CORRASION BY GRAVITY STREAMS. 273 



The presence of a mobile body such as water may now 

 ease its path downwards ; the yielding and rending of its 

 mass under gravitative stress may help it down declivities; 

 it may actually break in avalanche form over " steps" or 

 amphitheatre heads, but any work of corrasion accomplished 

 in this way will be iu the direction of the production of the 

 ''gravitative form," that peculiar geometrical form which 

 in the extreme case is a cylindrical cavity leading from 

 the surface of the earth to its centre. 



The glacier now appears incompetent as a corrader ; it 

 seems scarcely able to flow; its textural units reveal very 

 slight relative motion, the units themselves doubtless 

 change in character even ; its most halting phases are 

 shown where the greatest work of the flood glacier was 

 accomplished [Fig. 10 (c), Part I]; it creeps haltingly over 

 the masses of debris ; it lies stagnant in the basins of its 

 own flood making ; it is impounded by the debris ; its former 

 relatively clean and strongly moving surface is now a 

 sluggish and much jointed mass; it is dominated by the old 

 cliffs which express the flood corrasion; it is the sport of 

 avalanches ; it becomes a dumping ground for the cliff 

 talus; its sluggishness allows of its almost complete burial 

 in places under debris ; and the more the talus smothers its 

 front the more stagnant and incompetent it becomes. And 

 to emphasize the matter, at just those places where the 

 stream had been so strong, at those places does the weak 

 shrunken stream receive more debris. This has been 

 pointed out by Tarr, for a vertical cliff, determined by 

 undercutting and freshly exposed to air by diminution of 

 stream volume, must seek to find its atmospheric slope of 

 repose and thus give rise to much cliff debris. 



Yet even this stream at last, either by itself or by aid 

 from other more mobile, or from more dense streams, 

 assumes a general corrasive aspect, after having adjusted 



