816 



THE GEOGRAPHICAL SIGNIFICANCE OF FLOODS, 



(b) Caiion-nan'ows between exceptionally high and resistant 

 walls. 



(c) Areas of heavy thrust joining points of marked canon con- 

 vergence, or several points of maximum gravitative thrust. 



Fii 



• Cirque-formations. Compare with figs. 1, 2, and 3. 



To this case the writer would direct special attention, as 

 thereby main fiord-excavation is probably explicable. And, at a 

 glance, it will be seen that this is but a logical conclusion from 

 a study of an example as depicted in Fig 6. 



The trench depicted in Fig.G was formed by the establishment 

 of a point of heavy gravitative thrust downstream. This formed 

 a marked discordance of grade; and the trench resulted from the 

 recession of the initial waterfall. The higher the waterfall and 

 the greater the water volume, the greater the gravitative thrust, 

 that is, the deeper the basin excavated. 



Now in a preglacial canon we have, say, a marked canon- 

 convergence. A local basin is thus formed by the great ice- 

 flood. Upstream another marked convergence occurs, causing a 

 heavy down-canon thrust. In seeking to harmonise the grade 

 broken by the downstream ice-convergence, the ice-floods of the 

 feeding caiions cause the basin-head to retreat upstream as far as 



