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=_Reviews._=
_On the Glacial Succession in Europe._ By Prof. =James Geikie=.
Transactions of the Royal Society of Edinburgh, Vol. XXXVII.,
Part I. (No. 9), 1892, pp. 127-149 (with a map).
In this timely essay Prof. Geikie reaches the following conclusions:
1. The record of the first glacial epoch is found in the Weyborn Crag
of Britain, and the ground moraine beneath the "Lower Diluvium" of the
continent. During this epoch, the direction of the ice movement in
southern Sweden was from the south-east to the northwest. This first
glacial epoch of which direct evidence is adduced was followed by an
interglacial interval, during which the forest-bed of Cromer, the
breccia of Hötting, the lignites of Leffe and Pianico, and certain beds
in central France were deposited. During this interglacial epoch, the
climate is believed to have been very mild.
2. There followed a second epoch of glaciation, when the ice sheet of
Britain became confluent with that of the continent. This was the epoch
during which the ice sheet reached its southernmost extension. Its
depositions are found in the lower boulder clays of Britain, the lower
diluvium of Scandinavia and north Germany (in part), the lower glacial
deposits of south Germany and central Russia, the ground moraines and
high level gravel terraces of Alpine lands, and the terminal moraines
of the outer zone. During this second glacial epoch, Alpine glaciers
are believed to have attained their greatest development. This epoch
of extreme glaciation was followed by an interglacial interval, during
which Britain is believed to have been joined to the continent. During
this interval, the climate became temperate. In Russia (near Moscow)
there seems to be evidence that it was milder and more humid than that
of the same region at the present day. Toward the close of the mild
epoch, submergence seems to have been accompanied by an increasing
degree of cold, which finally ended in another glacial epoch.
3. The subsidence which marked the close of the second interglacial
interval, marked likewise the inauguration of the third glacial
epoch. Its work is represented in Britain by the upper boulder clay,
in Scandinavia and Germany by the lower diluvium (in part), in central
Russia by the upper glacial series, in Alpine lands by ground moraines
and gravel terraces. The ice sheets of Scandinavia and Britain were
again confluent, but did not extend quite so far south as during the
second glacial epoch. This third glacial epoch is believed to have
been followed by another interglacial interval, during which fresh
water alluvia, lignite and peat accumulations were made. These are
represented by the interglacial beds of north Germany, and by some
of the so-called post-glacial alluvia of Britain. There were also
marine deposits on the coasts of Britain and on the borders of the
Baltic. During this interglacial interval, Britain is believed to
have been continental. The climate was temperate, but in the course
of time became more severe. This increasing severity seems to have
been accompanied by submergence, which amounted to something like 100
ft. below the present sea-level on the coasts of Scotland. The Baltic
provinces of Germany were also invaded by the waters of the North Sea.
4. There followed a fourth period of glaciation, during which the major
part of the Scottish Highland was covered by an ice sheet. Local ice
sheets existed in the southern uplands of Scotland and in mountain
districts in other parts of Britain, and the great valley glaciers
sometimes coalesced on the low lands. Icebergs floated out at the
mouths of some of the highland sea-lochs. In some places, terminal
moraines were deposited upon marine beds which were then in process of
formation. These beds are now 100 ft. above the sea level. At this time
Scandinavia was covered by a great ice sheet, which yielded icebergs
to the sea along the whole west coast of Norway. The ground moraines
and terminal moraines of the mountain regions of Britain represent the
deposits of this ice epoch. The upper diluvium of Scandinavia, Finland,
and north Germany represent the work of the contemporaneous, but not
confluent, ice sheet of the continent. In the Alps, terminal moraines
in the large longitudinal valleys were made at the same time.
This fourth glacial epoch was followed by a fourth interglacial
interval, during which fresh water alluvial deposits were made, and
also the "lower buried forest and peat" of Britain and northwestern
Europe. At this time, Scotland seems to have stood 45 to 50 feet lower
than now, and Carse clays and raised beaches represent the work of the
sea. During this interglacial interval, Britain is believed to have
become again continental, while the climate became so far ameliorated
as to allow the growth of great forests. Subsequently the insulation
of Britain was effected, and this was followed by a climate which was
probably colder than the present.
5. The severity of the climate which marked the close of the fourth
interglacial interval was such as to bring about local glaciation in
some of the mountain valleys of Britain. Here and there the glaciers
projected their moraines so far down the mountains that they rest on
what is now the 45 to 50 feet beach. In the Alps, this fifth epoch of
glaciation is represented by the so-called post-glacial moraines in
the upper valleys. This is believed to have been the last appearance
of glaciers in Britain. The dissolution of these glaciers was again
followed by an emergence of the island, and by more genial climatic
conditions.
In support of his conclusions, Prof. Geikie cites some striking facts
which are not so widely known as they should be. For example, Swedish
geologists have found evidences that there was an ice sheet antedating
that which deposited the "lower diluvium," and that during this earlier
glaciation the direction of ice movement in southern Sweden was from
the south-east to the north-west. The ground moraine deposited by this
ice sheet is overlain by the "lower diluvium" which was produced by
an ice movement from the north north-east to the south south-west, or
nearly at right angles to the first. Again, near Moscow, there exist
interglacial beds whose plant remains indicate a climate milder and
more humid than that of the present time. These interglacial beds, it
will be observed, occur in the region of the "lower diluvium" quite
beyond the margin of the ice which produced the "upper diluvium" of
Germany and Scandinavia. During this interglacial interval, Prof.
Geikie maintains that no part of Russia could have been covered with
ice. If, then, within the limits of the area covered by the "lower
diluvium," and not by the "upper," distinct beds of glacial drift
are separated by such beds as those cited, there can be no question
but that such separation marks two distinct glacial epochs. If there
was an earlier glaciation when the movement of the ice in Sweden was
at right angles to that during which the lower part of the "lower
diluvium" was produced, this also would seem to be good evidence of
three ice epochs prior to the "upper diluvium." The epoch of the "upper
diluvium" would then constitute the fourth glacial epoch, and this is
the interpretation of Prof. Geikie.
Outside the area of the European continental ice sheet, facts are
adduced in striking confirmation of the multiple ice epoch theory.
These facts are found in Switzerland, where evidences of multiple
glaciation have been recognized, and in the Pyrenees where evidences
of three separate ice epochs have been found. In France, evidences
of an interglacial interval have been found in the region of the Puy
de Dôme of such duration as to allow the excavation of valleys to a
depth of 900 feet. The length of time which would be required for such
stupendous erosion must certainly be regarded as sufficient to allow
the preceding and succeeding glaciations to be considered as belonging
to two distinct epochs.
Another point of great significance and interest which Prof. Geikie's
essay brings out, is the correlation in Britain between epochs of
glaciation and epochs of subsidence on the one hand, and between
interglacial intervals and epochs of elevation on the other. If Prof.
Geikie's interpretation be well founded, and so far as we are able to
judge from the facts presented this is the case, his conclusions would
seem to be fatal to the hypothesis that glacial climate was produced by
northern elevation.
The map which Prof. Geikie gives, showing the limit of ice advance
during the fourth glacial epoch, seems to us open to criticism. On the
ground of personal observation, the writer believes that the ice sheet
of the glacial epoch here represented did not extend notably, if at
all, beyond the Baltic Ridge.[18]
[18] See _American Journal of Science_, May, 1887. In a recent letter,
Prof. Geikie indicates that he is convinced, from subsequent personal
observation, that his map is erroneous so far as the limit of the ice
of this epoch is concerned. The mapping given was based on the opinion
of others.
Prof. Geikie is an advocate of Dr. Croll's astronomical theory of
glacial climate, and thinks that even five is not the full number of
glacial epochs belonging to the Pleistocene period. He believes there
may have been a series of glacial epochs increasing in severity to a
maximum represented by what is now designated as the second glacial
epoch. This maximum was followed by a series of epochs of diminishing
severity, represented by what he designates the third, fourth and fifth
epochs. The essay is a timely contribution to glacial geology.
=Rollin D. Salisbury.=
=_Analytical Abstracts of Current Literature._=[19]
[19] Abstracts in this number are prepared by Henry B. Kummel, Chas. E.
Peet, J. A. Bownocker.
_The Sub-Glacial Origin of Certain Eskers._ By =William Morris
Davis=, Harvard University. (Proceedings of the Boston Society
of Natural History, Vol. XXV., May 18, 1892).
A critical discussion of the conditions under which it is conceived
certain eskers and sand plateaus (plains) were formed. The Auburndale
district, ten miles east of Boston, presents three classes of modified
drift deposits;--sand plateaus, eskers, and kames. These deposits are
well exposed.
The sand plateaus have the characteristics of delta deposits of
glacial streams,--even surfaces, well-bedded sands and gravels, the
beds sloping outward from the "head" at an angle of 12° to 20°, and in
close agreement with the slope of the plateau front, a lobate margin,
deposits distinctly coarser at the head than near the front, and a
series of nearly horizontal roughly cross-bedded gravels overlying the
sloping beds.
The eskers are essentially of the same material as that of the plateau,
often so poorly stratified as to render differentiation of the beds
difficult. The interstices between the pebbles are often unfilled,
although there is abundance of fine material in adjoining layers.
This "open work" is taken to indicate rapid deposition, and seems to
preclude the supposition that the gravels have settled down from a
superglacial position, or been traversed by currents of any volume.
In several instances the eskers can be followed to direct union with
sand plateaus. Towards its lower end the esker frequently "gives out
branches" and "the adjacent lowland surface becomes more or less
encumbered with sand mounds or kames," indicating a decayed margin of
the ice.
Prof. Davis' conclusions are:
"1. The eskers and sand plateaus of Auburndale and Newtonville were
formed by running water just inside and outside of the ice margin in
the closing stage of the last glacial epoch.
"2. The ice-sheet was a stagnant, decaying mass at the time of their
formation, as is shown by the ragged outline of its margin.
"3. Eskers and sand plateaus are genetically connected; the term,
feeding-esker, is fully warranted by the relation of the two in
position, structure, and composition.
"4. The sand plateaus were made rapidly; this is proved by the absence
of disordered beds at their heads, where space would have been opened
by the backward melting of the ice had the forward growth of the
plateau been slow. The eskers were also made rapidly, as is shown by
their 'open-work gravels.'
"5. The diversion of the feeding streams to other outlets left the
plateaus and the eskers without further energetic action as the ice
melted away from them.
"6. The present form and structure of the eskers are more accordant
with the supposition of a subglacial origin than of a superglacial
origin; but it is not intended to imply that other eskers of more
irregular form and different structure could not have been deposited in
superglacial channels."