THE NEW ZEALAND SOUND BASINS 29 



sides (Fig. 12). These features are also accentuated just below 

 points of deep canyon convergence. The central portions of these 

 rock basins are the deepest, and sounding lines 1,700 feet in length 

 have been employed without reaching bottom. 



Generally the sound entrances are much shallower and narrower 

 than portions much higher up-stream. 



PREGLACIAL HISTORY 



The plateau which survives now as ridges and mesas from Lake 

 Wakatipu to Preservation Inlet probably marks the old-age stage of 

 erosion of a surface originating in the Mesozoic folding^ of southern 

 New Zealand. Differential elevation then ensued, which carried 

 the plainlike surface (developed near sea-level) of the closing cycle 

 of erosion to considerable heights. The early Tertiary sedimentation 

 was probably induced by this deformation. Canyons early became 

 the expression of the cutting action of the streams on the raised area, 

 and during Eocene times a series of anastomosing and graded water- 

 courses were developed in the area. Subsidence ensued, closing the 

 Eocene erosion, the valleys were deeply drowned, and thick masses 

 of Oligocene age were deposited on the Eocene valley floors. This 

 has, it seems to me, been admirably demonstrated by Hutton,^ in 

 his reply to Von Haast's^ assertion that the canyons of the New 

 Zealand Alps are due entirely to ice-action ; for, according to Hutton, 

 Oligocene limestones occur on the lower slopes of Lake Wakatipu, 

 this same valley having been carved out of the schists by streams in 

 Eocene times on the uplifted plain. Therefore, taking Lake Waka- 

 tipu as a type of these lakes and fiords, we are driven to the con- 

 clusion that deep valleys had been carved in the hard Paleozoic com- 

 plex by Eocene streams, and that on subsidence ensuing at the close 

 of that period, the lower valleys were drowned and Oligocene lime- 

 stones deposited. 



Afterward elevation ensued, and the forces of subaerial erosion 

 once more came into play. Now, if stream-erosion here was com- 



1 F. W. Hutton, The Geology of Otago. 



2 The Geology of Otago, pp. 86-94. This volume is a real treasure-house of 

 general geological knowledge for southwestern New Zealand. 



3 Geology of Canterbury and Westland, pp. 177-92. 



