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The compression of a sheet
185
THE
COMPRESSION
OF A SHEET
(A STUDY IN TECTONICS). A. J. BULL, Ph.D., F.G.S. [Receil'ed 17th May. 1943.] [Read 71h May, 1943.1
f.
GENERAL COMPRESSION OF A SHEET IN ITS OWN PLANE.
H ERE has been a persistent idea that folded mountains were produced by the crust of the earth accommodating itself to a contracting core. This hypothesis originated in the conception that the earth was cooling by radiating energy into space, and that the heat so lost was not replaced. As the idea is so well known, and the literature abounds with interesting studies based upon it, there is no need to give references. In support of this hypothesis attempts have several times been made to relate an assumed amount of contraction of the spheroid to the shortening of the crust recorded in folded mountains; but, as the latter quantity is hardly capable of measurement and the former rests on assumption, no great certainty attaches to the results. It is usual in such arguments to ignore the observation that some parts of the crust have been in tension rather than compression for long periods of time; and also that folded mountains are not always in course of production, which would be the case once the crust had fractured under stress; for as the slow cooling and contraction progressed, so the broken and hence weak parts of the crust would continue to give relief by progressive folding and faulting. The contraction hypothesis easily held the field before the discovery of radium in 1899, but somewhat later it became known that in small quantities this element is scattered through the rocks of the earth's crust; and the quantities though minute are more than sufficient to generate as much heat as the earth loses by radiation into space. Early estimates were that the energy liberated by radium as heat was about 60 times greater than the heat which finds its way by conduction through the crust to the surface and is lost. One physicist went so far as to state that the earth must eventually explode, but this was an overstatement of the case. The discovery of radium was damaging to the contraction hypothesis, but its apologists were not easily discouraged; and when it was discovered that basic rocks contained less radium than the acid ones, a way was found of continuing the old idea, for the acid rocks are concentrated near the surface. It became necessary, however, to assume in the first place that mountains were due to contraction of the earth, and on this assumption it was calculated how rapidly the radium content must die away from the surface towards the centre of the earth, in order that the total calculated heat should be less than was lost by radiation. These physical speculations are interesting, but as a corrective to their value it must be pointed out that geologists possess the certain knowledge that conditions suitable for life, i.e., not greatly dissimilar from those of the present time, have persisted for a very long time. They can therefore view with a somewhat detached calmness statements either that the earth is only twenty million years old or that it will explode at no greatly distant date. Whatever are the forces that have caused earth movement in the past, they are still operative and will continue to operate. An important point with regard to the contraction hypothesis is that it has not yet been shown how exisiting tectonics can result from a general compression of the outer shell of the earth. All these various aspects of the problem lend little support to the hypothesis that mountains are produced in the earth's crust by a shrinking interior, and so in order to ascertain the validity of this hypothesis, it is desirable that a crucial test should be applied to it. Such a test might be to find out experimentally what structures are produced in a spherical sheet supported on a contracting interior with a fluid contact between the solid shell and the interior. It was Lord Avebury who realised that every small area in a contracting sphere could
T
186
A. J. BULL,
be imitated by a flat sheet in general compression in its own plane. T. T. Quirke [15*] has also recognised this point. It will therefore be of interest to consider what experiments have been made that approximate to these conditions. Reference may here be made to an obscure experiment made by A. E. B. Chancourtois about 1853, but which he did not describe until 25 years later. He used an inflated rubber balloon, the surface of which was oiled and then coated with wax, and this on partial deflation exhibited a pentagonal pattern of ridges, which he considered to be analogous to mountain chains. A somewhat similar experiment was made by R. T. Chamberlin and F. P. Shepard in 1923 [10], but this was without any fluid between the" crust" and the shrinking interior. Fifty years after Chancourtois, Lord Avebury [2] expressed the opinion that "If . . . folded mountains are caused by compression due to contraction of the earth, the compression must take place in two directions at right angles one to the other." This is the kind of compression that I have referred to as "general." Lord Avebury designed an apparatus to produce this stress in layers of cloth, etc., but his experiments did not have definite results, although his published figures show in places indications of three anticlines meeting at a point. In the discussion on Lord Avebury's paper, Dr. Johnston-Lavis made the suggestion that a better form of the experiment might be made with a sheet of rubber attached to a ring, the rubber to be stretched over the end of a cylinder and its contraction used to simulate the contracting crust of the earth in a layer of material resting upon it. Some ten years ago, I carried out this experiment [7], with the result that a definite pattern was obtained with a variety of materials. The first effect of the general contraction was the raising of round domes in the solid sheet under compression ; these then broke down into three anticlinal lines meeting at a point, and under favourable conditions many of these lines joined to form a network. These experimentally produced forms are not common tectonic structures in the earth's crust, and in my opinion are conclusive evidence that the contraction hypothesis is wrong. The subject of folded mountains is only referred to here because it has been thought to involve the general compression of a sheet. I am still inclined to the opinion that folded mountain ranges result from convective sub-crustal movements, which account for the folding being local in place and occasional in time. [4] [6]. II.
COMPRESSION OF A SHEET IN ONE DIRECTION IN ITS OWN PLANE.
Another idea has been that many tectonic structures were produced by the horizontal compression of strata between the" jaws of a vice." If a sheet of any suitable material is subjected to compression in one direction in its own plane, it will buckle into simple folds which may extend across the folded sheet. When, however, there are irregularities in the sheet or if it is curved along the direction of folding, then each elevation may pitch out and be succeeded by a depression along the axis common to both, PI. 4, A. This pattern is produced in a sheet that is free to fold in either direction from its plane. The geological case may be that of a superficial sheet of rock held down by gravity on to its support, when its folding will be chiefly upwards into the air, and the synclines remaining in contact with the support would tend to be flat-bottomed. Only in the case of a stratum with relatively soft material above and below would the pattern of PI. 4, A be produced. In compression of this type it is obvious that unless there is negligible friction between the rocks under stress and the base upon which they rest, the folding will be greatest at the sides of the sheet where the force is applied, for, however one may assume the friction to be reduced if the strata under compression are resting on a salt bed for instance, or even on a hot basaltic layer, the outer parts will be folded more than the centre, PI. 4, B. The idea of compression as between the jaws of a vice has been applied particularly to the case in which sediments, that have been accumulated in a sinking portion of the earth's crust, are subsequently folded and piled over each • For List of References see p. 188.
PROC. GEOL.
Assoc.,
VOL.
LIV (1943).
PLATS 4.
A .-PAffERN FORMED BY COMPRESSING A SH EET IN ONE DIRECTION IN ITS OWN PLANE.
B.-A
SHEET OF MATERIAL R ESTING ON GLASS AND COMPRESSED BY FORCES ApPLIED AT OPPOSITE EDGES.
(1'., f
THE COMPRESSION OF A SHEET.
187
other in great sheets. The study of Alpine tectonics has, however, shown that folding starts near the centre of the trough of sediments [1], and neither small scale experiments nor any mechanical scheme has so far been able to indicate how such folded mountain chains can be produced by a force applied from outside. Chamberlin and Shepard [10, p. 506] in their experimental study of folding attempted without success to imitate the nappes de recouvrement of the Western Alps by stresses applied from the side. There have been many experiments of this type from those of Sir James Hall to H. M. Cadell, Bailey Willis and other workers, but they always suggest to me that stress can be developed locally in the earth's crust without action and reaction being equal and opposite; on this point Oldham [14, p. lxxxv] has referred to a " widespread fallacy that pressure can be one sided." A simple case of the type of folding under discussion is that of the Jura Mountains, which are situated to the north-west of the Swiss Alps, and extend in an arcuate form from near Basel to the south-west of Geneva. The arc is bowed away from the Alps, and the axes of folding are roughly tangential. These mountains have a simple form of folding which has become known as the Jura type of structure. This consists of elongated anticlines arranged en echelon, each anticline pitching out and often being succeeded along its axis by a syncline [11, p. 134]. The rocks forming the Jura comprise Jurassic limestones and marls, some Cretaceous and Tertiary beds are preserved in the synclines, while Trias is exposed in the partly denuded anticlines. The early work on the tectonics by J. Thurman [17] and by H. D. Rogers [16] was largely concerned with ascertaining the direction of the movement by finding the direction from which the majority ofthe anticlines were inclined or overfolded; and though Rogers concluded that the movement had been southward, Thurman held the opposite view. The old idea that the form of a fold indicates the direction of pressure has been questioned by Oldham [14.] These early workers were dependent upon surface observations, and our fuller knowledge of the structures is due to Buxtorf's study of the railway tunnels at Grenchenberg and Hauenstein [8]. This work of Buxtorf has shown that a huge decollement has taken place and the folding is superficial. The whole of the beds above the Anhydrite Group of the Muschelkalk, Middle Trias, have moved on the plastic The foundation on which these beds rest is material of the Group. unaffected by the folding. The beds have moved to the north and northwest, that is outwards from the Alps. The anticlines are steep and sometimes faulted, but are on the whole little inclined; the synclines naturally tend to be flat bottomed, and in places are broad, where the beds have moved without folding. Some fractures of an imbricate character occur especially towards the northern margin [3] [11]. It is generally assumed that the Jura were produced by a pressure from the south consequent on the great Alpine folding. For instance J. W. Evans, in describing the Alpine Storm [13] speaks of a powerful north-western movement which gave rise to an important portion of the Alpine front and" formed wave after wave in the Jura." The context suggests that the Jura compression was due to a force applied from outside, and the consequences of the earlier work of Buxtorf are disregarded. Prof. Collett [11, p. 124]says "The Jura mountains are undoubtedly the result of a tangential push." To move a sheet of rock bodily merely by a force acting on one edge is hardly possible. This has been clearly recognised by some geologists; H. M. Cadell [9] mentioned it in 1888, and R. M. Deeley in 1918 wrote" A force applied at one end of-a rock sheet would merely buckle it up for a short distance." Further reasons on this point have been given by R. D. Oldham [14]. If therefore a force had been applied to the southern edge of the sheet of which the folded Jura are a part, then it would be the southern and not the northern edge of the sheet that would be folded; whereas it appears from the descriptions that it is the outer (northern and western) parts of the rock sheet that are folded, while the inner portion nearer the Alps is less disturbed. Apart from being carried by a movement of the sub-crustal material, the only obvious force that can move a rock-sheet bodily is gravity. This has been well expressed by Deeley [12, p. 116], who wrote" sheets of rock are not pushed along by pressure applied at one end ; they slide bodily down a slope under the
188
A. J. BULL,
action of gravity, every yard of the mass furnishing its own propulsive force," and the opinion has been expressed that the great Alpine nappes slid downhill [5, p. 154]. If the strata of the Jura were dipping to the north-west, then the weight of the sheet of rock that has moved would have had a component along the direction of the bedding; moreover, the tectonics indicate that the moving force was not localised and was sufficient to move the upper beds as a complete sheet on the salt beds, which played the part of a lubricant. Where the dip was uniform, there the rock sheet might slide as a whole; but at the outer edge, in those parts where the dip was decreasing, the rocks would be in compression owing to the component of the force of gravity in the plane of the bedding decreasing with the sine of the angle of dip, Fig. 24. Thus, in the outer part of the sliding rocksheet the conditions would favour folding, and this would also be produced by any irregularities in the floor over which the sheet moved. Imbricate structure also might be produced where the edge of the moving sheet met obstacles or reached the flatter ground. In this manner the observed tectonics of the Jura Mountains could have been produced. This hypothesis requires that the northwestern slopes of the Alps have been steeper than they are now, in order that the sediments should be able to move under the force of their own weight. Festoons with Jura type of folding have been seen in sheets of snow that have slipped down sliding roofs and also in paint that has been exposed to a nearby fire (PI. 5). Such structures like tectonic ones are often arcuate, because conditions can rarely be uniform enough to produce straight folds. There can be little doubt that, in the many observed cases in which rock sheets have moved bodily, the operative force was gravity. For these cases Dr. E. B. Bailey's slide-plane is a more apt term than thrust-plane.
FIG. 24.-Diagram showing a sheet of sediments coming on to less inclined ground. The concave portion, where the dip is changing, is under compression; because t.he resolved component of the force of gravity along the slide plane (= mg sin. dip) diminishes with the angle of dip and vanishes where the bed is horizontal.
III. REFERENCES. 1.
2.
3. 4. 5. 6. 7. 8. 9.
ARGAND, E. 1916. Sur rare des Alpes Occidentales, Eclogae Geol. Helv., xiv, pp. 145-191. AVEBURY, LORD. 1903. An Experiment in Mountain Building. Quart. Journ. Geo!. Soc., Ii x, p. 348. BAILEY, E. B. 1935. Tectonic Essays, Oxford. BULL, A. J. 1921. A Hypothesis of Mountain Building. Geol. Mag., lviii, pp. 364-7. - - - . 1927. Some Aspects of the Mountain Building Problem. Proc. Geol. . , .. Assoc., xxxviii, pp. 145-156. - - - . 1931. The Convection Current Hypothesis of Mountain Building. Geo!. Mag., lxviii, pp, 495-8. - - - . 1932. The Pattern of a Contracting Earth. Geo!. Mag., lxix, p. 73. BuxToRF, A. 1916. Prognosen und Befunden beim Hauensteinbasis and Grenchen bergtunnel und die Bedeutung der letztern fur die Geologie des Juragebirges, Verh, naturf. Gesellsch. Besel, xxvii, pp. 184-254. CADEll, H. M. 1890. Experimental Researches in Mountain Building. Edinb. Roy. Soc. Trans., xxxv, pp. 337-357.
PRoc. GaOL. Assoc., VOL. LIV (1943).
p.
188.
PLATE S.
lTo [ace
THE COMPRESSION OF A SHEET.
189
10.
CHAMBERLIN, R. T. and P. F. SHEPARD. 1923. Some Experiments in Folding Jour. Geoi. xxxi, pp. 490-512. II. COLLET, L. W. 1935. The Structure of the Alps, London. 12. DEELEY, R. M. 1918. Mountain Building. Ceo. Mag; Dec. vi, vol. v, p. Ill. 1:1. EVANS, J. W. 1926. Presidential Address. Quart, Journ. Geol. Soc., lxxxii, p. xcix. 14. OLDHAM, R. D. 192J. Presidential Address. Quart: Journ. Geol. Soc., lxxvii, pp. lxxvii-xcii. 15. QUIRKE, T. T. 1920. Concerning the Process of Thrust Faulting. Journ. Geol. xxviii, pp. 417-438. 16. ROGERS, H. D. 1849. On the Structural Features of the Appalachians, compared with those of the Alps and other disturbed districts of Europe. Proc. Am. Assoc., Rep. ii, p. 113. 17. THURMANN, J. 1857. Essai d'orographie jurassique, Geneva.
EXPLANATION OF PLATE 4. A.-Pattern formed by compressing a sheet in one direction in its own plane. In this experiment the sheet was a pile of soft paper supported within a mass of wet cotton wool. After compression the mass was allowed to dry slowly, when the paper became stiff and preserved the form taken by it and the cotton wool. The pattern consists of rounded folds which pitch out and reverse along the fold axes. A section across this would correspond closely in shape to the small folds to be seen in the cliffs of Coombe Martin, N. Devon. B.-A sheet of material resting on glass and compressed by forces applied at opposite edges. The sheet is supported on glass; this causes the synclines to be flat bottomed, and the friction prevents the folding from extending far into the sheet. The top fold in the lower part of the figure simulates the structure of island arcs.
DISCUSSION. MR. G. A. KELLAWAV : Dr. Bull has raised some interesting points in his paper. That the Jura structure he has just discussed may have been produced by the sliding and crumpling of relatively competent rocks as they moved over plastic salt measures under the influence of gravity seems to me quite probable. Similar structures on a very much smaller scale may be present in the Weald; for instance some of the sharp folds which have been noticed from time to time in the Hythe Beds, but further work may be necessary to establish this. Large scale structures in which the effort of gravity has played an important part have been described from Persia and elsewhere, but most of these originated subsequently to or were dependent on earlier folding and tilting, and in many cases on the development of erosional features as well. In order that gravity may have a chance to operate in the manner in which Dr. Bull has indicated, it is necessary to have an initial tilting of the basal plane down which the contorted sediments have presumably moved. What produced the initial tilting in this instance'? Differential loading as a result of deposition and erosion might produce sufficient warping to allow gravity-slide structures to develop, but it would be difficult to explain the structure of an area such as the main body of the Alps on this basis. DR. BULL, in reply, said : I thank Mr. Kellaway for his interesting remarks, and note that he has observed folds in the Hythe Beds which were presumably produced by sliding under gravity. I agree it may be difficult to conceive that the main body of the Alps originated ill this manner, but I hope to consider this possibility in the near future.
THE
COMPRESSION
OF A SHEET
(A STUDY IN TECTONICS). A. J. BULL, Ph.D., F.G.S. [Receil'ed 17th May. 1943.] [Read 71h May, 1943.1
f.
GENERAL COMPRESSION OF A SHEET IN ITS OWN PLANE.
H ERE has been a persistent idea that folded mountains were produced by the crust of the earth accommodating itself to a contracting core. This hypothesis originated in the conception that the earth was cooling by radiating energy into space, and that the heat so lost was not replaced. As the idea is so well known, and the literature abounds with interesting studies based upon it, there is no need to give references. In support of this hypothesis attempts have several times been made to relate an assumed amount of contraction of the spheroid to the shortening of the crust recorded in folded mountains; but, as the latter quantity is hardly capable of measurement and the former rests on assumption, no great certainty attaches to the results. It is usual in such arguments to ignore the observation that some parts of the crust have been in tension rather than compression for long periods of time; and also that folded mountains are not always in course of production, which would be the case once the crust had fractured under stress; for as the slow cooling and contraction progressed, so the broken and hence weak parts of the crust would continue to give relief by progressive folding and faulting. The contraction hypothesis easily held the field before the discovery of radium in 1899, but somewhat later it became known that in small quantities this element is scattered through the rocks of the earth's crust; and the quantities though minute are more than sufficient to generate as much heat as the earth loses by radiation into space. Early estimates were that the energy liberated by radium as heat was about 60 times greater than the heat which finds its way by conduction through the crust to the surface and is lost. One physicist went so far as to state that the earth must eventually explode, but this was an overstatement of the case. The discovery of radium was damaging to the contraction hypothesis, but its apologists were not easily discouraged; and when it was discovered that basic rocks contained less radium than the acid ones, a way was found of continuing the old idea, for the acid rocks are concentrated near the surface. It became necessary, however, to assume in the first place that mountains were due to contraction of the earth, and on this assumption it was calculated how rapidly the radium content must die away from the surface towards the centre of the earth, in order that the total calculated heat should be less than was lost by radiation. These physical speculations are interesting, but as a corrective to their value it must be pointed out that geologists possess the certain knowledge that conditions suitable for life, i.e., not greatly dissimilar from those of the present time, have persisted for a very long time. They can therefore view with a somewhat detached calmness statements either that the earth is only twenty million years old or that it will explode at no greatly distant date. Whatever are the forces that have caused earth movement in the past, they are still operative and will continue to operate. An important point with regard to the contraction hypothesis is that it has not yet been shown how exisiting tectonics can result from a general compression of the outer shell of the earth. All these various aspects of the problem lend little support to the hypothesis that mountains are produced in the earth's crust by a shrinking interior, and so in order to ascertain the validity of this hypothesis, it is desirable that a crucial test should be applied to it. Such a test might be to find out experimentally what structures are produced in a spherical sheet supported on a contracting interior with a fluid contact between the solid shell and the interior. It was Lord Avebury who realised that every small area in a contracting sphere could
T
186
A. J. BULL,
be imitated by a flat sheet in general compression in its own plane. T. T. Quirke [15*] has also recognised this point. It will therefore be of interest to consider what experiments have been made that approximate to these conditions. Reference may here be made to an obscure experiment made by A. E. B. Chancourtois about 1853, but which he did not describe until 25 years later. He used an inflated rubber balloon, the surface of which was oiled and then coated with wax, and this on partial deflation exhibited a pentagonal pattern of ridges, which he considered to be analogous to mountain chains. A somewhat similar experiment was made by R. T. Chamberlin and F. P. Shepard in 1923 [10], but this was without any fluid between the" crust" and the shrinking interior. Fifty years after Chancourtois, Lord Avebury [2] expressed the opinion that "If . . . folded mountains are caused by compression due to contraction of the earth, the compression must take place in two directions at right angles one to the other." This is the kind of compression that I have referred to as "general." Lord Avebury designed an apparatus to produce this stress in layers of cloth, etc., but his experiments did not have definite results, although his published figures show in places indications of three anticlines meeting at a point. In the discussion on Lord Avebury's paper, Dr. Johnston-Lavis made the suggestion that a better form of the experiment might be made with a sheet of rubber attached to a ring, the rubber to be stretched over the end of a cylinder and its contraction used to simulate the contracting crust of the earth in a layer of material resting upon it. Some ten years ago, I carried out this experiment [7], with the result that a definite pattern was obtained with a variety of materials. The first effect of the general contraction was the raising of round domes in the solid sheet under compression ; these then broke down into three anticlinal lines meeting at a point, and under favourable conditions many of these lines joined to form a network. These experimentally produced forms are not common tectonic structures in the earth's crust, and in my opinion are conclusive evidence that the contraction hypothesis is wrong. The subject of folded mountains is only referred to here because it has been thought to involve the general compression of a sheet. I am still inclined to the opinion that folded mountain ranges result from convective sub-crustal movements, which account for the folding being local in place and occasional in time. [4] [6]. II.
COMPRESSION OF A SHEET IN ONE DIRECTION IN ITS OWN PLANE.
Another idea has been that many tectonic structures were produced by the horizontal compression of strata between the" jaws of a vice." If a sheet of any suitable material is subjected to compression in one direction in its own plane, it will buckle into simple folds which may extend across the folded sheet. When, however, there are irregularities in the sheet or if it is curved along the direction of folding, then each elevation may pitch out and be succeeded by a depression along the axis common to both, PI. 4, A. This pattern is produced in a sheet that is free to fold in either direction from its plane. The geological case may be that of a superficial sheet of rock held down by gravity on to its support, when its folding will be chiefly upwards into the air, and the synclines remaining in contact with the support would tend to be flat-bottomed. Only in the case of a stratum with relatively soft material above and below would the pattern of PI. 4, A be produced. In compression of this type it is obvious that unless there is negligible friction between the rocks under stress and the base upon which they rest, the folding will be greatest at the sides of the sheet where the force is applied, for, however one may assume the friction to be reduced if the strata under compression are resting on a salt bed for instance, or even on a hot basaltic layer, the outer parts will be folded more than the centre, PI. 4, B. The idea of compression as between the jaws of a vice has been applied particularly to the case in which sediments, that have been accumulated in a sinking portion of the earth's crust, are subsequently folded and piled over each • For List of References see p. 188.
PROC. GEOL.
Assoc.,
VOL.
LIV (1943).
PLATS 4.
A .-PAffERN FORMED BY COMPRESSING A SH EET IN ONE DIRECTION IN ITS OWN PLANE.
B.-A
SHEET OF MATERIAL R ESTING ON GLASS AND COMPRESSED BY FORCES ApPLIED AT OPPOSITE EDGES.
(1'., f
THE COMPRESSION OF A SHEET.
187
other in great sheets. The study of Alpine tectonics has, however, shown that folding starts near the centre of the trough of sediments [1], and neither small scale experiments nor any mechanical scheme has so far been able to indicate how such folded mountain chains can be produced by a force applied from outside. Chamberlin and Shepard [10, p. 506] in their experimental study of folding attempted without success to imitate the nappes de recouvrement of the Western Alps by stresses applied from the side. There have been many experiments of this type from those of Sir James Hall to H. M. Cadell, Bailey Willis and other workers, but they always suggest to me that stress can be developed locally in the earth's crust without action and reaction being equal and opposite; on this point Oldham [14, p. lxxxv] has referred to a " widespread fallacy that pressure can be one sided." A simple case of the type of folding under discussion is that of the Jura Mountains, which are situated to the north-west of the Swiss Alps, and extend in an arcuate form from near Basel to the south-west of Geneva. The arc is bowed away from the Alps, and the axes of folding are roughly tangential. These mountains have a simple form of folding which has become known as the Jura type of structure. This consists of elongated anticlines arranged en echelon, each anticline pitching out and often being succeeded along its axis by a syncline [11, p. 134]. The rocks forming the Jura comprise Jurassic limestones and marls, some Cretaceous and Tertiary beds are preserved in the synclines, while Trias is exposed in the partly denuded anticlines. The early work on the tectonics by J. Thurman [17] and by H. D. Rogers [16] was largely concerned with ascertaining the direction of the movement by finding the direction from which the majority ofthe anticlines were inclined or overfolded; and though Rogers concluded that the movement had been southward, Thurman held the opposite view. The old idea that the form of a fold indicates the direction of pressure has been questioned by Oldham [14.] These early workers were dependent upon surface observations, and our fuller knowledge of the structures is due to Buxtorf's study of the railway tunnels at Grenchenberg and Hauenstein [8]. This work of Buxtorf has shown that a huge decollement has taken place and the folding is superficial. The whole of the beds above the Anhydrite Group of the Muschelkalk, Middle Trias, have moved on the plastic The foundation on which these beds rest is material of the Group. unaffected by the folding. The beds have moved to the north and northwest, that is outwards from the Alps. The anticlines are steep and sometimes faulted, but are on the whole little inclined; the synclines naturally tend to be flat bottomed, and in places are broad, where the beds have moved without folding. Some fractures of an imbricate character occur especially towards the northern margin [3] [11]. It is generally assumed that the Jura were produced by a pressure from the south consequent on the great Alpine folding. For instance J. W. Evans, in describing the Alpine Storm [13] speaks of a powerful north-western movement which gave rise to an important portion of the Alpine front and" formed wave after wave in the Jura." The context suggests that the Jura compression was due to a force applied from outside, and the consequences of the earlier work of Buxtorf are disregarded. Prof. Collett [11, p. 124]says "The Jura mountains are undoubtedly the result of a tangential push." To move a sheet of rock bodily merely by a force acting on one edge is hardly possible. This has been clearly recognised by some geologists; H. M. Cadell [9] mentioned it in 1888, and R. M. Deeley in 1918 wrote" A force applied at one end of-a rock sheet would merely buckle it up for a short distance." Further reasons on this point have been given by R. D. Oldham [14]. If therefore a force had been applied to the southern edge of the sheet of which the folded Jura are a part, then it would be the southern and not the northern edge of the sheet that would be folded; whereas it appears from the descriptions that it is the outer (northern and western) parts of the rock sheet that are folded, while the inner portion nearer the Alps is less disturbed. Apart from being carried by a movement of the sub-crustal material, the only obvious force that can move a rock-sheet bodily is gravity. This has been well expressed by Deeley [12, p. 116], who wrote" sheets of rock are not pushed along by pressure applied at one end ; they slide bodily down a slope under the
188
A. J. BULL,
action of gravity, every yard of the mass furnishing its own propulsive force," and the opinion has been expressed that the great Alpine nappes slid downhill [5, p. 154]. If the strata of the Jura were dipping to the north-west, then the weight of the sheet of rock that has moved would have had a component along the direction of the bedding; moreover, the tectonics indicate that the moving force was not localised and was sufficient to move the upper beds as a complete sheet on the salt beds, which played the part of a lubricant. Where the dip was uniform, there the rock sheet might slide as a whole; but at the outer edge, in those parts where the dip was decreasing, the rocks would be in compression owing to the component of the force of gravity in the plane of the bedding decreasing with the sine of the angle of dip, Fig. 24. Thus, in the outer part of the sliding rocksheet the conditions would favour folding, and this would also be produced by any irregularities in the floor over which the sheet moved. Imbricate structure also might be produced where the edge of the moving sheet met obstacles or reached the flatter ground. In this manner the observed tectonics of the Jura Mountains could have been produced. This hypothesis requires that the northwestern slopes of the Alps have been steeper than they are now, in order that the sediments should be able to move under the force of their own weight. Festoons with Jura type of folding have been seen in sheets of snow that have slipped down sliding roofs and also in paint that has been exposed to a nearby fire (PI. 5). Such structures like tectonic ones are often arcuate, because conditions can rarely be uniform enough to produce straight folds. There can be little doubt that, in the many observed cases in which rock sheets have moved bodily, the operative force was gravity. For these cases Dr. E. B. Bailey's slide-plane is a more apt term than thrust-plane.
FIG. 24.-Diagram showing a sheet of sediments coming on to less inclined ground. The concave portion, where the dip is changing, is under compression; because t.he resolved component of the force of gravity along the slide plane (= mg sin. dip) diminishes with the angle of dip and vanishes where the bed is horizontal.
III. REFERENCES. 1.
2.
3. 4. 5. 6. 7. 8. 9.
ARGAND, E. 1916. Sur rare des Alpes Occidentales, Eclogae Geol. Helv., xiv, pp. 145-191. AVEBURY, LORD. 1903. An Experiment in Mountain Building. Quart. Journ. Geo!. Soc., Ii x, p. 348. BAILEY, E. B. 1935. Tectonic Essays, Oxford. BULL, A. J. 1921. A Hypothesis of Mountain Building. Geol. Mag., lviii, pp. 364-7. - - - . 1927. Some Aspects of the Mountain Building Problem. Proc. Geol. . , .. Assoc., xxxviii, pp. 145-156. - - - . 1931. The Convection Current Hypothesis of Mountain Building. Geo!. Mag., lxviii, pp, 495-8. - - - . 1932. The Pattern of a Contracting Earth. Geo!. Mag., lxix, p. 73. BuxToRF, A. 1916. Prognosen und Befunden beim Hauensteinbasis and Grenchen bergtunnel und die Bedeutung der letztern fur die Geologie des Juragebirges, Verh, naturf. Gesellsch. Besel, xxvii, pp. 184-254. CADEll, H. M. 1890. Experimental Researches in Mountain Building. Edinb. Roy. Soc. Trans., xxxv, pp. 337-357.
PRoc. GaOL. Assoc., VOL. LIV (1943).
p.
188.
PLATE S.
lTo [ace
THE COMPRESSION OF A SHEET.
189
10.
CHAMBERLIN, R. T. and P. F. SHEPARD. 1923. Some Experiments in Folding Jour. Geoi. xxxi, pp. 490-512. II. COLLET, L. W. 1935. The Structure of the Alps, London. 12. DEELEY, R. M. 1918. Mountain Building. Ceo. Mag; Dec. vi, vol. v, p. Ill. 1:1. EVANS, J. W. 1926. Presidential Address. Quart, Journ. Geol. Soc., lxxxii, p. xcix. 14. OLDHAM, R. D. 192J. Presidential Address. Quart: Journ. Geol. Soc., lxxvii, pp. lxxvii-xcii. 15. QUIRKE, T. T. 1920. Concerning the Process of Thrust Faulting. Journ. Geol. xxviii, pp. 417-438. 16. ROGERS, H. D. 1849. On the Structural Features of the Appalachians, compared with those of the Alps and other disturbed districts of Europe. Proc. Am. Assoc., Rep. ii, p. 113. 17. THURMANN, J. 1857. Essai d'orographie jurassique, Geneva.
EXPLANATION OF PLATE 4. A.-Pattern formed by compressing a sheet in one direction in its own plane. In this experiment the sheet was a pile of soft paper supported within a mass of wet cotton wool. After compression the mass was allowed to dry slowly, when the paper became stiff and preserved the form taken by it and the cotton wool. The pattern consists of rounded folds which pitch out and reverse along the fold axes. A section across this would correspond closely in shape to the small folds to be seen in the cliffs of Coombe Martin, N. Devon. B.-A sheet of material resting on glass and compressed by forces applied at opposite edges. The sheet is supported on glass; this causes the synclines to be flat bottomed, and the friction prevents the folding from extending far into the sheet. The top fold in the lower part of the figure simulates the structure of island arcs.
DISCUSSION. MR. G. A. KELLAWAV : Dr. Bull has raised some interesting points in his paper. That the Jura structure he has just discussed may have been produced by the sliding and crumpling of relatively competent rocks as they moved over plastic salt measures under the influence of gravity seems to me quite probable. Similar structures on a very much smaller scale may be present in the Weald; for instance some of the sharp folds which have been noticed from time to time in the Hythe Beds, but further work may be necessary to establish this. Large scale structures in which the effort of gravity has played an important part have been described from Persia and elsewhere, but most of these originated subsequently to or were dependent on earlier folding and tilting, and in many cases on the development of erosional features as well. In order that gravity may have a chance to operate in the manner in which Dr. Bull has indicated, it is necessary to have an initial tilting of the basal plane down which the contorted sediments have presumably moved. What produced the initial tilting in this instance'? Differential loading as a result of deposition and erosion might produce sufficient warping to allow gravity-slide structures to develop, but it would be difficult to explain the structure of an area such as the main body of the Alps on this basis. DR. BULL, in reply, said : I thank Mr. Kellaway for his interesting remarks, and note that he has observed folds in the Hythe Beds which were presumably produced by sliding under gravity. I agree it may be difficult to conceive that the main body of the Alps originated ill this manner, but I hope to consider this possibility in the near future.