ice-sheet having covered so large a part of Europe, which a few years
ago was looked upon as a wild dream, has been amply justified by the
labours of those who are so assiduously investigating the peripheral
areas of the "great northern drift." And perhaps I may be allowed
to express my own belief that the drifts of middle and southern
England, which exhibit the same complexity as the Lower Diluvium of
the Continent, will eventually be generally acknowledged to have had
a similar origin. I have often thought that whilst politically we are
happy in having the sea all round us, geologically we should have
gained perhaps by its greater distance. At all events we should have
been less ready to invoke its assistance to explain every puzzling
appearance presented by our glacial accumulations.
I now pass on to review some of the general results obtained by
continental geologists as to the extent of area occupied by inland-ice
during the last great extension of glacier-ice in Europe. It is
well known that this latest ice-sheet did not overflow nearly so
wide a region as that underneath which the lowest boulder-clay was
accumulated. This is shown not only by the geographical distribution
of the youngest boulder-clay, but by the direction of rock-striæ, the
trend of erratics, and the position of well-marked terminal moraines.
Gerard de Geer has given a summary[AH] of the general results obtained
by himself and his fellow-workers in Sweden and Norway; and these have
been supplemented by the labours of Berendt, E. Geinitz, Hauchecorne,
Keilhack, Klockmann, Schröder, Wahnschaffe, and others in Germany, and
by Sederholm in Finland.[AI] From them we learn that the end-moraines
of the ice circle round the southern coasts of Norway, from whence
they sweep south-east by east across the province of Gottland in
Sweden, passing through the lower ends of Lakes Wener and Wetter, while
similar moraines mark out for us the terminal front of the inland-ice
in Finland--at least two parallel frontal moraines passing inland from
Hango Head on the Gulf of Finland through the southern part of that
province to the north of Lake Ladoga. Further north-east than this
they have not been traced; but, from some observations by Helmersen,
Sederholm thinks it probable that the terminal ice-front extended
north-east by the north of Lake Onega to the eastern shores of the
White Sea. Between Sweden and Finland lies the basin of the Baltic,
which at the period in question was filled with ice, forming a great
Baltic glacier which overflowed the Öland Islands, Gottland, and Öland,
and which, fanning-out as it passed towards the south-west, invaded, on
the south side, the Baltic provinces of Germany, while, on the north,
it crossed the southern part of Scania in Sweden and the Danish islands
to enter Jutland.
[AH] _Zeitschrift d. deutsch. geolog. Ges._, Bd. xxxvii., p. 177.
[AI] For papers by Berendt and his associates see especially the
_Jahrbuch d. k. preuss. geol. Landesanstalt_, and the _Zaitschr. d.
deutsch. geol. Ges._ for the past few years. Geinitz: _Forsch. z. d.
Lands- u. Volkskunde_, i. 5; _Leopoldina_, xxii., p. 37; I. _Beitrag z.
Geologie Mecklenburgs_, 1880, pp. 46, 56. Sederholm: _Fennia_, I. No.
7.
The upper boulder-clay of those regions is now recognised as the
ground-moraine of this latest ice-sheet. In many places it is separated
from the older boulder-clay by interglacial deposits--some of which are
marine, while others are of freshwater and terrestrial origin. During
interglacial times the sea that overflowed a considerable portion of
north Germany was evidently continuous with the North Sea, as is shown
not only by the geographical distribution of the interglacial marine
deposits, but by their North Sea fauna. German geologists generally
group all the interglacial deposits together, as if they belonged to
one and the same interglacial epoch. This perhaps we must look upon
as only a provisional arrangement. Certain it is that the freshwater
and terrestrial beds which frequently occur on the same or a lower
level, and at no great distance from the marine deposits, cannot in
all cases be contemporaneous with the latter. Possibly, however, such
discordances may be accounted for by oscillations in the level of the
interglacial sea--land and water having alternately prevailed over the
same area. Two boulder-clays, as we have seen, have been recognised
over a wide region in the north of Germany. In some places, however,
three or more such boulder-clays have been observed overlying one
another throughout considerable areas, and these clays are described
as being distinctly separate and distinguishable the one from the
other.[AJ] Whether they, with their intercalated aqueous deposits,
indicate great oscillations of one and the same ice-sheet--now
advancing, now retreating--or whether the stony clays may not be the
ground-moraines of so many different ice-sheets, separated the one from
the other by true interglacial conditions, future investigations must
be left to decide.
[AJ] H. Schröder: _Jahrb. d. k. preuss. geol. Landesanstalt für 1887_ ,
p. 360.
The general conclusions arrived at by those who are at present
investigating the glacial accumulations of northern Europe may be
summarised as follows:--
1. Before the invasion of northern Germany by the inland-ice the
low-grounds bordering on the Baltic were overflowed by a sea which
contained a boreal and arctic fauna. These marine conditions are
indicated by the presence under the lower boulder-clay of more or
less well-bedded fossiliferous deposits. On the same horizon occur
also beds of sand, containing freshwater shells, and now and again
mammalian remains, some of which imply cold and others temperate
climatic conditions. Obviously all these deposits may pertain to one
and the same period, or more properly to different stages of the
same period--some dating back to a time when the climate was still
temperate, while others clearly indicate the prevalence of cold
conditions, and are therefore probably somewhat younger.
2. The next geological horizon in ascending order is that which
is marked by the Lower Diluvium--the glacial and fluvio-glacial
detritus of the great ice-sheet which flowed south to the foot of
the Harz Mountains. The boulder-clay on this horizon now and again
contains marine, freshwater, and terrestrial organic remains--derived
undoubtedly from the so-called pre-glacial beds already referred
to. These latter, it would appear, were ploughed up and largely
incorporated with the old ground-moraine.
3. The interglacial beds which next succeed contain remains of a
well-marked temperate fauna and flora, which point to something
more than a mere partial or local retreat of the inland-ice. The
geographical distribution of the beds, and the presence in these of
such forms as _Elephas antiquus_, _Cervus elephas_, _C. megaceros_,
and a flora comparable to that now existing in northern Germany,
justify geologists in concluding that the interglacial epoch was one
of long duration, and characterised in Germany by climatic conditions
apparently not less temperate than those that now obtain. One of the
phases of that interglacial epoch, as we have seen, was the overflowing
of the Baltic provinces by the waters of the North Sea.
4. To this well-marked interglacial epoch succeeded another epoch of
arctic conditions, when the Scandinavian inland-ice once more invaded
Germany, ploughing through the interglacial deposits, and working
these up in its ground-moraine. So far as I can learn, the prevalent
belief among geologists in north Germany is that there was only one
interglacial epoch; but, as already stated, doubt has been expressed
whether all the facts can be thus accounted for. There must always be
great difficulty in the correlation of widely-separated interglacial
deposits, and the time does not seem to me to have yet come when we can
definitely assert that all those interglacial beds belong to one and
the same geological horizon.
I have dwelt upon the recent work of geologists in the peripheral areas
of the drift-covered regions of northern Europe, because I think the
results obtained are of great interest to glacialists in this country.
And for the same reason I wish next to call attention to what has been
done of late years in elucidating the glacial geology of the Alpine
Lands of central Europe--and more particularly of the low-grounds that
stretch out from the foot of the mountains. Any observations that
tend to throw light upon the history of the complex drifts of our own
peripheral areas cannot but be of service. It is quite impossible to do
justice in this brief sketch to the labours of the many enthusiastic
geologists who within recent years have increased our knowledge of the
glaciation of the Alpine Lands. At present, however, I am not so much
concerned with the proofs of general glaciation as with the evidence
that goes to show how the Alpine ground-moraines have been formed,
and with the facts which have led certain observers to conclude that
the Alps have endured several distinct glaciations within Pleistocene
times. Swiss geologists are agreed that the ground-moraines which
clothe the bottoms of the great Alpine valleys, and extend outwards
sometimes for many miles upon the low-grounds beyond, are of true
glacial origin. Now these ground-moraines are closely similar to the
boulder-clays of this country and northern Europe--like them, they are
frequently tough and hard-pressed, but now and again somewhat looser,
and less firmly coherent. Frequently also they contain lenticular
beds, and more or less thick sheets of aqueous deposits--in some places
the stony clays even exhibiting a kind of stratification--and ever and
anon such water-assorted materials are commingled with stony clay in
the most complex manner. These latter appearances are, however, upon
the whole best developed upon the low-grounds that sweep out from the
base of the Alps. The only question concerning the ground-moraines
that has recently given rise to much discussion is the origin of the
materials themselves. It is obvious that there are only three possible
modes in which those materials could have been introduced to the
ground-moraine: either they consist of superficial morainic débris
which has found its way down to the bottom of the old glaciers by
crevasses; or they may be made up of the rock-rubbish, shingle, gravel,
etc., which doubtless strewed the valleys before these were occupied by
ice; or, lastly, they may have been derived in chief measure from the
underlying rocks themselves by the action of the ice that overflowed
them. The investigations of Penck, Blaas, Böhm, and Brückner appear to
me to have demonstrated that the ground-moraines are composed mostly
of materials which have been detached from the underlying rocks by
the erosive action of the glaciers themselves. Their observations
show that the regions studied by them in great detail were almost
completely buried under ice--so that the accumulation of superficial
moraines was for the most part impossible; and they advance a number
of facts which prove positively that the ground-moraines were formed
and accumulated under ice. I cannot here recapitulate the evidence,
but must content myself by a reference to the papers in which this is
fully discussed.[AK] These geologists do not deny that some of the
material may occasionally have come from above, nor do they doubt that
pre-existing masses of rock-rubbish and alluvial accumulations may also
have been incorporated with the ground-moraines; but the enormous
extent of the latter, and the direction of transport and distribution
of the erratics which they contain cannot be thus accounted for, while
all the facts are readily explained by the action of the ice itself,
which used its sub-glacial débris as tools with which to carry on the
work of erosion.
[AK] Penck: _Die Vergletscherung der deutschen Alpen._ Blaas:
_Zeitschrift d. Ferdinandeums_, 1885. Böhm: _Jahrb. d. k. k. geol.
Reichsanstalt_, 1885, Bd. xxxv., Heft 3. Brückner: _Die Vergletscherung
d. Salzachgebietes, etc._, 1886.
Professor Heim and others have frequently asserted that glaciers
have little or no eroding power, since at the lower ends of existing
glaciers we find no evidence of such erosion being in operation. But
the chief work of a glacier cannot be carried on at its lower end,
where motion is reduced to a minimum, and where the ice is perforated
by sub-glacial tunnels and arches, underneath which no glacial erosion
can possibly take place; and yet it is upon observations made in
just such places that the principal arguments against the erosive
action of glaciers have been based. If all that we could ever know of
glacial action were confined to what we can learn from peering into
the grottoes at the terminal fronts of existing glaciers, we should
indeed come to the conclusion that glaciers do not erode their rocky
beds to any appreciable extent. But as we do not look for the strongest
evidence of fluviatile erosion at the mouth of a river, but in its
valley--and mountain-tracks, so if we wish to learn what glacier-ice
can accomplish, we must study in detail some wide region from which the
ice has completely disappeared. When this plan has been followed, it
has happened that some of the strongest opponents of glacial erosion
have been compelled by the force of the evidence to go over to the
other camp. Dr. Blaas, for example, has been led by his observations
on the glacial formations of the Inn valley to recant his former
views, and to become a formidable advocate of the very theory which
he formerly opposed. To his work and the memoirs by Penck, Brückner,
and Böhm already cited, and especially to the admirable chapter on
glacier-erosion by the last-named author, I would refer those who may
be anxious to know the last word on this much-debated question.
The evidence of interglacial conditions within the Alpine lands
continues to increase. These are represented by alluvial deposits of
silt, sand, gravel, conglomerate, breccia, and lignites. Penck, Böhm,
and Brückner find evidence of two interglacial epochs, and maintain
that there have been three distinct and separate epochs of glaciation
in the Alps. No mere temporary retreat and re-advance of the glaciers,
according to them, will account for the various phenomena presented by
the interglacial deposits and associated morainic accumulations. During
interglacial times the glaciers disappeared from the lower valleys
of the Alps--the climate was temperate, and probably the snow-fields
and glaciers approximated in extent to those of the present day. All
the evidence conspires to show that an interglacial epoch was of
prolonged duration. Dr. Brückner has observed that the moraines of
the last glacial epoch rest here and there upon löss, and he confirms
Penck's observations in south Bavaria that this remarkable formation
never overlies the morainic accumulations of the latest glacial epoch.
According to Penck and Brückner, therefore, the löss is of interglacial
age. There can be little doubt, however, that löss does not belong to
any one particular horizon. Wahnschaffe[AL] and others have shown that
throughout wide areas in north Germany it is the equivalent in age of
the Upper Diluvium, while Schumacher[AM] points out that in the Rhine
valley it occurs on two separate and distinct horizons. Professor
Andreæ has likewise shown[AN] that there is an upper and lower löss in
Alsace--each characterised by its own special fauna.
[AL] _Abhandl. z. geol. Specialkarte v. Preussen_, etc., Bd. vii. Heft
1; _Zeitschr. d. Zeutsch. geol. Ges._, 1885, p. 904; 1886, p. 367.
[AM] _Hygienische Topographie von Strassburg i. E._, 1885.
[AN] _Abhandl. z. geol. Specialkarte a. Elsass-Lothringen_, Bd. iv.
Heft 2.
There is still considerable difference of opinion as to the mode of
formation of this remarkable accumulation. By many it is considered to
be an aqueous deposit; others, following Richthofen, are of opinion
that it is a wind-blown accumulation; while some incline to the
belief that it is partly the one and partly the other. Nor do the
upholders of these various hypotheses agree amongst themselves as to
the precise manner in which water or wind has worked to produce the
observed results. Thus, amongst the supporters of the aqueous origin
of the löss, we find this attributed to the action of heavy rains
washing over and rearranging the material of the boulder-clays.[AO]
Many, again, have held it probable that löss is simply the finest loam
distributed over the low-grounds by the flood-waters that escaped from
the northern inland-ice and the _mers de glace_ of the Alpine lands of
central Europe. Another suggestion is that much of the material of the
löss may have been derived from the denudation of the boulder-clays by
flood-water, during the closing stages of the last cold period. It is
pointed out that in some regions, at least, the löss is underlaid by a
layer of erratics, which are believed to be the residue of the denuded
boulder-clay. We are reminded by Klockmann[AP] and Wahnschaffe[AQ] that
the inland-ice must have acted as a great dam, and that wide areas
in Germany, etc., would be flooded, partly by water derived from the
melting inland-ice, and partly by waters flowing north from the hilly
tracts of middle Germany. In the great basins thus formed there would
be a commingling of fine silt material derived from north and south,
which would necessarily come to form a deposit having much the same
character throughout.
[AO] Laspeyres: _Erläuterungen z. geol Specialkaret v Preussen_, etc.,
_Blatt. Gröbzig, Zörbig, und Petersberg_.
[AP] Klockmann: _Jahrb. d. k. preuss. geol. Landesanstalt für 1883_, p.
262.
[AQ] Wahnschaffe: _Op. cit._, and _Zeitschr. d. deutsch. geol. Ges._,
1886, p. 367.
From what I have myself seen of the löss in various parts of Germany,
and from all that I have gathered from reading and in conversation
with those who have worked over löss-covered regions, I incline to
the opinion that löss is for the most part of aqueous origin. In many
cases this can be demonstrated, as by the occurrence of bedding and
the intercalation of layers of stones, sand, gravel, etc., in the
deposit; again, by the not infrequent appearance of freshwater shells;
but, perhaps, chiefly by the remarkable uniformity of character which
the löss itself displays. It seems to me reasonable also to believe
that the flood-waters of glacial times must needs have been highly
charged with finely-divided sediment, and that such sediment would be
spread over wide regions in the low-grounds--in the slackwaters of the
great rivers and in the innumerable temporary lakes which occupied,
or partly occupied, many of the valleys and depressions of the land.
There are different kinds of löss or löss-like deposits, however,
and all need not have been formed in the same way. Probably some may
have been derived, as Wahnschaffe has suggested, from denudation of
boulder-clay. Possibly also, some löss may owe its origin to the
action of rain on the stony clays, producing what we in this country
would call "rain-wash." There are other accumulations, however, which
no aqueous theory will satisfactorily explain. Under this category
comes much of the so-called _Berglöss_, with its abundant land-shells,
and its generally unstratified character. It seems likely that such
löss is simply the result of sub-aërial action, and owes its origin
to rain, frost, and wind acting upon the superficial formations, and
rearranging their finer-grained constituents. And it is quite possible
that the upper portion of much of the löss of the lower-grounds may
have been re-worked in the same way. But I confess I cannot yet find in
the facts adduced by German geologists any evidence of a dry-as-dust
epoch having obtained in Europe during any stage of the Pleistocene
period. The geographical position of our Continent seems to me to
forbid the possibility of such climatic conditions, while all the
positive evidence we have points to humidity rather than dryness as
the prevalent feature of Pleistocene climates. It is obvious, however,
that after the flood-waters had disappeared from the low-grounds of
the Continent, sub-aërial action would come into play over the wide
regions covered by the glacial and fluvio-glacial deposits. Thus, in
the course of time, these deposits would become modified,--just as
similar accumulations in these islands have been top-dressed, as it
were, and to some extent even rearranged. I am strengthened in these
views by the conclusions arrived at by M. Falsan--the eminent French
glacialist. Covering the plateaux of the Dombes, and widely spread
throughout the valleys of the Rhone, the Ain, the Isère, etc., in
France there is a deposit of löss, he says, which has been derived from
the washing of the ancient moraines. At the foot of the Alps, where
black schists are largely developed, the löss is dark grey, but west of
the secondary chain the same deposit is yellowish, and composed almost
entirely of silicious materials, with only a very little carbonate of
lime. This _limon_ or löss, however, is very generally modified towards
the top by the chemical action of rain--the yellow löss acquiring a
red colour. Sometimes it is crowded with calcareous concretions, but
at other times it has been deprived of its calcareous element and
converted into a kind of pulverulent silica or quartz. This, the true
löss, is distinguished from another _lehm_, which Falsan recognises as
the product of atmospheric action--formed, in fact, _in situ_, from the
disintegration and decomposition of the subjacent rocks. Even this lehm
has been modified by running water--dispersed or accumulated locally,
as the case may be.[AR]
[AR] Falsan: _La Période glaciaire_, p. 81.