

(41 



VALLEYS. 



VALLEYS. 



642 



smooth and level the valleys, to fill up the lakes, which are a part of 

 the valleys ; in a word, to change gulfs into lakes, and chasms into 

 vales, by a mere surface action on forms which had been more boldly 

 marked in earlier eras of nature. 



We must therefore believe that immediately after the desiccation of 

 the land, its grand chains of mountains and long continuous vales 

 were more firmly outlined, more roughly and strongly modelled, deeper 

 and higher than they now appear, and it only remains to inquire to 

 what known geological causes this can be justly ascribed. 



We must remember, first, that the surfaces of stratification on the 

 sea's bed were once continuous, but on the land they are now inter- 

 rupted by excavated valleys and left dismembered on residual hills : 

 secondly, that these surfaces were plains or nearly so, and horizontal 

 or nearly so; but now they are found contorted, ruptured, placed 

 in angular positions, vertical, or even reversed in particular regions. 

 The latter class of effects depends on the violent nature of the elevating 

 movements to which the land was subject ; the former is often inde- 

 pendent of local disturbance, and seems to be due to the mere action 

 of powerful currents of water. But it is often seen that the line of 

 these valleys is the line of a fault, of a synclinal basin, or anticlinal 

 ridge, that is to say, a line of weakness, a line of least resistance, deter- 

 mined by causes anterior to the current of water which, flowing up or 

 down the line, or both up and down, has worn it into a valley. 



Now if we remember that the most powerful mechanical action of 

 water takes place on the seacoast ; if we remember that, by the con- 

 tinual or the periodical rising of the land, this littoral action has been 

 transferred from point to point over every part of the area of the land, 

 beginning among the mountains at the source of the present rivers, 

 and successively washing and wasting every part; we shall readily 

 admit in this one universal and powerful agency the principal cause 

 which broke the continuity of the planes of strata, washed away the 

 least resisting and left the hardest parts, and, by successively retiring 

 lines of action, gradually completed the main features of the valleys 

 and hills which had not been previously impressed by violent subter- 

 ranean movements. 



Atmospheric agencies must be admitted to have greatly co-operated 

 in this result, especially if, as geologists suppose, there were grounds 

 for believing these to have been more powerful in the earlier [or in 

 some past] eras of the world, when the temperature was perhaps 

 higher and the atmosphere in consequence more highly vaporous. 

 Nor must we undervalue the eroding power of modern streams, or the 

 volume of the disintegrated earthy masses which they transport away. 

 It is past a question that modern rivers have cut then- own channels 

 through lava (Lyell, ' Principles of GeoL'), through diluvial gravel and 

 clay drifted from other regions (Phillips, ' Sections of the Yorkshire 

 Coast'), and through trap thrown up by the Eifel volcanoes ([Phillips], 

 MS., 1829). But in each of these latter instances the valley of diluvial 

 gravel and clay lies in and conceals in part an older valley of ruder 

 aspect, excavated in the stratified rocks of sandstone or limestone or 

 argillaceous slate ; and we may often contemplate in the course of one 

 stream the fragmentary state of the rocks as left by elevatory forces, 

 the wasting of these when they formed part of an ancient shore, the 

 obliteration of the old valleys oy come yet ill-understood cause of local 

 accumulation, and the final adjustment of levels and slopes by causes 

 which are still continuing this beneficent process, enlarging and en- 

 ricliing our meadows, contracting the areas of our lakes, and softening 

 for the future wants of mankind the rugged features of hills which 

 will not always defy the hand of industry. 



(The reader who desires to follow out this large subject may consult 

 with great advantage De Luc's works as, ' Letters on Geology," 

 ' Lettres sur 1'Hixt. de la Terre et do 1'Homme ; ' Play fair, ' Illustrations 

 of Huttonian Theory:' Buckland's ' Reliquiae Diluvianas ; ' Lyell, 

 'Principles' and 'Manual of Geology;' Murchison ; Darwin; [John 

 Phillips, ' Rivers, Mountains, and Seacoast of Yorkshire '] ; and other 

 modern writers. The article PARALLEL ROADS [NAT. HIST. Div., 

 with addition in the present article], may aluo be read. M. Agassiz's 

 ' Speculations on Glaciers,' have several points of important bearing on 

 the subject of Valleys.) 



Allusion has been made in the article SURFACE OP THE EARTH, of 

 which subject valleys constitute BO important a feature, to the 

 researches on the subject of their excavation of Mr. G. Poulett Scrope, 

 V.P.G.S., a not less philosophical, if less popular advocate of the 

 sufficiency of existing causes in geological dynamics than Sir C. Lyell, 

 and to the views which his inductions from their results have led him 

 to form. In the first volume of the ' Proceedings of the Geological 

 Society of London,' p. 170, the reading (now above thirty years since) 

 is recorded of a paper by that geologist, hitherto, we think, unappre- 

 ciated, in which attention is drawn to the value which would attach to 

 a test by which any one v.illey could be ascertained to be the result 

 either of a rapid and violent, or of a slow and gradual excavatory 

 process ; since the forces of aqueous erosion are of a general nature, 

 and while in activity in one river channel, were probably not idle in 

 others. Such a test was previously pointed out by Mr. JScrope, in his 

 work on the ' Geology of Central France,' where lava-currents which 

 have flowed into valleys at intervals of time appear now at different 

 heights above the actual river bed, marking the successive steps of the 

 progress of excavation. In the paper here cited he finds another, and 

 on equally valuable test in the extreme sinuosities of some valleys. 



Any sudden, violent, and transient rush of water of a diluvial character, 

 that is, a flood of wide area, could only produce straight trough-shaped 

 channels in the direction of the current, and could never wear out a 

 series of tortuous flexures, through which some rivers now twist about, 

 and often flow for a time in an exactly opposite direction to the general 

 straight line of descent, which a deluge or debftcle would naturally 

 have taken. Curvatures of this extreme kind are frequent in the 

 channels of rivers flowing lazily through flat alluvial plains ; these 

 curves are gradually deepened and extended, till the extreme of 

 aberration is corrected, and the direct line of descent restored, by 

 the river cutting through the isthmus which separates two neigh- 

 bouring curves. There are occasional instances, where the bias of the 

 river, or direction of its lateral force of excavation has remained so 

 constant, as to give the valley itself the utmost degree of sinuosity. 

 But such examples must be immensely rarer than those of the con- 

 figuration previously described ; because the frequent shiftings of the 

 channels of streams tend to obliterate their windangs, and reduce the 

 sum of the several successive excavations that is, the valley to a 

 more or less straight form. 



The valley of the Moselle, between Berncastle and Roarn, excavated 

 to a depth of from 600 to 800 feet through an elevated platform of 

 later palaeozoic formerly termed transition rocks, constitutes a 

 striking instance of the former class. Its windings are often so 

 extreme, that the river returns after a course of seventeen miles in one 

 instance, and nearly as much in two others, to within a distance 

 of a few hundred yards of the spot it passed before ; wearing away on 

 either side the base of the ridge-shaped isthmus separating the curves, 

 and enclosing a peninsula of elevated land five or six hundred feet 

 high ; but sloping towards the bottom of the curves, where it is 

 strewed with boulders, left there, it may be presumed, by the river, 

 as it gradually deepened its channel and extended its lateral curvature. 

 The valley of the Meuse near Givet offers, through a great distance, a 

 number of similar windings, and the same character is seen at intervals 

 in many of the other rivers of the same physical district of Europe. 

 Parts of the Seine below Paris, and the valley of the Wye between 

 Hereford and Chepstow, are examples nearer home. 



Valleys which like these twist about in the same regular curves as 

 the channel of a brook meandering through a meadow, can only be 

 accounted for by the slow and long continued erosion of the streams 

 that still flow in them, increased at intervals by wintry floods. To 

 attribute them to a transient and tremendous rush of water in the 

 main direction of the valley appears to be impossible. Whilst these 

 valleys were slowly excavated, other rivers, during the same protracted 

 period, will have produced likewise an amount of excavation propor- 

 tioned to their volume and velocity, and the nature of the rocks they 

 flowed over. In the examples cited above, the rocks are mostly hard 

 strata, yet the valleys are wide and deep. Where softer strata, as 

 sands, clays, and marls, were the materials worked upon, the valleys 

 excavated may be expected, as they are found to be, far wider in pro- 

 portion to the volume of water flowing through them. The comparative 

 softness of the materials also, by accelerating the lateral erosion of 

 the stream, will have multiplied the shiftings of its channel, and 

 reduced their sum with greater certainty to one average direction. 

 Hence the deeply sinuous valleys, such as those particularised, are only 

 found penetrating the more solid rock formations. Mr. Scrope con- 

 cludes by a confirmation of his opinion that extreme curvature of 

 channel can only be produced by a slow and comparatively tranquil 

 process of excavation, by a reference to mountainous districts, in 

 which, where the torrents and rivers are most rapid, their course is 

 nearly straight ; from which a legitimate converse induction arises, 

 that a certain subdued velocity in the stream is necessary to produce 

 the former result. 



It may deserve investigation, whether the spiral course of the rivers 

 and valleys of eastern Africa, noticed in the geographical portion of 

 this article, may not have originated in the windings of the rivers in a 

 former and very different geological condition of the country, of which 

 the actual curves are the remains, after the elevation of the land into 

 its present form. 



In a paper on the elevation and denudation of the lake district of 

 Cumberland and Westmoreland (' Quart. Journ. of Geol. Soc.,' vol. iv., 

 pp. 70-98), Mr. Hopkins has investigated and apparently solved the 

 problem of the origin and process of formation of the lake valleys; 

 throwing great light on the history of similar valleys in other countries. 

 The probable origin of the lakes, it may be remarked, that is, in the 

 first instance, of their valleys, in diverging dislocations, was first 

 suggested by Professor Sedgwick, but Mr. Hopkins has placed the 

 argument in its favour, for the first time, in a determinate and demon- 

 strative form. 



It would appear impossible not to ascribe the origin of the 

 lakes of Coniston and Windermere, for example, to the disloca- 

 tions of strata, with which they are so immediately associated. Mr. 

 Hopkins has described an enormous dislocation of the band of lime- 

 stone interstratified with the older palseozoic rocks seen just above 

 Coniston Water, producing a horizontal displacement of about a mile. 

 The direction of the fault, as determined by a line joining the extremi- 

 ties of the dislocated portion of the limestone band, passes exactly 

 down the lake. Another fault ranges down the valley of Troutbeck, 

 as indicated by a dislocation of the limestone band, and a great hori- 



