium of North Germany,
etc., the inland-ice of Norway calved its icebergs at the mouths of
the great fiords. Thus, during the so-called "second" glacial epoch of
Scandinavian and German geologists, the Norwegian inland-ice did not
coalesce with any British _mer de glace_. The true equivalent in this
country of the upper diluvium is not our upper boulder-clay, but the
great valley-moraines of our mountain-regions. It is our epoch of large
valley-glaciers which corresponds to that of the great Baltic ice-flow.
Our upper and lower boulder-clays are on the horizon of the lower
diluvium of Germany and the glacial deposits of central Russia.
It will now be seen that the evidence in Britain is fully borne out
by what is known of the glacial succession in the corresponding
latitudes of the Continent. I had inferred that our epoch of large
valley-glaciers formed a distinct stage by itself, and was probably
separated from that of the preceding ice-sheet by a prolonged interval
of interglacial conditions. One link in the chain of evidence, however,
was wanting: I could not point to the occurrence of interglacial
deposits underneath the great valley-moraines. But these, as we have
seen, form a well-marked horizon on the Continent, and we cannot doubt
that a similar interglacial stage obtained in these islands. We may
feel confident, in fact, that genial climatic conditions supervened on
the dissolution of the last great _mer de glace_ in Britain, and that
the subsequent development of extensive snow-fields and glaciers in our
mountain-regions was contemporaneous with the appearance of the last
great Baltic glacier.
We need not be surprised that interglacial beds should be well
developed underneath the bottom-moraine of that great glacier, while
they have not yet been recognised below the corresponding morainic
accumulations of our Highlands and Uplands. The conditions in the
low-grounds of the Baltic coast-lands favoured their preservation,
for the ice in those regions formed a broad _mer de glace_, under
the peripheral areas of which sub-glacial erosion was necessarily
at a minimum and the accumulation at a maximum. In our Scottish
mountain-valleys, however, the very opposite was the case. The
conditions obtaining there were not at all comparable to those that
characterised the low-grounds of northern Germany, etc., but were quite
analogous to those of Norway, where, as in our own mountain-regions,
interglacial beds are similarly wanting. It is quite possible,
however, that patches of such deposits may yet be met with underneath
our younger moraines, and they ought certainly to be looked for. But
whether they occur or not in our mountain-valleys, it is certain
that some of the older alluvia of our Lowlands must belong to this
horizon. Hitherto all alluvial beds that overlie our upper boulder-clay
have been classified as post-glacial; but since we have ascertained
that our latest _mer de glace_ was succeeded by genial interglacial
conditions, we may be sure that records of that temperate epoch will
yet be recognised in such Lowland tracts as were never reached by the
glaciers of the succeeding cold epoch. Hence, I believe that some of
our so-called "post-glacial" alluvia will eventually be assigned to
an interglacial horizon. Amongst these may be cited the old peat and
freshwater beds that rest upon the upper boulder-clay at Hailes Quarry,
near Edinburgh. To the same horizon, in all probability, belong the
clays, with Megaceros, etc., which occur so frequently underneath
the peat-bogs of Ireland. An interesting account of these was given
some years ago by Mr. Williams,[CH] who, as a collector of Megaceros
remains, had the best opportunity of ascertaining the nature of the
deposits in which these occur. He gives a section of Ballybetagh Bog,
nine miles south-east of Dublin, which is as follows:--
1. Boulder-clay.
2. Fine tenacious clay, without stones.
3. Yellowish clay, largely composed of vegetable matter.
4. Brownish clay, with remains of Megaceros.
5. Greyish clay.
6. Peat.
[CH] _Geol. Mag._, 1881, p. 354.
The beds overlying the boulder-clay are evidently of lacustrine
origin. The fine clay (No. 2), according to Mr. Williams, is simply
reconstructed boulder-clay. After the disappearance of the _mer de
glace_ the land would for some time be practically destitute of any
vegetable covering, and rain would thus be enabled to wash down the
finer ingredients of the boulder-clay that covered the adjacent slopes,
and sweep them into the lake. The clay formed in this way is described
as attaining a considerable thickness near the centre of the old
lake, but it thins off towards the sides. The succeeding bed (No. 3)
consists so largely of vegetable débris that it can hardly be called
a clay. Mr. Williams describes it as a "bed of pure vegetable remains
that has been ages under pressure." He notes that there is a total
absence in this bed of any tenacious clay like that of the underlying
stratum, and infers, therefore, that the rainfall during the growth
of the lacustrine vegetation was not so great as when the subjacent
clay was being accumulated. The remains of Megaceros occur resting on
the surface of the plant-bed and at various levels in the overlying
brownish clay, which attains a thickness of three to four feet. The
latter is a true lacustrine sediment, containing a considerable
proportion of vegetable matter, interstratified with seams of clay
and fine quartz-sand. According to Mr. Williams, it was accumulated
under genial or temperate climatic conditions like the present.
Between this bed and the overlying greyish clay (from 30 inches to
3 feet thick) there is always in all the bog deposits examined by
Mr. Williams a strongly-marked line of separation. The greyish clay
consists exclusively of mineral matter, and has evidently been derived
from the disintegration of the adjacent granitic hills. Mr. Williams
is of opinion that this clay is of aqueo-glacial formation. This he
infers from its nature and texture, and from its abundance. "Why,"
he asks, "did not this mineral matter come down in like quantity all
the time of the deposit of the brown clay which underlies it? Simply
because, during the genial conditions which then existed, the hills
were everywhere covered with vegetation; when the rain fell it soaked
into the soil, and the clay being bound together by the roots of the
grasses, was not washed down, just as at the present time, when there
is hardly any degradation of these hills taking place." He mentions,
further, that in the grey clay he obtained the antler of a reindeer,
and that in one case the antlers of a Megaceros, found embedded in
the upper surface of the brown clay, immediately under the grey clay,
were scored like a striated boulder, while the under side showed no
markings. Mr. Williams also emphasises the fact that the antlers of
Megaceros frequently occur in a broken state--those near the surface
of the brown clay being most broken, while those at greater depths are
much less so. He shows that this could not be the result of tumultuous
river-action--the elevation of the valley precluding the possibility
of its receiving a river capable of producing such effects. Moreover,
the remains show no trace of having been water-worn, the edges of the
teeth of the great deer being as sharp as if the animal had died but
yesterday. Mr. Williams thinks that the broken state of the antlers
is due to the "pressure of great masses of ice on the surface of the
clay in which they were embedded, the wide expanse of the palms of the
antlers exposing them to pressure and liability to breakage; and even,
in many instances, when there was 12 or 14 inches in circumference of
solid bone almost as hard and sound as ivory, it was snapped across."
It is remarkable that in this one small bog nearly one hundred heads of
Megaceros have been dug up.
Mr. Williams' observations show us that the Megaceros-beds are
certainly older than the peat-bogs with their buried timber. When he
first informed me of the result of his researches (1880), I did not
believe the Megaceros-beds could be older than the latest cold phase
of the Ice Age. I thought that they were later in date than our last
general _mer de glace_, and I think so still, for they obviously rest
upon its ground-moraine. But since I now recognise that our upper
boulder-clay is not the product of the last glacial epoch, it seems
to me highly probable that the Megaceros-beds are of interglacial
age--that, in short, they occupy the horizon of the interglacial
deposits of north Germany, etc. The appearances described by Mr.
Williams in connection with the "grey clay" seem strongly suggestive of
ice-action. Ballybetagh Bog occurs at an elevation of 800 feet above
the sea, in the neighbourhood of the Three Rock Mountain (1479 feet),
and during the epoch of great valley-glaciers the climatic conditions
of that region must have been severe. But without having visited the
locality in question I should hesitate to say that the phenomena
necessarily point to local glaciation. Probably frost, lake-ice, and
thick accumulations of snow and _névé_ might suffice to account for the
various facts cited by Mr. Williams.
I have called special attention to these Irish lacustrine beds,
because it is highly probable that the post-glacial age of similar
alluvia occurring in many other places in these islands has hitherto
been assumed and not proved. Now that we know, however, that a long
interglacial stage succeeded the disappearance of the last general
_mer de glace_, we may feel sure that the older alluvia of our Lowland
districts cannot belong exclusively to post-glacial times. The local
ice-sheets and great glaciers of our "third" glacial epoch were
confined to our mountain-regions; and in the Lowlands, therefore,
which were not invaded, we ought to have the lacustrine and fluviatile
accumulations of the preceding interglacial stage. A fresh interest now
attaches to our older alluvia, which must be carefully re-examined in
the new light thus thrown upon them.
Turning next to the Alpine Lands of central Europe, we find that
geologists there have for many years recognised two glacial epochs.
Hence, like their _confrères_ in northern Europe, they speak of
"first" and "second" glacial epochs.[CI] Within recent years, however,
Professor Penck has shown that the Alps have experienced at least
three separate periods of glaciation. He describes three distinct
ground-moraines, with associated river-terraces and interglacial
deposits in the valleys of the Bavarian Alps, and his observations
have been confirmed by Professor Brückner and Dr. Böhm.[CJ] The same
glacialists, I understand, have nearly completed an elaborate survey
of the eastern Alps, of which they intend shortly to publish an
extended account. The results obtained by them are very interesting,
and fully bear out the conclusions already arrived at from their
exploration of the Bavarian Alps.[CK] A similar succession of
glacial epochs has quite recently been determined by Dr. Du Pasquier
in north Switzerland.[CL] Nor is this kind of evidence confined to
the north side of the Alps. On the shores of Lake Garda, between
Salò and Brescia, three ground-moraines, separated by interglacial
accumulations, are seen in section. The interglacial deposits consist
chiefly of loams--the result of sub-aërial weathering--and attain a
considerable thickness. From this Penck infers that the time which has
elapsed since the latest glaciation is less than that required for the
accumulation of either of the two interglacial series--a conclusion
which, he says, is borne out by similar observations in other parts of
the Alpine region.[CM]
[CI] Morlot: _Bulletin de la Soc. Vaud. d. Sciences nat._, 1854, 1858,
1860. Deicke: _Bericht. d. St. Gall. naturf. ges._, 1858. Heer: _Urwelt
der Schweiz._ Mühlberg: _Festschrift d. aarg. naturf. Ges. z. Feier
ihrer 500 Sitz._, 1869. Rothpletz: _Denkschr. d. schweizer. Ges. f.
d. ges. Naturwissensch._, Bd. xxviii., 1881. Wettstein: _Geologie v.
Zurich u. Umgebung_, 1885. Baltzer: _Mitteil. d. naturf. Ges. Bern_,
1887. Renevier: _Bull. de la Soc. helvèt. d. Sciences nat._, 1887.
[CJ] Penck: _Die Vergletscherung d. deutschen Alpen_, 1882. Brückner:
"Die Vergletscherung des Salzachgebietes," _Geogr. Abhandl. Wien_, Bd.
i. Böhm: _Jahrb. der k. k. geol. Reichsanst._, 1884, 1885. See also O.
Fraas, _Neues Jahrb. f. Min. Geol. u. Palæont._, 1880, Bd. i. p. 218;
E. Fugger and C. Kastner, _Verhandl. d. k. k. geol. Reichsanst._, 1883,
p. 136.
[CK] _Mittheil. des deutsch. u. oesterreich. Alpenvereins_, 1890, No.
20 u. 23.
[CL] _Beiträge z. geolog. Karte der Schweiz_, 31 Lief., 1891; _Archiv.
d. Sciences phys. et nat._, 1891, p. 44.
[CM] "Die grosse Eiszeit," _Himmel u. Erde_.
Although the occurrence of such sub-äerial products intercalated
between separate morainic accumulations is evidence of climatic
changes, still it does not tell us how far the glaciers retreated
during an interglacial stage. Fortunately, however, lignite beds and
other deposits charged with plant remains are met with occupying a
similar position, and from these we gather that during interglacial
times the glaciers sometimes retired to the very heads of the
mountain-valleys, and must have been smaller than their present
representatives. Of such interglacial plant-beds, which have been met
with in some twenty localities, the most interesting, perhaps, is the
breccia of Hötting, in the neighbourhood of Innsbruck.[CN] This breccia
rests upon old morainic accumulations, and is again overlaid by the
later moraines of the great Inn glacier. From the fact that the breccia
yielded a number of supposed extinct species of plants, palæontologists
were inclined to assign it to the Pliocene. Professor Penck, however,
prefers to include it in the Pleistocene system, along with all the
glacial and interglacial deposits of the Alpine Lands. According to
Dr. von Wettstein, the flora in question is not Alpine but Pontic. At
the time of the formation of the breccia the large-leaved _Rhododendron
ponticum_ flourished in the Inn Valley at a height of 1200 metres above
the sea; the whole character of the flora, in short, indicates a warmer
climate than is now experienced in the neighbourhood of Innsbruck. It
is obvious, therefore, that in interglacial times the glaciers must
have shrunk back, as Professor Penck remarks, to the highest ridges of
the mountains.
[CN] Penck: _Die Vergletscherung der deutschen Alpen_, p. 228.
_Verhandl. d. k. k. geol. Reichsanst._, 1887, No. 5; _Himmel und Erde_,
1891. Böhm: _Jahrb. d. k. k. geol. Reichsanst._, 1884, p. 147. Blaas:
_Ferdinandeums Zeitschr._, iv. Folge; _Bericht. d. naturwissensch.
Vereins_, 1889, p. 97.