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Show , SINtTOSITY OF RIVERS. 170 . bt 'ned by the attrition of h · 1 power IS o at wa ter , a new mec amb ca along W'lt h v t'olence by . a stream. san d and pebbles, odrn we ith fore i gn .m gredients bemg .t hrown Running water charge it b mechanical force, sappmg an~ against a rock, excavates . y bent portion is at length precl· . . t'll the supermcum undermmmg 1 Tl bstruction causes a temporary pita ted into the stream .lich t~e~ sweeps down the ~arrier. By increase of the water, wl . th ravine is widened mto a small, a I·epetition of these land-sltps, .. e re caused by the deflexion of . 1 · 1 inuosttles a narrow valley' m w nc ~ s d tl to the other. The unequal the stream first to one s.t de an ou lehn w hich the channe1 I·S ero de d ' hardness of the materta1s t~1f !S to the lateral force of ex- tends also to gt.v e ne w dtrectiO. nbs y accidental sht. ftm· gs of h t e cavation. Wl1 en by t·h ese, or h 1 d numerous other causes, t e alluvt. al matter . the channe ' an . t m 111·11. its genera e of descent, It eat.s ou a current lS made to ~ross k the side of the hill boundmg the curve in the op~osite ban it o:s turned back again at an equal valley' from whtcl~ curt~ee line of descent, it gradually h~llows ancrle and rccrossmg d . the opposite bank, ttll the o ' lower own, m . f out another curve . bed present a success10n o hole sides of the valley, or river- ' :alient and retiring angl;sdeviation from a straight course by Among the cause~ o tend to widen the valleys through which torrents and rtvers t' d the confluence of lateral fl ay be men 10ne . which they ow, m . d'~r rent seasons in mountamous 1 . gularly at tue . torrents, swo n ~rre d discharging at different ttmes . b partlal storms, an . 1 regiOns y . . bris into the mam channe . unequal quantities of de f a river are extremely great, h t us flexures o . d When t e tor uo . 1' f descent IS often restore · f the direct me 0 t the aberr. ation ro. m through t h e I.S thmus which separates who by the riv~r cuttmg bus in the annexed diagram, t e neighbourmg curves. T ' No.2. d . t return for a . f h river has cause It o that extreme sinuosity o t e d. t' to its main course, so ed brief space in. a contr~y at:~ect~~nisthmus (at a). is consum a peninsula ·IS forme ' SINUOSITY OF RIVERS. 171 on both sides by currents flowing in opposite directions. In this case an island is soon formed,-on either side of which a portion of the stream usua1ly remains*. These windings occur not only in the channels of rivers flowing through flat alluvial plains, but large valleys also are excavated to a great depth through solid rocks in this serpentine form. In the valley of the Moselle, between 13erncastle and Roarn, which is sunk to a depth of from six to eight hundred feet through an elevated platform of transition rocks, the curves are so considerable 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 pas&ed beforet. The valley of the Meuse, near Givet, and many others in different countries, offer similar windings. Mr. Scrope has remarked, that these tortuous flexures are decisively opposed to the hypothesis, that any violent and transient rush of water suddenly swept out such va1leys; for great floods would produce straight channels in the direction of the current, not sinuous excavations, wherein rivers flow back again in an opposite direction to their general line of descent. , Our present purpose, however, relates to the force of aqueous erosion, and the transportation of materials by running water, considered separately, and not to the question so much controverted respecting the formation of va1leys in general. '!.,his subject cannot be fully discussed without referring to all the powers .to which the inequalities of the earth's surface, and the very existence of land above the level of the sea, are due. Nor even when we have described the influence of all the chemical and mechanical agents which operate at one period in effecting changes in the external form of the land, shall we be enabled to present the reader with a comprehensive theory of the origin of the present valleys. It will be necessary to consider the complicated ~ffec!s of all these causes at distinct geological epochs, and to ~nqmre how particular regions, after having remained for ages In a state of comparative tranquillity, and under the influence .of a certain state of the atmosphere, may be subsequently .remodelled by another series of subterranean movements,-how "P"' See. a paper on the Excavation of Valleys, &c., by G. Poulett Scrope, Esq. roceedmgs of Geol, Soc. No, 14 1830, t Ibid. ' ' |
