508 NEW ZEALAND SOUND (AND LAKE) BASINS, 



(2) The apparent lack of corrasive power displayed by even 

 such great glaciers as those of present day Greenland. 



With regard to the first-named difficulty, viz., the great size of 

 the sound and lake basins, it appears to be merely a question of 

 the size and velocity of the agent employed. In the case of the 

 ordinary river, long holes over 50 feet deep are ploughed out 

 below temporary base-level during high floods. The enormous 

 flood sea-wave also can apparently carve out a rock platform to 

 depths exceeding 100 feet below sea-level.- A strongly converging 

 ice-stream, say 5,000 to 6,000 feet in depth such as once occupied 

 Milford Sound, should be able, also, at such points of canon con- 

 vergence, to corrade to great depths below base-level. For the 

 efficiency of a glacier, as regards vertical corrasion, is not neces- 

 sarily limited by the level of the temporary base on to which it 

 descends any more than the flood streams of roadside gutters, 

 ordinary rivers, or the ocean are. As a matter of fact one would 

 naturally expect the flood glacier to exert an influence comparable 

 at least with its own depth. Thus if the river flood (say 50 feet 

 in depth) could produce a hollow in its bed some 50 feet deep, and 

 the storm wave, less than 100 feet in height, carve a rock 

 platform deeply below sea-level; so also we should expect the 

 glacier 5,000 to 6,000 feet in depth to not necessarily lose its 

 corrasive power at points of marked canon convergence, until it 

 had produced a basin comparable in depth with its own height 

 above base-level. And it is just at such points that we find deep 

 sound and lake basins. Thus at Milford Sound, to take one- 

 out of many illustrations the old glacier had a short and steep 

 run to base-level, and the main fiord canon has but little larger 

 area in cross-section than that of either of its two converging 

 feeders, viz., the Arthur and Cleddau Canons. The floors of the 

 canons themselves are but slightly raised above base-level, but 

 the sound formed by their sharp convergence is a rock basin with 

 a floor nearly 2,000 feet below base-level. Lower down, at the 

 junction with Harrison Cove Canon, the depth is even more 

 accentuated. Moreover, the Sound walls are enormously high; 

 so that the ice-flood was prevented from spreading and was forced 



