THE NEW ZEALAND SOUND BASINS 51 



For a little reflection will show — as with ordinary streams — that 

 at these points the valleys are too broad, and their grades have been 

 too reduced, to permit of work other than aggradation. 



As with streams, again, so with the retreating glaciers. On the least 

 reduced slopes a little cutting will be still accomplished, but aggrada- 

 tion, moraine-overriding, and general ice-stagnation will now char- 

 acterize these old channel grades which formerly expressed the slopes 

 in which the glaciers of mighty ice-floods only could effect corrasion. 



Hence, in my opinion, arise the present stagnant, although pos- 

 sibly large, glaciers of regions such as the Alaskan and Norwegian 

 fiords; for along the old flooded channels of such localities, before they 

 can again resume cutting, they must readjust their channel grades. 

 Until such time they will be engaged removing excess of load, in 

 filling up of rock basins. Even a slight ice-flood at the present stage 

 would have its operations mostly confined to aggrading. 



"Hanging" valleys mark the differential erosion of main and 

 tributary channels during the height of the ice-flood. Undermining 

 of canyon walls and truncating of spurs near convergence of narrow 

 valleys would cause recession of tributary graded channels, and at 

 these points one would naturally look for fine examples of "hanging" 

 valleys. A magnificent example for study is afforded by Milford 

 Sound along its northern wall. Here, just below the junction ojf the 

 Arthur and Cleddau canyons, the "hanging" valleys, double wall 

 slopes, and aligned walls are pronounced. A little lower down, the 

 Harrison Cove canyon comes in, and immediately below this the deep 

 rock basin, actually overhanging lower wall, and magnificent "hang- 

 ing" valley occur. On the more protected southern side these result- 

 ant thrust forms are not nearly so pronounced. 



Subsidence as an explanation of the great depths of the south- 

 western New Zealand sounds is utterly opposed to the evidence 

 yielded by topographic studies in non-glaciated Australia and New 

 Zealand. Rock-basin excavation below baselevel by thrusts from 

 convergent ice-masses^ is a sufficient explanation, and accounts also 

 for the disposition and shapes of all the associated forms. 



Cirques arise from ice-sapping action, as with ordinary waterfalls.^ 



I Of course, here, as in New Zealand and eastern Australia generally, a post- 

 glacial subsidence to the extent of several hundreds of feet must be admitted. 

 ' See also A. Penck, and W. D. Johnson, ante. 



