CORRASION BY GRAVITY STREAMS. 271 



of an ordinary river flood. It lies stagnant, rises higher 

 and higher, ever ready to cause devastation, but lacking 

 the chance to do so except at extremely localised spots. 



And again., even along the valley bases proper, aggrada- 

 tion may be going on hand in hand with corrasion. As the 

 increased power of corrasion in channel narrows and other 

 favourable spots allows the mighty stream to overcome the 

 strength of the rock structures ; to burst off large frag- 

 ments; to wedge out slabs along or across joint systems; 

 to abrade the scars so made with larger or smaller stream 

 debris, so on reaching a lower valley "broad" the velocity 

 is much checked; work is disproportionately decreased, 

 and dropping out of the largest blocks occurs from time to 

 time. 



Thus very large basins may be associated with debris 

 strewn "broads" containing islands or hummocks rising 

 above the general level. This point will receive further 

 attention later on. 



Another interesting point here follows as a corollary. 

 Seeing that pressure increases flow, all other things being 

 equal, and that accelerated flow is attended by a wonder- 

 fully increased power of corrasion, it follows that the same 

 glacial thickness which we have been considering — capable 

 of accomplishing such corrasive wonders when the cross- 

 sections are so reduced in strong walled canons many 

 thousands of feet deep — could not accomplish any such 

 marked differential corrasion in areas of senile topography. 

 The load for unit surface is just the same, but gravitative 

 stress has here no lines of deep descent, no marked valley 

 constrictions, no such profound depths to follow. In a 

 word, the action is more even and the channel bases are 

 much less interrupted. 



Before considering the next point, namely, the effect of 

 a decreasing glaciation, we might note at the height of the 



