288 E. C. ANDREWS. 



drainage of such lakes would reveal their rock basin origin 

 higher upstream in, and just below, the constriction, while 

 their morainic origin lower down would also be seen. 



(f) Moraines. — These would, by analogy with gravity 

 streams, be almost wanting in the canon constrictions, 

 and the greater the stream volume and the greater the 

 velocity the less opportunity for any material to exist in, 

 or near, the defile. 



In a pronounced " broad" the law of the cross-sections 

 is emphasised, there will be a great decrease in velocity; 

 corrasion will be at a minimum and partial falling out of 

 the ground moraine may be experienced even at this flood 

 stage. 



Thus the same stream may produce mighty fiord basins 

 at one spot, while at another spot along its channel, it may 

 not only be incompetent to continue the deep basin form- 

 ation, but even be unable to remove hillocks or incise itself 

 into lowlying plains. Moraine dumping may even be in 

 progress hereabouts. 



But in every case a glance at the land surface, all other 

 things being equal, would suggest the location of minimum 

 and maximum action, and furthermore, not only the loca- 

 tion of such maximum and minimum glacial corrasion, but 

 would also suggest the quantitative variations of such 

 corrasive action. This is probably the explanation for the 

 differential action of glaciers as found by Gilbert in Alaska 

 and cited by Fairchild. 



Annette Island and the lowland around Sitka apparently 

 lie in valley "broads," possibly even at or beyond valley 

 mouths. For example, they may represent terraces of 

 marine erosion. As such the glaciers here must follow the 

 law of the increased cross-sections which give great general 

 slackening of velocity. This is associated with geometrical 



