CORRASION BY GRAVITY STREAMS. 241 



course growing all the time, while from M to O' the work is 

 done without increase of stream strength, energy decreases 

 rapidly, and motion is more in the nature of flowage. 



The excavation above M will therefore be influenced by 

 the form assumed by the material in homogeneous struc- 

 tures falling under the influence of gravity toward a central 

 gash. This is the amphitheatre or cirque form with curve 

 slope decreasing with depth. (For exceptional forms see 

 Appendix). 



From M to O' the horizontal velocity is increased by 

 flowage setting in under pressure and vertical velocity is 

 rapidly decreased. The time factor for the whole corrasive 

 curve A'MO' is the same. The curve M O' tends therefore 

 to possess a comparatively small angular value. It will be 

 important to remember this difference in excavation 

 methods when considering the farther history of such basins. 



B. Subsequent history of channels and channel basins 

 determined by increase of stream velocity. 



Several cases need to be considered here, namely: — 

 (i) Basins etc. in channel constrictions, the channel 



slope being negligible, 

 (ii) Basins and associated forms at channel confluences 

 (iii) Basin and overdeepened channel slopes, 

 (iv) Basins and overdeepening of channels in weak 

 local structures. 



The key to the situation here again is the knowledge 

 that variable stream velocity, all other things being equal, 

 produces very variable corrasive results. If the time factor 

 has had a moderate value only, then a careful examination 

 of the channel base and sides will furnish an epitome of the 

 results of the variable stream velocities along that channel. 

 Each stream considered from point to point of intake of its 

 larger tributaries must seek to render its velocity uniform. 



