1869. Boule: _Revue d'Anthropologie_, 1879.
[DE] _Prehistoric Europe_, p. 306. Professor Penck writes me that he
and the Swiss glacialist, Dr. Du Pasquier, have recently examined
these deposits, and are able to confirm my conclusion as to their
interglacial position.
There can be no doubt, then--indeed it is generally admitted--that
the cold conditions that culminated in our Glacial period began to
manifest themselves in Pliocene times. Moreover, as it can be shown
that _Elephas meridionalis_ and its congeners lived in central Europe
after an epoch of extensive glaciation, it is highly probable that
the Forest-bed, which contains the relics of the same mammalian
fauna, is equivalent in age to the early interglacial beds of France
and the Alpine Lands. We seem, therefore, justified in concluding
that the alternation of genial and cold climates that succeeded the
disappearance of the greatest of our ice-sheets was preceded by
analogous climatic changes in late Pliocene times.
I shall now briefly summarise what seems to have been the glacial
succession in Europe:--
{1. Weybourn Crag; ground-moraine of great Baltic
{ glacier underlying lower diluvium; the oldest
{ recognised ground-moraines of central Europe.
{
Glacial { These accumulations represent the earliest
{ glacial epoch of which any trace has been
{ discovered. It would appear to have been one of
{ considerable severity, but not so severe as the
{ cold period that followed.
{2. Forest-bed of Cromer; Hötting breccia; lignites
{ of Leffe and Pianico; interglacial beds of
Interglacial { central France.
{
{ Earliest recognised interglacial epoch; climate
{ very genial.
{3. Lower boulder-clays of Britain; lower diluvium
{ of Scandinavia and north Germany (in part);
{ lower glacial deposits of south Germany and
{ central Russia; ground-moraines and high-level
{ gravel-terraces of Alpine Lands, etc.;
Glacial { terminal moraines of outer zone.
{
{ The epoch of maximum glaciation; the
{ British and Scandinavian ice-sheets confluent;
{ the Alpine glaciers attain their greatest development.
{4. Interglacial freshwater alluvia, peat, lignite, etc.,
{ with mammalian remains (Britain, Germany,
{ etc., central Russia, Alpine Lands, etc.); and
{ marine deposits (Britain, Baltic coast-lands).
Interglacial {
{ Continental condition of British area; climate
{ at first cold, but eventually temperate. Submergence
{ ensued towards close of the period,
{ with conditions passing from temperate to
{ arctic.
{5. Upper boulder-clay of Britain; lower diluvium
{ of Scandinavia, Germany, etc., in part; upper
{ glacial series in central Russia; ground-moraines
{ and gravel-terraces in Alpine Lands.
{
{ Scandinavian and British ice-sheets again
Glacial { confluent, but _mer de glace_ does not extend
{ quite so far as that of the preceding cold epoch.
{ Conditions, however, much more severe than
{ those of the next succeeding cold epoch.
{ Alpine glaciers deposit the moraines of the
{ inner zone.
{6. Freshwater alluvia, lignite, peat, etc. (some of the
{ so-called post-glacial alluvia of Britain;
{ interglacial beds of north Germany, etc.; Alpine
{ Lands(?); marine deposits of Britain and Baltic
{ coast-lands).
Interglacial {
{ Britain probably again continental; climate at
{ first temperate and somewhat insular; submergence
{ ensues with cold climatic conditions--Scotland
{ depressed for 100 feet; Baltic provinces
{ of Germany, etc., invaded by the waters of
{ the North Sea.
{7. Ground-moraines, terminal moraines, etc., of the
{ mountain regions of Britain; upper diluvium
{ of Scandinavia, Finland, north Germany, etc.;
{ great terminal moraines of same regions; terminal
{ moraines in the large longitudinal valleys
{ of the Alps (Penck).
{
{ Major portion of Scottish Highlands covered
Glacial { by ice-sheet; local ice-sheets in Southern Uplands
{ of Scotland and mountain districts in
{ other parts of Britain; great valley-glaciers
{ sometimes coalesce on low-grounds; icebergs
{ calved at mouths of Highland sea-lochs; terminal
{ moraines dropped upon marine deposits
{ then forming (100-feet beach). Scandinavia
{ shrouded in a great ice-sheet, which broke
{ away in icebergs along the whole west coast of
{ Norway. Epoch of the last great Baltic glacier.
{8. Freshwater alluvia (with arctic plants); "lower
{ buried forest and peat" (Britain and north-west
{ Europe generally). Carse-clays and raised
{ beaches of 45 to 50-feet level in Scotland.
Interglacial {
{ Britain again continental; climate at first
{ cold, subsequently becoming temperate: great
{ forests. Eventual insulation of Britain; climate
{ humid, and probably colder than now.
{9. Local moraines in mountain-valleys of Britain,
{ here and there resting on 45 to 50-feet beach;
{ so-called "post-glacial" moraines in the upper
{ valleys of the Alps.
{
{ Probably final appearance of glaciers in our
Glacial { islands. Some of these glaciers attained a
{ considerable size, reaching the sea and shedding
{ icebergs. It may be noted here that the decay
{ of these latest glaciers was again followed by
{ emergence of the land and a recrudescence of
{ forest-growth ("upper buried forest").
A word of reference may now be made to that remarkable association
of evidence of submergence, with proofs of glacial conditions, which
has so frequently been noted by geologists. Take, for example, the
succession in Scotland, and observe how each glacial epoch was preceded
and apparently accompanied by partial submergence of the land:--
1. _Epoch of Greatest Mer de Glace_ (lower boulder-clay); British and
Scandinavian ice-sheets coalescent. Followed by wide land-surface
= Continental Britain, with genial climate. Submergence of land--to
what extent is uncertain, but apparently to 500 feet or so.
2. _Epoch of Lesser Mer de Glace_ (upper boulder-clay); British and
Scandinavian ice-sheets coalescent. Followed by wide land-surface
= Continental Britain, with genial climate. Submergence of land for
100 feet or thereabout.
3. _Epoch of Local Ice-sheets in Mountain Districts;_ glaciers here
and there coalesce on the low-grounds; icebergs calved at mouths of
Highland sea-lochs (moraines on 100-feet beach). Followed by wide
land-surface = Continental Britain, with genial climate. Submergence
of land for 50 feet or thereabout.
4. _Epoch of Small Local Glaciers_, here and there descending to sea
(moraines on 50-feet beach).
These oscillations of the sea-level did not terminate with the
emergence of the land after the formation of the 50-feet beach. There
is evidence to show that subsequent to the retreat of the small local
glaciers (4) and the emergence of the land, our shores extended
seawards beyond their present limits, but how far we cannot tell. With
this epoch of re-emergence the climate again became more genial, our
forests once more attaining a greater vertical and horizontal range.
Submergence then followed (the 25 to 30-feet beach), accompanied
by colder and more humid conditions, which, while unfavourable to
forest-growth, tended greatly to increase the spread of peat-bogs. We
have no evidence, however, to show that small local glaciers again
appeared. Finally the sea retired, and the present conditions ensued.
It will be seen that the submergence which preceded and probably
accompanied the advent of the lesser _mer de glace_ (2) was greater
than that which heralded the appearance of the local ice-sheets (3),
as that in turn exceeded the depression that accompanied the latest
local glaciers (4). There would seem, therefore, to be some causal
connection between cold climatic conditions and submergence. This is
shown by the fact that not only did depression immediately precede and
accompany the appearance of ice-sheets and glaciers, but the degree of
submergence bore a remarkable relation to the extent of glaciation.
Many speculations have been indulged in as to the cause of this curious
connection between glaciation and depression; these, however, I will
not consider here. None of the explanations hitherto advanced is
satisfactory, but the question is one well deserving the attention of
physicists, and its solution would be of great service to geology.
A still larger question which the history of these times suggests
is the cause of climatic oscillations. I have maintained that the
well-known theory advanced by James Croll is the only one that seems
to throw any light upon the subject, and the observations which have
been made since I discussed the question at length, some fifteen years
ago, have added strength to that conviction. As Sir Robert Ball has
remarked, the astronomical theory is really much stronger than Croll
made it out to be. In his recently-published work, _The Cause of an Ice
Age_, Sir Robert says that the theory is so thoroughly well based that
there is no longer any ground for doubting its truth. "We have even
shown," he continues, "that the astronomical conditions are so definite
that astronomers are entitled to direct that vigorous search be
instituted on this globe to discover the traces of those vast climatic
changes through which astronomy declares that our earth must have
passed." In concluding this paper, therefore, I may shortly indicate
how far the geological evidence seems to answer the requirements of the
theory.
Following Croll, we find that the last period of great eccentricity
of the earth's orbit extended over 160,000 years--the eccentricity
reaching its highest value in the earlier stages of the cycle. It is
obvious that during this long cycle the precession of the equinox must
have completed seven revolutions. We might therefore expect to meet
with geological evidence of recurrent cold or glacial and genial or
interglacial epochs; and not only so, but the records ought to show
that the earlier glacial epoch or epochs were colder than those that
followed. Now we find that the epoch of maximum glaciation supervened
in early Pleistocene times, and that three separate and distinct
glacial epochs of diminished severity followed. Of these three, the
first would appear to have been almost as severe as that which preceded
it, and it certainly much surpassed in severity the cold epochs of the
later stages. But the epoch of maximum glaciation, or the first of
the Pleistocene series, was not the earliest glacial epoch. It seems
to have been preceded by one of somewhat less severity than itself,
but which nevertheless, as we gather from the observations of Penck
and his collaborators, was about as important as that which came
after the epoch of maximum glaciation. Hence it would appear that the
correspondence of the geological evidence with the requirements of
the astronomical theory is as close as we could expect it to be. Four
glacial with intervening genial epochs appear to have fallen within
Pleistocene times; while towards the close of the Pliocene, or at the
beginning of the Pleistocene period, according as we choose to classify
the deposits, an earlier glacial epoch followed by genial interglacial
conditions, supervened.
In this outline of a large subject it has not been possible to do more
than indicate very briefly the general nature of the evidence upon
which the chief conclusions are based. I hope, however, to have an
opportunity ere long of dealing with the whole question in detail.
[Note.--Since the original publication of this Essay, renewed
investigation and study have led me to conclude that the correlation
of the British and Continental glacial series is even more simple
than I had supposed. I believe the use of the terms "Lower"
and "Upper" in connection with the "Diluvial" deposits of the
Continent has hitherto blinded us to the obvious succession of
the boulder-clays. In Britain we have, as shown above, a "lower
boulder-clay," an "upper boulder-clay," and the still younger
boulder-clays (ground-moraines), and terminal moraines of our
district ice-sheets and valley-glaciers. In the low-grounds of
the Continent the succession is precisely similar. Thus the lower
boulder-clay that sweeps south into Saxony represents the lower
boulder-clay of Britain. In like manner, the upper boulder-clay of
western and middle Germany, of Poland, and western and north-western
Russia, is the equivalent of our own upper boulder-clay. Lastly, the
so-called "upper diluvium" and the great terminal moraines of the
Baltic coast-lands are on the horizon of the younger boulder-clays
and terminal moraines of the mountainous areas of the British
Islands. The so-called "lower diluvium" of the Baltic coast-lands
thus represents not the _lower_ but the _upper_ diluvium of western
and middle Germany, Poland, etc. German geologists are of opinion
that the upper boulder-clays of the Baltic coast-lands and of the
valley of the Elbe are the ground-moraines of one and the same
ice-sheet, which, on its retreat, piled up the terminal moraines
of the Baltic Ridge. I believe the two boulder-clays in question
are quite distinct, and that the terminal moraines referred to
mark the furthest advance of the last great Baltic glacier. The
contemporaneity of the two boulder-clays has been taken for granted
simply because they are each underlaid by a lower boulder-clay. But,
as we have seen, the upper boulder-clay of the Baltic coast-lands is
underlaid not by one only, but by two, and in some places even by
three other boulder-clays--phenomena which never present themselves
in the regions not invaded by the last great Baltic glacier. Three
or four boulder-clays occur in the coast-lands of the Baltic because
those regions were overflowed successively by three or four separate
ice-sheets. Only two boulder-clays are met with south and east of
the Baltic Ridge, because the tracts lying south and south-east of
that ridge were traversed by only two _mers de glace_--namely, by
that of the epoch of maximum glaciation and by the less extensive
ice-sheet of the next succeeding cold period. In the region between
the Elbe and the mountains of middle Germany only one boulder-clay
appears, because that region has never been invaded by more than
one ice-sheet. The succession thus indicated may be tabulated as
follows:--
1. _Epoch of Earliest Baltic Glacier._ Lowest boulder-clay of
southern Sweden; lowest boulder-clay of Baltic provinces of
Prussia; horizon of the Weybourn Crag.
2. _Epoch of Greatest Mer de Glace._ Lower boulder-clays of middle
and southern Germany, central Russia, British Islands; second
boulder-clay of Baltic provinces of Prussia.
3. _Epoch of Lesser Mer de Glace._ Upper boulder-clay of western and
middle Germany, Poland, and west central Russia; upper boulder-clay
of Britain; third boulder-clay of Baltic provinces of Prussia.
4. _Epoch of Last Great Baltic Glacier._ Upper boulder-clay and
terminal moraines of Baltic coast-lands; district and valley-moraines
of Highlands and Uplands of British Islands.
5. _Epoch of Small Local Glaciers._ Valley-moraines in mountainous
regions of Britain, etc.
The evidence on which these conclusions are based is set forth at some
length in a forthcoming re-issue of my _Great Ice Age_.--Nov. 1, 1892.]
[Illustration:
PLATE IV
SKETCH MAP OF NORTHERN EUROPE SHOWING AREAS COVERED BY
ICE DURING THE EPOCH OF MAXIMUM GLACIATION, AND BY THE GREAT BALTIC
GLACIER AND THE LOCAL ICE-SHEETS OF BRITAIN AT A LATER DATE.
The Edinburgh Geographical Institute J. G. Bartholomew F.R.G.S
]
* * * * *
Explanation of Plate IV.
Map of Europe showing the areas occupied by ice during the Epoch of
Maximum Glaciation (Second Glacial Epoch), and the extent of glaciation
in Scandinavia, Finland, Baltic coast-lands, etc., and the British
Islands during the Fourth Glacial Epoch. For the limits of the greater
glaciation on the Continent, Habenicht, Penck, Nikitin, and Nathorst
have been followed. The Great Baltic Glacier is chiefly after De Geer.
XI.
The Geographical Evolution of Europe.[DF]
[DF] _The Scottish Geographical Magazine_, vol. ii., 1886.
It is one of the commonplaces of geology that the Present is built
up out of the ruins of the Past. Every rock beneath our feet has its
story of change to tell us. Mountains, valleys, and plains, continents
and islands, have passed through vicissitudes innumerable, and bear
within them the evidence of a gradual development or evolution. Looking
back through the vista of the past one sees the dry lands gradually
separating from the ocean, and gathering together into continental
masses according to a definite plan. It is this slow growth, this
august evolution, carried on through countless æons, which most
impresses the student of physical geology. The earth seems for the time
as if endowed with life, and like a plant or animal to pass through its
successive stages of development until it culminates in the present
beautiful world. This conception is one of comparatively recent growth
in the history of geological science. Hutton, the father of physical
geology, had indeed clearly perceived that the dry lands of the globe
were largely composed of the débris of former land-surfaces--that
there had been alternate elevations and depressions of the earth's
crust, causing now sea and now land to predominate over given areas.
But the facts known in this day could not possibly have suggested
those modern ideas of geographical evolution, which are the outcome
of the multifarious observation and research of later years. It is to
Professor Dana, the eminent American geologist, that we are indebted
for the first clear enunciation of the views which I am now about to
illustrate. According to him the great oceanic basins and continental
ridges are of primeval antiquity--their origin is older than that
of our oldest sedimentary formations. It is not maintained that the
present lands have always continued above the level of the sea. On the
contrary, it can be proved that many oscillations of level have taken
place within each continental area, by which the extent and outline
of the land have been modified again and again. Notwithstanding such
changes, however, the great continental ridges would appear to have
persisted from the earliest geological times as dominant elevations of
the earth's crust. Some portions of these, as Dana remarks, may have
been submerged for thousands of feet, but the continents have never
changed places with the oceans.
I shall presently indicate the nature of the evidence by which it is
sought to prove the vast age of our continental masses, but before
doing so it will be well to give an outline of the facts which go to
show that the oceanic depressions of the globe are likewise of primeval
antiquity.
The memorable voyage of the _Challenger_ has done much to increase
our knowledge of the deep seas and the accumulations forming therein.
The researches of the scientific staff of the expedition, and more
particularly those of Mr. Murray, have indeed given a new impulse
to the study of the larger questions of physical geology, and have
lent strong support to the doctrine of the permanence of the oceanic
basins and continental ridges. One of the most important facts brought
before our attention by Mr. Murray is the absence of any land-derived
materials from the sediments now gathering in the deeper abysses of the
ocean. The coasts of continents and continental islands are strewn, as
every one knows, with the wreck of the land--with gravel, sand, and
mud, derived from the demolition of our rocks and soils. The coarser
débris accumulates upon beaches and in shallow littoral waters, while
the finer materials are swept further out to sea by tidal and other
currents--the sediment being gradually sifted as it is borne outwards
into deeper water, until only the finest mud and silt remain to be
swept forward. As the floor of the ocean shelves down to greater depths
the transporting power of currents gradually lessens, and finally
land-derived sediment ceases to appear. Such terrigenous materials
may be said to extend from the littoral zone down to depths of 2000
feet and more, and to a distance of 60 to 300 miles from shore. They
are confined, therefore, to a comparatively narrow belt round the
margins of continents and islands. And thus there are vast regions
of the oceanic depressions over which no terrigenous or land-derived
materials are accumulating. Instead of these we meet with a remarkable
red clay and various kinds of ooze, made up largely of the shells of
foraminifera, pelagic mollusca, and radiolarians, and the frustules
of diatoms. The red clay is the most widely distributed of abysmal
deposits. Indeed, it seems to form a certain proportion of all the
deep-sea organic oozes, and may be said, therefore, to exist everywhere
in the abysmal regions of the oceanic basins. It is extremely
fine-grained, and owes its deep brown or red colour to the presence of
the oxides of manganese and iron. Scattered through the deposit occur
particles of various minerals of volcanic origin, together with lapilli
and fragments of pumice, _i.e._, volcanic _ejectamenta_. Such materials
may have been thrown out from terrestrial volcanoes and carried by
the winds or floated by currents until they became water-logged and
sank; or they may to some extent be the relics of submarine eruptions.
Whatever may have been their immediate source, they are unquestionably
of volcanic origin, and are not associated with any truly terrigenous
sediment. The red clay is evidently the result of the chemical action
of sea-water on volcanic products; and many facts conspire to show
that its formation is an extremely slow process. Thus, remains of
vertebrates, consisting of the ear-bones of whales, beaks of ziphius,
and teeth of sharks, are often plentifully present, and there is no
reason to suppose, as MM. Murray and Renard point out, that the parts
of the ocean where these remains occur are more frequented by whales
and sharks than other regions where similar relics are rarely or never
dredged up. Of these remains some have all the appearance of having
lain upon the sea-bottom for a very long time, for they belong to
extinct species, and are either partially coated or entirely surrounded
with thick layers of manganese-iron. In the same red clay occur small
metallic spherules which are of cosmic origin--in other words, meteoric
dust. The accumulation of all these substances in such relatively great
abundance shows us that the oceanic basins have remained unchanged for
a vast period of time, and assures us that the formation of the abysmal
red clay is extremely slow.
When we come to examine the rocks which enter into the framework of
our continents, we find that they may be roughly classed under these
heads:--
1st, Terrestrial and Aqueous Rocks.
2d, Igneous Rocks.
3d, Crystalline Schists.