Geological Evidences of the Antiquity of Man/Chapter 17
CHAPTER XVII.
POST-GLACIAL DISLOCATIONS AND FOLDINGS OF CRETACEOUS AND DRIFT STRATA IN THE ISLAND OF MÖEN, IN DENMARK.
GEOLOGICAL STRUCTURES OF THE ISLAND OF MÖEN—GREAT DISTURBANCES OF THE CHALK POSTERIOR IN DATE TO THE GLACIAL DRIFT, WITH RECENT SHELLS—M. PUGGAARD'S SECTIONS OF THE CLIFFS OF MÖEN—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 post-pliocene 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 post-pliocene 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 Möen, Denmark.
A striking illustration of such convulsions of post-pliocene date may be seen in the Danish island of Möen, which is situated about fifty miles south of Copenhagen. The island is about sixty 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 sixty 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 Möen, 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 Möen in 1835, after devoting several days in company with Dr. Forchhammer to its examination,[1] 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 Möens Klint,[2] 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.
In the low part of the island, at a, fig. 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 No. 1 soon makes its appearance from beneath the overlying members of the drift Nos. 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:
No. 2. Stratified loam and sand, five feet thick, containing at one spot, near the base of the cliff at s, fig. 48, Cardium edule, Tellina solidula, and Turritella, with fragments of other shells. Between No. 2 and the chalk No. 1, there usually intervenes a breccia of broken chalk flints.
No. 3. Unstratified blue clay or till, with small pebbles
Fig. 47
Southern extremity of Möens 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.
Fig. 48
Section of Möens Klint (Puggaard), continued from fig. 47.
s Fossil shells of recent species in the drift at this point.
g Greatest height near g, 280 feet.
and fragments of Scandinavian rocks occasionally scattered through it, twenty feet thick.
No. 4. A second unstratified mass of yellow and more sandy clay forty feet thick, with pebbles and angular polished and striated blocks of granite and other Scandinavian rocks, transported from a distance.
No. 5. Stratified sands and gravel, with occasionally large erratic blocks; the whole mass varying from forty to a hundred feet in thickness, but this only in a few spots.
The angularity of many of the blocks in Nos. 3 and 4, and the glaciated surfaces of others, and the transportation from a distance attested by their crystalline nature, proves 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, fig. 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, fig. 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 No. 2, which, at the point s in this diagram, contains the shells of recent species before alluded to.
Near the northern end of the Möens 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.
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 fig. 49 (p. 346), 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.
Fig. 49
Post-glacial disturbances of vertical, folded, and shifted strata of chalk and drift, in the Dronningestol Möen, 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.
M. Puggaard has deduced the following conclusions from his study of these cliffs.
1st. 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 No. 2, with their littoral shells, were superimposed on the chalk in a shallow sea.
2nd. The overlying unstratified boulder clays 3 and 4 were thrown down in deeper water by the aid of floating ice coming from the north.
3rd. 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 No 5, in some parts of the island, lie huge erratics twenty feet or more in diameter, which imply that they were carried by ice in a sea of sufficient depth to float large ice-bergs.
4th. 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 Möen, 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 ESE. to WNW., with lines of fracture at right angles to them; the second running from SSE. to NNW., also with fractures in a transverse direction; and lastly, a sinking in a N. and S. direction, with other subsidences of contemporaneous date running at right angles, or E. and W.
When we approach the north-west end of Möens 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 Möens 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 Möen 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, where there has been so much denudation, have been missing, even if it had once existed, we might have referred the vertically 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 older 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. Hörbye on the mountains of the Dovrefjeld in lat. 62° N., where the striation and friction is said to be independent of the present shape and slope of the mountains.[3] 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, p. 235), would not account for the deviation of the furrows and striæ 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 Altenfiord, in Finmark, between lat. 70° and 71° N. 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 thirty miles from south to north along the borders of a fiord rising gradually from a height of eighty-five feet to an elevation of 220 feet above the sea, or at the rate of about four feet in a mile.[4]
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 nine feet vertically, while it declined gradually from this maximum of upheaval in a distance of about twenty-three 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 ninety 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 paleozoic rocks.[5]
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 Möen, for the principal movements in any given period seem to be of that more uniform kind spoken of at p. 334, 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 paleozoic 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 Möen 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 No. 5, p. 346, with its large erratic blocks, were deposited, as some of those beds occurring in the disturbed parts of Möen appear to have escaped the convulsions to which Nos. 2, 3, and 4 were subjected. If this be so, the whole derangement, although post-pliocene, 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.
- ↑ Lyell, Geological Transactions, 2nd series, vol. ii. p. 243.
- ↑ Puggaard, Geologie d. Insel Möen, Bern, 1851; and Bulletin de la Société Géologique de France, 1851.
- ↑ Observations sur les Phénomènes d'Érosion en Norwège, 1857.
- ↑ Proceedings of the Geological Society, 1845, vol. iv. p. 94.
- ↑ Bulletin de la Société Géologique de France, vol. xiii. p. 660, 1856, where I have described the facts communicated to me by Messrs. Roberts and Walter Mantell.