this accumulation as a product of the Pleistocene period. It is not
of post-glacial age--even much of what one may call the "remodified
löss" being of late Glacial or Pleistocene age. I cannot attempt to
give here a summary of what has been learned within recent years as
to the fauna of the löss. The researches of Nehring and Liebe have
familiarised us with the fact that, at some particular stage in the
Pleistocene period, a fauna like that of the alpine steppe-lands
of western Asia was indigenous to middle Europe, and the recent
investigations by Woldrich have increased our knowledge of this fauna.
At what horizon, then, does this steppe-fauna make its appearance?
At Thiede Dr. Nehring discovered in so-called löss three successive
horizons, each characterised by a special fauna. The lowest of these
faunas was decidedly arctic in type; above that came a steppe-fauna,
which last was succeeded by a fauna comprising such forms as mammoth,
woolly rhinoceros, _Bos_, _Cervus_, horse, hyæna, and lion. Now, if
we compare this last fauna with the forms which have been obtained
from true post-glacial deposits--those deposits, namely, which overlie
the younger boulder-clays and flood-accumulations of the latest
glacial epoch, we find little in common. The lion, the mammoth, and
the rhinoceros are conspicuous by their absence from the post-glacial
beds of Europe. In place of them we meet with a more or less arctic
fauna, and a high-alpine and arctic flora, which as we all know
eventually gave place to the flora and fauna with which Neolithic man
was contemporaneous. As this is the case throughout north-western and
central Europe, we seem justified in assigning the Thiede beds to the
Pleistocene period, and to that interglacial stage which preceded and
gradually merged into the last glacial epoch. That the steppe-fauna
indicates relatively drier conditions of climate than obtained when
perennial snow and ice covered wide areas of the low-ground goes
without saying, but I am unable to agree with those who maintain
that it implies a dry-as-dust climate, like that of some of the
steppe-regions of our own day. The remarkable commingling of arctic-
and steppe-faunas discovered in the Böhmer-Wald[AS] by Woldrich shows,
I think, that the jerboas, marmots, and hamster-rats were not incapable
of living in the same regions contemporaneously with lemmings,
arctic hares, Siberian social voles, etc. But when a cold epoch was
passing away the steppe-forms probably gradually replaced their
arctic congeners, as these migrated northwards during the continuous
amelioration of the climate.
[AS] Woldrich: _Sitzungsb. d. kais. Akad. d. W. math. nat. Cl._, 1880,
p. 7; 1881, p. 177; 1883, p. 978.
If the student of the Pleistocene faunas has certain advantages in the
fact that he has to deal with forms many of which are still living,
he labours at the same time under disadvantages which are unknown to
his colleagues who are engaged in the study of the life of far older
periods. The Pleistocene period was distinguished above all things
by its great oscillations of climate--the successive changes being
repeated and producing correlative migrations of floras and faunas.
We know that arctic and temperate faunas and floras flourished during
interglacial times, and a like succession of life-forms followed the
final disappearance of glacial conditions. A study of the organic
remains met with in any particular deposit will not necessarily,
therefore, enable us to assign these to their proper horizon. The
geographical position of the deposit, and its relation to Pleistocene
accumulations elsewhere, must clearly be taken into account. Already,
however, much has been done in this direction, and it is probable that
ere long we shall be able to arrive at a fair knowledge of the various
modifications which the Pleistocene floras and faunas experienced
during that protracted period of climatic changes of which I have been
speaking. We shall even possibly learn how often the arctic, steppe-,
prairie-, and forest-faunas, as they have been defined by Woldrich,
replaced each other. Even now some approximation to this better
knowledge has been made. Dr. Pohlig,[AT] for example, has compared the
remains of the Pleistocene faunas obtained at many different places
in Europe, and has presented us with a classification which, although
confessedly incomplete, yet serves to show the direction in which we
must look for further advances in this department of inquiry.
[AT] Pohlig: _Sitzungsb. d. niederrheinischen Gesellschaft zu Bonn_,
1884; _Zeitschr. d. deutsch. geolog. Ges._, 1887, p. 798. For a
very full account of the diluvial European and northern Asiatic
mammalian faunas by Woldrich, see _Mém. de l'Acad. des Sciences de St.
Pétersbourg_, vii^e sér., t. xxxv., 1887.
During the last twenty years the evidence of interglacial conditions
both in Europe and America has so increased that geologists generally
no longer doubt that the Pleistocene period was characterised by great
changes of climate. The occurrence at many different localities on the
Continent of beds of lignite and freshwater alluvia, containing remains
of Pleistocene mammalia, intercalated between separate and distinct
boulder-clays has left us no other alternative. The interglacial
beds of the Alpine Lands of central Europe are paralleled by similar
deposits in Britain, Scandinavia, Germany, and France. But opinions
differ as to the number of glacial and interglacial epochs--many
holding that we have evidence of only two cold stages and one general
interglacial stage. This, as I have said, is the view entertained
by most geologists who are at work on the glacial accumulations of
Scandinavia and north Germany. On the other hand, Dr. Penck and others,
from a study of drifts of the German Alpine Lands, believe that they
have met with evidence of three distinct epochs of glaciation, and two
epochs of interglacial conditions. In France, while some observers
are of opinion that there have been only two epochs of general
glaciation, others, as, for example, M. Tardy, find what they consider
to be evidence of several such epochs. Others again, as M. Falsan,
do not believe in the existence of any interglacial stages, although
they readily admit that there were great advances and retreats of
the ice during the Glacial period. M. Falsan, in short, believes in
oscillations, but is of opinion that these were not so extensive as
others have maintained. It is, therefore, simply a question of degree,
and whether we speak of oscillations or of epochs, we must needs
admit the fact that throughout all the glaciated tracts of Europe,
fossiliferous deposits occur intercalated among glacial accumulations.
The successive advance and retreat of the ice, therefore, was not a
local phenomenon, but characterised all the glaciated areas. And the
evidence shows that the oscillations referred to were on a gigantic
scale.
The relation borne to the glacial accumulations by the old river
alluvia which contain relics of palæolithic man early attracted
attention. From the fact that these alluvia in some places overlie
glacial deposits, the general opinion (still held by some) was that
palæolithic man must needs be of post-glacial age. But since we have
learned that all boulder-clay does not belong to one and the same
geological horizon--that, in short, there have been at least two,
and probably more, epochs of glaciation--it is obvious that the mere
occurrence of glacial deposits underneath palæolithic gravels does not
prove these latter to be post-glacial. All that we are entitled in such
a case to say is simply that the implement-bearing beds are younger
than the glacial accumulations upon which they rest. Their horizon
must be determined by first ascertaining the relative position in the
glacial series of the underlying deposits. Now, it is a remarkable
fact that the boulder-clays which underlie such old alluvia belong,
without exception, to the earlier stages of the Glacial period. This
has been proved again and again, not only for this country but for
Europe generally. I am sorry to reflect that some twenty years have now
elapsed since I was led to suspect that the palæolithic deposits were
not of post-glacial but of glacial and interglacial age. In 1871-72 I
published a series of papers in the _Geological Magazine_ in which were
set forth the views I had come to form upon this interesting question.
In these papers it was maintained that the alluvia and cave-deposits
could not be of post-glacial age, but must be assigned to pre-glacial
and interglacial times, and in chief measure to the latter. Evidence
was led to show that the latest great development of glacier-ice
in Europe took place after the southern pachyderms and palæolithic
man had vacated England--that during this last stage of the Glacial
period man lived contemporaneously with a northern and alpine fauna
in such regions as southern France--and lastly, that palæolithic man
and the southern mammalia never revisited north-western Europe after
extreme glacial conditions had disappeared. These conclusions were
arrived at after a somewhat detailed examination of all the evidence
then available--the remarkable distribution of the palæolithic and
ossiferous alluvia having, as I have said, particularly impressed me.
I coloured a map to show at once the areas covered by the glacial and
fluvio-glacial deposits of the last glacial epoch, and the regions in
which the implement-bearing and ossiferous alluvia had been met with,
when it became apparent that the latter never occurred at the surface
within the regions occupied by the former. If ossiferous alluvia did
here and there appear within the recently glaciated areas it was always
either in caves, or as infra- or interglacial deposits. Since the date
of these researches our knowledge of the geographical distribution of
Pleistocene deposits has greatly increased, and implements and other
relics of palæolithic man have been recorded from many new localities
throughout Europe. But none of this fresh evidence contradicts the
conclusions I had previously arrived at; on the contrary, it has
greatly strengthened my general argument.
Professor Penck was, I think, the first on the Continent to adopt the
views referred to. He was among the earliest to recognise the evidence
of interglacial conditions in the drift-covered regions of northern
Germany, and it was the reflections which those remarkable interglacial
beds were so well calculated to suggest that led him into the same path
as myself. Dr. Penck has published a map[AU] showing the areas covered
by the earlier and later glacial deposits in northern Europe and the
Alpine Lands, and indicating at the same time the various localities
where palæolithic finds have occurred, and in not a single case do any
of the latter appear within the areas covered by the accumulations of
the last glacial epoch.
[AU] _Archiv für Anthropologie_, Bd. xv. Heft 3, 1884.
A glance at the papers which have been published in Germany within
the last few years will show how greatly students of the Pleistocene
ossiferous beds have been influenced by what is now known of
the interglacial deposits and their organic remains. Professors
Rothpletz[AV] and Andreæ,[AW] Dr. Pohlig[AX] and others, do not
now hesitate to correlate with those beds the old ossiferous and
implement-bearing alluvia which lie altogether outside of glaciated
regions.
[AV] Rothpletz: _Denkschrift d. schweizer. Ges. für d. gesammt. Nat._,
Bd. xxviii. 1881.
[AW] Andreæ: _Abhandl. z. geolog. Specialkarte v. Elsass-Lothringen_,
Bd. iv. Heft 2, 1884.
[AX] Pohlig: _op. cit._
The relation of the Pleistocene alluvia of France to the glacial
deposits of that and other countries has been especially canvassed.
Rothpletz, in the paper I have cited, includes these alluvia amongst
the interglacial deposits, and in the present year (1889) we have an
interesting essay on the same subject by the accomplished secretary of
the Anthropological and Archæological Congress which met recently in
Paris. M. Boule[AY] correlates the palæolithic cave- and river-deposits
of France with those of other countries, and shows that they must be
of interglacial age. His classification, I am gratified to find, does
not materially differ from that given by myself a number of years ago.
He is satisfied that in France there is evidence of three glacial
epochs and two well-marked interglacial horizons. The oldest of the
palæolithic stages of Mortillet (Chelléenne) culminated according
to Boule during the last interglacial epoch, while the more recent
palæolithic stages (Moustérienne, Solutréenne, and Magdalénienne)
coincided with the last great development of glacier-ice. The
Palæolithic age, so far as Europe is concerned, came to a close during
this last cold phase of the Glacial period.
[AY] Boule: _Revue d'Anthropologie_, 1889, t. 1.
There are many other points relating to glacial geology which have
of late years been canvassed by Continental workers, but these I
cannot discuss here. I have purposely indeed restricted my remarks
to such parts of a wide subject as I thought might have interest for
glacialists in this country, some of whom may not have had their
attention directed to the results which have recently been attained
by their fellow-labourers in other lands. Had time permitted I should
gladly have dwelt upon the noteworthy advances made by our American
brethren in the same department of inquiry. Especially should I have
wished to direct attention to the remarkable evidence adduced in
favour of the periodicity of glacial action. Thus Messrs. Chamberlin
and Salisbury, after a general review of that evidence, maintain
that the Ice Age was interrupted by one chief interglacial epoch and
also by three interglacial sub-epochs or episodes of deglaciation.
These authors discuss at some length the origin of the löss, and come
to the general conclusion that while deposits of this character may
have been formed at different stages of the Glacial period, and under
different conditions, yet upon the whole they are best explained by
aqueous action. Indeed a perusal of the recent geological literature of
America shows a close accord between the theoretical opinions of many
Transatlantic and European geologists.
Thus as years advance the picture of Pleistocene times becomes more and
more clearly developed. The conditions under which our old palæolithic
predecessors lived--the climatic and geographical changes of which they
were the witnesses--are gradually being revealed with a precision that
only a few years ago might well have seemed impossible. This of itself
is extremely interesting, but I feel sure that I speak the conviction
of many workers in this field of labour when I say that the clearing
up of the history of Pleistocene times is not the only end which they
have in view. One can hardly doubt that when the conditions of that
period and the causes which gave rise to these have been more fully
and definitely ascertained we shall have advanced some way towards
the better understanding of the climatic conditions of still earlier
periods. For it cannot be denied that our knowledge of Palæozoic,
Mesozoic, and even early Cainozoic climates is unsatisfactory. But
we may look forward to the time when much of this uncertainty will
disappear. Meteorologists are every day acquiring a clearer conception
of the distribution of atmospheric pressure and temperature and the
causes by which that distribution is determined, and the day is
approaching when we shall be better able than we are now to apply this
extended meteorological knowledge to the explanation of the climates
of former periods in the world's history. One of the chief factors in
the present distribution of atmospheric temperature and pressure is
doubtless the relative position of the great land- and water-areas;
and if this be true of the present, it must be true also of the past.
It would almost seem, then, as if all one had to do to ascertain the
climatic conditions of any particular period, was to prepare a map
depicting with some approach to accuracy the former relative position
of land and sea. With such a map could our meteorologists infer what
the climatic conditions must have been? Yes, provided we could assure
them that in other respects the physical conditions did not differ
from the present. Now there is no period in the past history of our
globe the geographical conditions of which are better known than the
Pleistocene. And yet, when we have indicated these upon a map, we find
that they do not give the results which we might have expected. The
climatic conditions which they seem to imply are not such as we know
did actually obtain. It is obvious, therefore, that some additional
and perhaps exceptional factor was at work to produce the recognised
results. What was this disturbing element, and have we any evidence of
its interference with the operation of the normal agents of climatic
change in earlier periods of the world's history? We all know that
various answers have been given to such questions. Whether amongst
these the correct solution of the enigma is to be found, time will
show. Meanwhile, as all hypothesis and theory must starve without facts
to feed on, it behoves us as working geologists to do our best to add
to the supply. The success with which other problems have been attacked
by geologists forbids us to doubt that ere long we shall have done much
to dispel some of the mystery which still envelopes the question of
geological climates.
IX.
The Glacial Period and the Earth-Movement Hypothesis.[AZ]
[AZ] This article contains the substance of two papers, one read before
the Victoria Institute, in 1892; the other an address delivered to the
Geological Society of Edinburgh, in 1891.
Perhaps no portion of the geological record has been more assiduously
studied during the last quarter of a century than its closing chapters.
We are now in possession of manifold data concerning the interpretation
of which there seems to be general agreement. But while that is the
case, there remain, nevertheless, certain facts or groups of facts
which are variously accounted for. Nor have all the phenomena of
the Pleistocene period received equal attention from those who have
recently speculated and generalised on the subject of Pleistocene
climate and geography. Yet, we may be sure, geologists are not likely
to arrive at any safe conclusions as to the conditions that obtained
in Pleistocene times, unless the evidence be candidly considered
in all its bearings. No interpretation of that evidence which does
not recognise every outstanding group of facts can be expected to
endure. It may be possible to frame a plausible theory to account for
some particular conspicuous phenomena, but should that theory leave
unexplained a residuum of less conspicuous but nevertheless well-proved
facts, then, however strongly it may be fortified, it must assuredly
fall.
As already remarked, there are many phenomena in the interpretation of
which geologists are generally agreed. It is, for example, no longer
disputed that in Pleistocene times vast sheets of ice--continental
_mers de glace_--covered broad areas in Europe and North America, and
that extensive snow-fields and large local glaciers existed in many
mountain-regions where snow-fields and glaciers are now unknown, or
only meagrely developed. It is quite unnecessary, however, that I
should give even the slightest sketch of the aspect presented by the
glaciated tracts of our hemisphere at the climax of the Ice Age. The
geographical distribution and extent of the old snow-fields, glaciers,
and ice-sheets is matter now of common knowledge. It will be well,
however, to understand clearly the nature of the conditions which
obtained at the climax of glacial cold--at that stage, namely, when the
Alpine glaciers reached their greatest development, and when so much of
Europe was cased in snow and ice. This we shall best do by comparing
the present with the past. Now in our day the limits of perennial snow
are attained at heights that necessarily vary with the latitude. This
is shown as follows:--
_Region._ _N. Lat._ _Height of Snow-Line._
Iceland, 65° 3,070 feet.
Norway, 61° 5,180-5,570 "
N. Urals, 59° 30' 4,790 "
Alps, 46° 8,884 or 9,000 "
Caucasus, 43° 10,600-11,000 "
Apennines, 42° 30' 9,520 "
Etna, 37° 30' 9,530 "
Sierra Nevada, 37° 11,187 "
Thus in traversing Europe from north to south the snow-line may be
said to rise from 3000 feet to 11,000 feet in round numbers. It is
possible from such data to draw across the map a series of isochional
lines, or lines of equal perennial snow, and this has been done by my
friend, Professor Penck of Vienna.[BA] It will be understood that each
isochional line traverses those regions above which the line of névé
is estimated to occur at the same height. Thus the isochional line of
1000 metres (3280 feet) runs from the north of Norway down to lat. 64°
on the west coast, whence it must pass west to the south of Iceland.
The line of 1500 metres (4920 ft.) is traced from the north end of the
Urals in a westerly direction. It then follows the back-bone of the
Scandinavian peninsula, passes over to Scotland, and thence strikes
west along lat. 55°. For each of these lines good data are obtainable.
The line of 2000 metres (6560 ft.) is, however, hypothetical. It is
estimated to extend from the Ural Mountains, about the lat. of 57°,
over the mountains of middle Germany and above the north of France. The
line of 2500 metres (8200 ft.) passes from the southern termination of
the Urals, in lat. 51°, to the east Carpathians, thence along the north
face of the Alps, thereafter south-west across the Cevennes to the
north-west end of the Pyrenees; and thence above the Cantabrian and the
Portuguese Highlands to the coast in lat. 39°. The line of 3000 metres
(9840 ft.) is estimated to occur above the Caspian Sea, near lat. 44°,
and extends west through the north end of the Caucasus to the Balkans.
Thence it is traced north-west to the Alps, south-west to the Pyrenees,
which range it follows to the west, and thereafter sweeps south above
the coast at Cadiz. The line of 3500 metres (11,480 ft.) runs from
the Caucasus south-west across Asia Minor to the Lebanon Mountains;
thence it follows the direction of the Mediterranean, and traverses
Morocco above the north face of the Atlas range. Finally the line of
4000 metres (13,120 feet) is estimated to trend in the same general
direction as the last-mentioned line, but, of course, further to the
south. Although these isochional lines are to some extent conjectural,
yet the data upon which they are based are sufficiently numerous and
well-known to prevent any great error, and we may admit that the lines
represent with tolerable accuracy the general position of the snow-line
over our Continent. So greatly has our knowledge of the glaciation of
Europe increased during recent years, that the height of the snow-line
of the Glacial period has been determined by MM. Simony, Partsch,
Penck, and Höfer. Their method is simple enough. They first ascertain
the lowest parts of a glaciated region from which independent glaciers
have flowed. This gives the maximum height of the old snow-line. Next
they determine the lowest point reached by such glaciers. It is obvious
that the snow-line would occur higher up than that, but at a lower
level than the actual source of the glaciers; and thus the minimum
height of the former snow-line is approximately ascertained. The
lowest level from which independent glaciers formerly flowed, and the
terminal point reached by the highest-lying glaciers having been duly
ascertained, it is possible to determine with sufficient accuracy the
mean height of the old snow-line. The required data are best obtained,
as one might have expected, in the Pyrenees and amongst the mountains
of middle and southern Europe. In those regions the snow-line would
seem to have been some 3000 feet or so lower than now. From such
data Professor Penck has constructed a map showing the isochional
lines of the Glacial period. These lines are, I need hardly say, only
approximations, but they are sufficiently near the truth to bring out
the contrast between the Ice Age and the present. Thus the isochional
of 1000 metres, which at present lies above northern Scandinavia,
was pushed south to the latitude of southern France and north Italy;
while the isochional of 2000 metres (now overlying the extreme north
of France and north Germany) passed in glacial times over the northern
part of the Mediterranean.[BB]
[BA] "Geographische Wirkungen der Eiszeit," _Verhandl. d. vierten
deutschen Geographentages zu München_, 1884.
[BB] It is interesting to note that while in the Tatra (north
Carpathians) the snow-line was depressed in glacial times to the extent
of 2700 feet only, in the Alps it descended some 4000 feet or more
below its present level. With the snow-line of that great chain at such
an elevation it is obvious that only a few of the higher points of the
Apennines could rise into the region of _névé_. This is the reason why
moraines are met with in only the higher valleys of that range.