818 THE GEOGRAPHICAL SIGNIFICANCE OF FLOODS, 



riding the old flood-moraines : in a word, by analogy with 

 ordinary stream-action, they will be found hopelessly incompetent 

 to effect corrasion of their beds. 



Thus is deduced a most significant truth that, while present- 

 day glaciers may certainly be studied to great advantage with a 

 view to ascertaining methods of ice-motion on slopes, the work 

 of the recent ice age glaciers cannot be appreciated by such 

 study alone. On the contrary, unless one has a firm grasp of 

 Gilbert's principle, the study of present-day glaciers would 

 actually tend to cause disbelief in the competency of any ice-mass 

 to efficiently corrade its channel. Experiments should be con- 

 ducted in small stream-formed caiions by forcing strong ice- 

 streams along the same for a considerable period. The removal 

 of the ice should then reveal contours such as those just described. 



Application. — 1 . New Zealand: Milford Sound. The 

 convergence of Harrison Cove caiion with the main valley occurs 

 near the Sound-mouth. Several miles higher up is the marked 

 convergence of the Arthur and Cleddau canons. Probably a 

 basin was formed by the gravitative thrust at Harrison Cove 

 mouth. This basin-excavation was continued upstream by the 

 heav}'' ice-rush resulting from the marked convergence of the 

 Arthur and Cleddau glaciers. This trough, in the line of fiercest 

 thrust, is, as we should expect, free from morainic material : in 

 fact, the trough itself demonstrates corrasion as opposed to 

 aggradation. The marked shallowing of the main Sound outlet, 

 as revealed b}^ hj'drographic surveys, indicates either "dumping" 

 of material here or a slackening off, during the ice-flood, of rock- 

 corrading power. Deltas now occupy the lower portions of the 

 Cleduau and Arthur Rivers. These represent the action of 

 insignificant postglacial water-streams. The resultant set of the 

 former monstrous glaciers of the Arthur and Cleddau caiions 

 was, approximately, to the north-west; therefore the northern 

 wall of Milford should have been specially selected for attack. 

 Undercutting forces would here be at a maximum, and hanging 

 valleys would be correspondingly well "hung up": this is the 

 actual state of affairs at Milford. 



