C0RRASI0N BY GRAVITY STREAMS. 



247 



original portion was formed. In other words the floor will 

 tend to experience aggradation at such points down-stream. 



Briefly, if a stream of constant volume eut away the 

 block P P'V V'V'V'" [Fig. 8 (a)] so as to enlarge the channel 

 cross-section by the area V V'V'V"' then the stream 

 velocity is measurably reduced, and as such, its capacity 

 for basining below the plane P P'V V" is lessened, Fig. 8 

 (a). A number of such recessions may thus commence 

 simultaneously (owing to altered stream conditions) on a 

 declivity [Fig. 6 (b)] and a channel floor may be converted 

 into a series of "steps" with interstep "treads" (Fig. 9). 

 This is, in fact, very common in nature. During a severe 

 rainstorm such forms may be observed to form simultane- 

 ously on a slope, lagging behind or pursuing others. 

 Fig. 9. 



Figures illustrating the origin 

 of "steps" and interstep "treads" 

 along the channel base of a 

 stream. 



Johnson (pp. 570 - 571) calls 

 attention also to this peculiar 

 "step" and interstep "tread" 

 like appearances of high 

 Alpine valleys- valleys which 

 undoubtedly possess high 

 channel slopes, and which 

 have been recently visited by 

 great ice streams. But an 

 immediate corollary of our 

 theorem is that all such points where velocity is increased 

 yield similar corrasive results, and therefore a whole 

 channel declivity may be cut up into a series of " steps " 

 and interstep " treads " with basined floors or flattish floors 

 diversified by shallow basins and having amphitheatrical 



