162 



KNOWLEDGE 



[July 1, 1896. 



THE FOLDINGS OF THE ROCKS. 



By ProI'. J. LoOAN LOBLEY, F.G.S. 



ALT1H)UGII most observers of nature are fully 

 cognizant of the inclination or "dip" of the 

 stratified rocks, and well know that oblique beds 

 seen in an exposure or section of these rocks are 

 but parts of groat folds, yet few are aware of the 

 extent to which the sedimentary rocks have been disturbed 

 from their original horizontality. It may even bo said 

 that few geologists, to whom rock-foldings are familiar, 

 realize the aggregate magnitude, much less the momentous 

 significance, of the plications of the stratified rocks. It 

 may, therefore, be useful to present a few facts to bring 

 home to the mind the vast extent to which the once 

 horizontally deposited strata, forming the exterior rind of 

 the globe, have been folded and plicated since they were 

 completed sheets of rock. 



Cultivated, verdure-covered, or wooded land so generally 

 forms the surface, and conceals the underlying beds from 

 observation in almost all generally known regions, that 

 exposures of the rocks beneath the surface have to be 

 sought for ; but yet they are sufficiently numerous to 

 reveal the general structure of the ground beneath us. 

 Natural sections are seen in mountain precipices, toi-rent 

 ravines, banks of rivers, and cliffs of the sea coast ; 

 while mining, quarrying, well sinking and boring, and 

 road and )-ailway engineering, with its excavations 

 and tunnelling, supply many and most valuable artificial 

 sections. 



Although these exposures of the rocks forming the 

 exterior crust of the earth show in many cases horizontal 

 or only slightly inclined strata, yet so many present to 

 view highly inclined beds, and these in so many widely 

 separated parts of the world, that, in the words of Sir 

 Archibald Geikie, " we may readily perceive that the 

 normal structure of the visible part of the earth's crust 

 is one of innumerable foldings of rocks.' The examples 

 that follow must, therefore, be regarded merely as typical 

 illustrations, and not by any means as an exhaustive list 

 of those that have been observed. 



Our own country of England affords many remarkable 

 examples of highly inclined and folded rocks. In Shrop- 

 shire the Cambrian rocks of the Longmynd Hills are 

 actually vertical, with beds of conglomerate in which the 

 longer axes of the pebbles are upright ; and so it is evident 

 that these massive beds are but remnants of enormous 

 folds, the upper parts of which — the crowns of the arches 

 as it were — have been entirely removed. Such is also the 

 case in the Isle of Wight, where, at both the east and the 

 west end of the island, the stratification of the Chalk may 

 be seen by the lines of flints to be almost vertical. In 

 other places the rocks, though not so nearly vertical as 

 ill the above-named localities, are yet very highly inclined. 

 In the Vallis Valloy, near Frome in Somersetshire, the 

 Carboniferous Limestone is at a very high angle — about 

 seventy degi'ees — but overlaid by Jurassic rocks that are 

 almost horizontal. The grand section of the Carboniferous 

 Limestone in the gorge of the Avon below Bristol shows a 

 dip of forty degrees throughout the whole section, which 

 is fully a mile in length, giving the thickness of the 

 formation. In the Mendip Hills the compression of the 

 horizontal extension of the rocks by folding has been 

 estimated to be as much as half of its original length. In 

 many localities, too, the rocks are so plicated that they 

 can only be called contorted, the bendings being so nume- 

 rous in a small space and the angles so acute. Such are 

 the Purbecks of Lulworth, in Dorsetshire, and the Lias of 

 some sections in the Midlands. The remarkable Silurian 



inlier in the midst of the Dudley coalfield, forming the 

 Wren's Nest, is but the summit of a fold of Wenlock 

 Limestone that rises through the Coal Measures. Of large 

 but less acute foldings there is an example to the west of 

 the Malvern Hills, whore the Wenlock Limestone is in a 

 great synclinal that passes under the Ijudlow, which at 

 Ledbury becomes an anticlinal ; and in the same county of 

 Hereford the remarkable "valley of elevation " at Wool- 

 hope gives a conspicuous anticlinal of Upper Silurian rocks. 

 A very fine example of folding, showing both the synclinal 

 and anticlinal fold on a large scale, is presented by the 

 London Basin and the great valley of the Weald to the 

 south, London being over a synclinal of the Chalk which 

 rises to the south and forms the North Downs that over- 

 look the great Weald Vale, through which passes from east 

 to west an anticlinal axis from which the beds dip to the 

 north and south. 



In Wales the examples of inclined and folded rocks are 

 almost as numerous as the sections, both North and South 

 \\'ales being formed for the most part by greatly folded 

 Palfeozoic rocks. A remnant of a vast synclinal fold of 

 Caradoc rocks forms the upper part of Snowdon, in North 

 Wales, and a great anticlinal of Ludlow rocks gives the 

 Alt Fawr and Corw-y-Fan of Brecon, in South Wales; while 

 marvellously contorted rocks, most acutely bent, may be 

 seen at Holyhead Island, opposite the South Stack. Kock 

 foldings and acute plications are as conspicuously dis- 

 played in Scotland and Ireland. Indeed, in some parts of 

 Scotland there is seen an actual inversion of strata, the 

 beds being bent back on themselves. On the coast of 

 Berwickshire the Silurian rocks are highly contorted, and so 

 also are the rocks near the Old Head of Kinsale, in Ireland. 



When the English Channel is crossed the rock foldings 

 are found to be on a still greater scale in surface extension 

 than is indicated by the observation of British rocks alone. 

 Along the whole distance, from Westphalia on the east to 

 Somersetshire on the west, and even further, to South 

 Wales, the Palneozoic rocks are folded in a succession of 

 synclinals and anticlinals ; and in the Ardennes, forming 

 part of this line, enormous masses of these folded rocks 

 have been removed by denudation, yet hills approaching 

 mountains in elevation are left which are the remnants of 

 the original vast plications. In the Eifel district of 

 Germany the Devonian rocks are greatly plicated, and of 

 the same age are the rocks forming a great anticlinal in the 

 Department of the Sarthe, in France ; while Silurian rocks 

 show foldings on an extensive scale in Bohemia. 



Those who have travelled by the railway from Macon 

 to Geneva may remember the rock foldings displayed 

 i by the cuttings along the base of the Jura range of 

 mountains. These foldings of the Secondary rocks in the 

 Jura are, or have been, regularly alternating synclinals 

 and anticlinals, though now showing much denudation, 

 which in many cases has transformed the synclinal folds 

 into elevations and the anticlinals into depressions. But 

 when the Alps themselves are reached, rock folding is on 

 a truly stupendous scale. Even to the ordinary tourist, 

 the precipitous sile of the Rhigi towards Lake Lucerne 

 shows, by the inclined beds there conspicuously displayed, 

 the enormous uptilting and folding to which the Alpine 

 rocks have been subjected; and at the southern end of the 

 same lake, near Fluelen, highly plicated rocks will arreat 

 his attention. It is, however, by the perforation of the main 

 axis of the Alps by the great tunnel of the St. Gothard that 

 we have become acquainted with rock folding in perhaps 

 its greatest manifestation. Thus it has been found that 

 the rocks forming the central portions of this great 

 mountain range are mainly the vertical portions of vast 

 plications, and at Andermatt and Airolo these rocks rise 



