V.—The Permanence of River Valleys

Extract

A 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, but with a fall in the same direction, tributaries in the same position, and the same catchment basin as before. Rain 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.