1 7^ Proceedings of the Royal Physical Society. 



K1VER-TERRA.CING — Its Methods. 



Ill the excavation of valleys, streams do not generally con- 

 line themselves to the narrow limits of a channel along the 

 middle. They extend their operations over some breadth of 

 ground, upon which they wind and shift, erode and deposit ; 

 keeping open for their own use only a narrow channel, and 

 flooring the rest of the bottom of the valley with alluvial 

 deposits. The methods by which flat-bottomed valleys result 

 from the movements of a single winding thread of flowing 

 water within them, are also the methods by which the 

 level alluvium-strewn surfaces are ranged into tiers of planes 

 separated by those successive steps or embankments that con- 

 vert them into terraces. 



The powers of running water to waste, excavate, and trans- 

 j)ort, need no demonstration here. The works of Hutton and 

 Playfair, De la Beche, Lyell, Eamsay, Jukes, Arch. Geikie, 

 and many others whose names belong to the history of 

 geology, have rendered them part of the simple alphabet of 

 the science. We say nothing, therefore, of the importance of 

 rivers as graving tools in earth-sculpture ; it is only necessary 

 to refer to some of the turns and touches, as it were, of the 

 point of the instrument. The section of river terraces con- 

 sists (Fig. 1) of surface planes, gravels and alluvia spread 

 thereon, and edges or terrace-fronts facing the river. We 

 limit ourselves to some account of the methods by which 

 these several parts are formed and combined. 



Formation of River-Curves — Deflection-Pools. 



It was pointed out by De la Beche in his " Geological 

 Observer," and is one of the most familiar of facts, that 

 streams, winding as they go, tend to scoop at the elbows of 

 the curves. A straight course does not suit the conditions of 

 a mobile fluid passing over a variable bed. Some inequality in 

 the channel or weakness in the bank is sure to direct the force 

 of the stream against one side, and it then begins to cut side- 



