DRIFT STRATA IN THE ISLAND OF MOEN, IN DENMARK.
Geological Structure of the Island of Moen.
Great Disturbances of the Chalk posterior in Date to the
Glacial Drift, with Recent Shells.
M. Puggaard's Sections of the Cliffs of Moen.
Flexures and Faults common to the Chalk and Glacial Drift.
Different Direction of the Lines of successive Movement,
Fracture, and Flexure.
Undisturbed Condition of the Rocks in the adjoining Danish Islands.
Unequal Movements of Upheaval in Finmark.
Earthquake of New Zealand in 1855.
Predominance in all Ages of uniform Continental Movements over
those by which the Rocks are locally convulsed.
In the preceding chapters I have endeavoured to show that the study of
the successive phases of the glacial period in Europe, and the enduring
marks which they have left on many of the solid rocks and on the
character of the superficial drift are of great assistance in enabling
us to appreciate the vast lapse of ages which are comprised in the
Pleistocene epoch. They enlarge at the same time our conception of the
antiquity, not only of the living species of animals and plants but
of their present geographical distribution, and throw light on the
chronological relations of these species to the earliest date yet
ascertained for the existence of the human race. That date, it will be
seen, is very remote if compared to the times of history and tradition,
yet very modern if contrasted with the length of time during which all
the living testacea, and even many of the mammalia, have inhabited the
globe.
In order to render my account of the phenomena of the glacial epoch
more complete, I shall describe in this chapter some other changes in
physical geography and in the internal structure of the earth's crust,
which have happened in the Pleistocene period, because they differ
in kind from any previously alluded to, and are of a class which were
thought by the earlier geologists to belong exclusively to epochs
anterior to the origin of the existing fauna and flora. Of this nature
are those faults and violent local dislocations of the rocks, and those
sharp bendings and foldings of the strata, which we so often behold in
mountain chains, and sometimes in low countries also, especially where
the rock-formations are of ancient date.
POST-GLACIAL DISLOCATIONS AND FOLDINGS OF CRETACEOUS AND DRIFT STRATA IN
THE ISLAND OF MOEN, DENMARK.
A striking illustration of such convulsions of Pleistocene date may
be seen in the Danish island of Moen, which is situated about 50 miles
south of Copenhagen. The island is about 60 miles in circumference, and
consists of white Chalk, several hundred feet thick, overlaid by
boulder clay and sand, or glacial drift which is made up of several
subdivisions, some unstratified and others stratified, the whole having
a mean thickness of 60 feet, but sometimes attaining nearly twice that
thickness. In one of the oldest members of the formation fossil marine
shells of existing species have been found.
Throughout the greater part of Moen the strata of the drift are
undisturbed and horizontal, as are those of the subjacent Chalk; but on
the north-eastern coast they have been throughout a certain area bent,
folded, and shifted, together with the beds of the underlying Cretaceous
formation. Within this area they have been even more deranged than is
the English Chalk-with-flints along the central axis of the Isle of
Wight in Hampshire, or of Purbeck in Dorsetshire. The whole displacement
of the Chalk is evidently posterior in date to the origin of the drift,
since the beds of the latter are horizontal where the fundamental
Chalk is horizontal, and inclined, curved, or vertical where the Chalk
displays signs of similar derangement. Although I had come to these
conclusions respecting the structure of Moen in 1835, after devoting
several days in company with Dr. Forchhammer to its examination,*
(* Lyell, "Geological Transactions" 2nd series volume 2 page
243.)
I should have hesitated to cite the spot as exemplifying convulsions on
so grand a scale, of such extremely modern date, had not the island been
since thoroughly investigated by a most able and reliable authority,
the Danish geologist, Professor Puggaard, who has published a series of
detailed sections of the cliffs.
These cliffs extend through the north-eastern coast of the island,
called Moens Klint,* where the Chalk precipices are bold and
picturesque, being 300 and 400 feet high, with tall beech-trees growing
on their summits, and covered here and there at their base with huge
taluses of fallen drift, verdant with wild shrubs and grass, by which
the monotony of a continuous range of white Chalk cliffs is prevented.
(* Puggaard, "Geologie d. Insel Moen" Bern 1851; and
"Bulletin de la Societe Geologique de France" 1851.)
[Illustration: Figure 47 and 48. Southern Extremity Of Moens Klint]
(FIGURE 47. SOUTHERN EXTREMITY OF MOENS KLINT (PUGGAARD).
A. Horizontal drift.
B. Chalk and overlying drift beginning to rise.
C. First flexure and fault. Height of cliff at this point,
180 feet.)
(FIGURE 48. SECTION OF MOENS KLINT (PUGGAARD), CONTINUED
FROM FIGURE 47.
S. Fossil shells of recent species in the drift at this point.
G. Greatest height near G, 280 feet.)
In the low part of the island, at A, Figure 47, or the southern
extremity of the line of section above alluded to, the drift is
horizontal, but when we reach B, a change, both in the height of the
cliffs and in the inclination of the strata, begins to be perceptible,
and the Chalk Number 1 soon makes its appearance from beneath the
overlying members of the drift Numbers 2, 3, 4, and 5.
This Chalk, with its layers of flints, is so like that of England as to
require no description. The incumbent drift consists of the following
subdivisions, beginning with the lowest:
Number 2. Stratified loam and sand, 5 feet thick, containing at one
spot near the base of the cliff, at s, Figure 48, Cardium edule, Tellina
solidula, and Turritella, with fragments of other shells. Between Number
2 and the Chalk Number 1, there usually intervenes a breccia of broken
flints.
Number 3. Unstratified blue clay or till, with small pebbles and
fragments of Scandinavian rocks occasionally scattered through it, 20
feet thick.
Number 4. A second unstratified mass of yellow and more sandy clay 40
feet thick, with pebbles and angular polished and striated blocks of
granite and other Scandinavian rocks, transported from a distance.
Number 5. Stratified sands and gravel, with occasionally large erratic
blocks; the whole mass varying from 40 to 100 feet in thickness, but
this only in a few spots.
The angularity of many of the blocks in Numbers 3 and 4, the glaciated
surfaces of others, and the transportation from a distance attested by
their crystalline nature, prove them to belong to the northern drift or
glacial period.
It will be seen that the four subdivisions 2, 3, 4, and 5 begin to rise
at B, Figure 47, and that at C, where the cliff is 180 feet high, there
is a sharp flexure shared equally by the Chalk and the incumbent drift.
Between D and G, Figure 48, we observe a great fracture in the rocks
with synclinal and anticlinal folds, exhibited in cliffs nearly 300 feet
high, the drift beds participating in all the bendings of the Chalk;
that is to say, the three lower members of the drift, including Number
2, which, at the point S in this diagram, contains the shells of Recent
species before alluded to.
Near the northern end of the Moens Klint, at a place called "Taler,"
more than 300 feet high, are seen similar folds, so sharp that there
is an appearance of four distinct alternations of the glacial and
Cretaceous formations in vertical or highly inclined beds; the Chalk at
one point bending over so that the position of all the beds is reversed.
[Illustration: Figure 49. Post-Glacial Disturbances]
(FIGURE 49. POST-GLACIAL DISTURBANCES OF VERTICAL, FOLDED,
AND SHIFTED STRATA OF CHALK AND DRIFT, IN THE DRONNINGESTOL,
MOEN, HEIGHT 400 FEET (PUGGAARD).
1. Chalk with flints.
2. Marine stratified loam, lowest member of glacial formation.
3. Blue clay or till, with erratic blocks unstratified.
4. Yellow sandy till, with pebbles and glaciated boulders.
5. Stratified sand and gravel with erratics.)
But the most wonderful shiftings and faultings of the beds are
observable in the Dronningestol part of the same cliff, 400 feet
in perpendicular height, where, as shown in Figure 49, the drift is
thoroughly entangled and mixed up with the dislocated Chalk.
If we follow the lines of fault, we may see, says M. Puggaard, along
the planes of contact of the shifted beds, the marks of polishing and
rubbing which the Chalk flints have undergone, as have many stones in
the gravel of the drift, and some of these have also been forced into
the soft Chalk. The manner in which the top of some of the arches of
bent Chalk have been cut off in this and several adjoining sections,
attests the great denudation which accompanied the disturbances,
portions of the bent strata having been removed, probably while they
were emerging from beneath the sea.
M. Puggaard has deduced the following conclusions from his study of
these cliffs.
First. The white Chalk, when it was still in horizontal stratification,
but after it had suffered considerable denudation, subsided gradually,
so that the lower beds of drift Number 2, with their littoral shells,
were superimposed on the Chalk in a shallow sea.
Second. The overlying unstratified boulder clays 3 and 4 were thrown
down in deeper water by the aid of floating ice coming from the north.
Third. Irregular subsidences then began, and occasionally partial
failures of support, causing the bending and sometimes the engulfment
of overlying masses both of the Chalk and drift, and causing the
various dislocations above described and depicted. The downward movement
continued till it exceeded 400 feet, for upon the surface even of Number
5, in some parts of the island, lie huge erratics 20 feet or more
in diameter, which imply that they were carried by ice in a sea of
sufficient depth to float large icebergs. But these big erratics, says
Puggaard, never enter into the fissures as they would have done had they
been of date anterior to the convulsions.
Fourth. After this subsidence, the re-elevation and partial denudation
of the Cretaceous and glacial beds took place during a general upward
movement, like that now experienced in parts of Sweden and Norway.
In regard to the lines of movement in Moen, M. Puggaard believes, after
an elaborate comparison of the cliffs with the interior of the island,
that they took at least three distinct directions at as many
successive eras, all of post-glacial date; the first line running from
east-south-east to west-north-west, with lines of fracture at
right angles to them; the second running from south-south-east to
north-north-west, also with fractures in a transverse direction; and
lastly, a sinking in a north and south direction, with other subsidences
of contemporaneous date running at right angles or east and west.
When we approach the north-west end of Moens Klint, or the range of
coast above described, the strata begin to be less bent and broken,
and after travelling for a short distance beyond we find the Chalk and
overlying drift in the same horizontal position as at the southern end
of the Moens Klint. What makes these convulsions the more striking is
the fact that in the other adjoining Danish islands, as well as in a
large part of Moen itself, both the Secondary and Tertiary formations
are quite undisturbed.
It is impossible to behold such effects of reiterated local movements,
all of post-Tertiary date, without reflecting that, but for the
accidental presence of the stratified drift, all of which might easily
have been missing, where there has been so much denudation, even if it
had once existed, we might have referred the verticality and flexures
and faults of the rocks to an ancient period, such as the era between
the Chalk with flints and the Maestricht Chalk, or to the time of the
latter formation, or to the Eocene, or Miocene, or Pliocene eras, even
the last of them long prior to the commencement of the glacial epoch.
Hence we may be permitted to suspect that in some other regions, where
we have no such means at our command for testing the exact date of
certain movements, the time of their occurrence may be far more modern
than we usually suppose. In this way some apparent anomalies in the
position of erratic blocks, seen occasionally at great heights above the
parent rocks from which they have been detached, might be explained, as
well as the irregular direction of certain glacial furrows like those
described by Professor Keilhau and Mr. Horbye on the mountains of
the Dovrefjeld in latitude 62 degrees north, where the striation and
friction is said to be independent of the present shape and slope of the
mountains.*
(* "Observations sur les Phenomenes d'Erosion en Norwege"
1857.)
Although even in such cases it remains to be proved whether a general
crust of continental ice, like that of Greenland described by Rink (see
above, Chapter 13), would not account for the deviation of the furrows
and striae from the normal directions which they ought to have followed
had they been due to separate glaciers filling the existing valleys.
It appears that in general the upward movements in Scandinavia, which
have raised sea-beaches containing marine shells of Recent species to
the height of several hundred feet, have been tolerably uniform over
very wide spaces; yet a remarkable exception to this rule was observed
by M. Bravais at Altenfjord in Finmark, between latitude 70 and 71
degrees north. An ancient water-level, indicated by a sandy deposit
forming a terrace and by marks of the erosion of the waves, can be
followed for 30 miles from south to north along the borders of a fjord
rising gradually from a height of 85 feet to an elevation of 220 feet
above the sea, or at the rate of about 4 feet in a mile.*
(* "Proceedings of the Geological Society" 1845 volume 4
page 94.)
To pass to another and very remote part of the world, we have witnessed
so late as January 1855 in the northern island of New Zealand a sudden
and permanent rise of land on the northern shores of Cook's Straits,
which at one point, called Muko-muka, was so unequal as to amount to
9 feet vertically, while it declined gradually from this maximum of
upheaval in a distance of about 23 miles north-west of the greatest
rise, to a point where no change of level was perceptible. Mr. Edward
Roberts of the Royal Engineers, employed by the British Government
at the time of the shock in executing public works on the coast,
ascertained that the extreme upheaval of certain ancient rocks followed
a line of fault running at least 90 miles from south to north into the
interior; and what is of great geological interest, immediately to the
east of this fault the country, consisting of Tertiary strata, remained
unmoved or stationary; a fact well established by the position of a line
of Nullipores marking the sea-level before the earthquake, both on the
surface of the Tertiary and Palaeozoic rocks.*
(* "Bulletin de la Societe Geologique de France" volume 13
1856 page 660, where I have described the facts communicated
to me by Messrs. Roberts and Walter Mantell.)
The repetition of such unequal movements, especially if they recurred at
intervals along the same lines of fracture, would in the course of ages
cause the strata to dip at a high angle in one direction, while towards
the opposite point of the compass they would terminate abruptly in a
steep escarpment.
But it is probable that the multiplication of such movements in the
post-Tertiary period has rarely been so great as to produce results like
those above described in Moen, for the principal movements in any given
period seem to be of a more uniform kind, by which the topography of
limited districts and the position of the strata are not visibly altered
except in their height relatively to the sea. Were it otherwise we
should not find conformable strata of all ages, including the primary
fossiliferous of shallow-water origin, which must have remained
horizontal throughout vast areas during downward movements of several
thousand feet going on at the period of their accumulation. Still less
should we find the same primary strata, such as the Carboniferous,
Devonian, or Silurian, still remaining horizontal over thousands of
square leagues, as in parts of North America and Russia, having escaped
dislocation and flexure throughout the entire series of epochs
which separate Palaeozoic from Recent times. Not that they have been
motionless, for they have undergone so much denudation, and of such
a kind, as can only be explained by supposing the strata to have been
subjected to great oscillations of level, and exposed in some cases
repeatedly to the destroying and planing action of the waves of the sea.
It seems probable that the successive convulsions in Moen were
contemporary with those upward and downward movements of the glacial
period which were described in the thirteenth and some of the following
chapters, and that they ended before the upper beds of Number 5, Figure
49, with its large erratic blocks, were deposited, as some of those
beds occurring in the disturbed parts of Moen appear to have escaped the
convulsions to which Numbers 2, 3, and 4 were subjected. If this be so,
the whole derangement, although Pleistocene, may have been anterior to
the human epoch, or rather to the earliest date to which the existence
of man has as yet been traced back.