70 Rev. E. Sill — Permanence of River Valleys. 



A 



V. — The Permanence of Eiver Valleys. 



By the Kev. E. Hill, F.G.S. 



GOOD deal has been written on the permanence of ocean 

 basins, but about the permanence of river valleys I have 

 never noticed any discussion. The causes which tend to maintain 

 the existence of a river valley are indeed obvious and simple ; 

 probably this simplicity and obviousness are the very reasons why 

 so little has been said. Yet even if superfluous it may be interesting 

 to bring together some of these causes, such at least as have come 

 into my mind. 



What actions can even temporarily obliterate a channel cut into 

 the surface of the land ? Submergence followed by re-elevation : 

 what will that effect ? During submergence sediment may be 

 deposited. If this be deposited as a mantle of equal thickness over 

 the whole bottom, the surface on re-elevation would have the same 

 shape as before ; hills and valleys would reappear of equal relative 

 heights and depths. Probably, however, the amount of sediment 

 would be greater in greater depths : more would be deposited in 

 valleys than over hills. Then on re-elevation the surface would 

 appear with features smoothed down, but not obliterated. Where 

 a valley had been, a valley would again be ; less deep, hut with 

 a fall in the same direction, tributaries in the same position, and 

 the same catchment basin as before. Eain and rivers would 

 resume their old courses and their old work ; the valleys would be 

 the same as before. Imagine even the most improbable case of 

 a sedimentation such as should bury hills and valleys alike ; let 

 every feature vanish as a path is hidden beneath a sheet of snow ; 

 even then the valley would reassert itself. For the sediment would 

 be moist, and on re-elevation would dry and shrink. Shrinkage 

 would be most where depth was greatest : that would be where 

 the valley had been. Thus shrinkage would restore the outlines of 

 the old drainage system ; water would follow the same courses as 

 before ; each valley would reappear and its excavation begin anew. 



If obliteration be effected subaerially there arises a different 

 state of things, Messrs. Anderson and Flett describe ' the Souffriere 

 eruptions of 1902 in St. Vincent as having with their ashes filled 

 up the stream valleys level with their banks: "those who visited 

 the country shortly after the first eruption describe it as having 

 a smooth, gently rolling surface like that of blown sand." Blown 

 sand itself or steppe dust may obliterate channels. One may 

 imagine a complicated river system thus replaced by an almost 

 featureless surface of seolian loess. Yet even in this case the 

 original relief of the ground will surely seldom fail to influence 

 its subsequent shape. In the paper cited above it is said" that after 

 rain had fallen "the knife-edges between the valleys were the only 

 parts retaining the original smooth surface . . . already many 

 of the streams have thoroughly cleaned out the ash from the upper 



' Eoyal Society Proceedings, vol. Ixx, p. 432. 

 2 Ibid., p. 435. 



