Vol. 62.] GEOLOGY OP DUNED1N (NEW ZEALAND). 385 



In the Lower Harbour, or north-eastern part of the harbour, nearly 

 all the bays point northward or eastward : as, for instance, Portobello 

 Bay and Deborah Bay ; though a few, such as Hamilton Bay, offer 

 rather distinct exceptions. These northern and eastern directions 

 are the natural directions that would result if the bays were the 

 valleys of small tributary streams feedingthe larger one that occupied 

 the present depression of the lower harbour. 



Similarly, the bays in the inner or Upper Harbour, such as 

 Sauers Bay and Broad Bay, are directed nearly due south or west, 

 and may easily represent the submerged valleys of streams tributary 

 to the main river that flowed from Port Chalmers to St. Clair. 



To the general rule in this district (that the upper portion of the 

 stream- valleys are steep and rocky, and the lower portions open 

 gravel-flats) there is one remarkable exception. This is found in 

 the valley of the Leith Water. For the upper 2 miles of its 

 course, this stream runs in the typical Y-shaped steep rocky valley. 

 For the next 4 miles the stream flows quietly over a gently-sloping 

 valley with slowly-rising sides. Thence almost to its mouth, a 

 distance of 5 miles, it flows with numerous cascades in a deep 

 gorge, but for the last mile the bottom of this gorge is filled with 

 recent gravels. In other words, the upper portion of this valley is 

 still developing in the same way as all the other valleys of the 

 district. Then there is a portion the development of which is 

 mature, but the lower part is a gorge still in the very earliest 

 stage of development. It is apparent that the course of this stream 

 has been interrupted by some cause the effect of which has not yet 

 spread back to that part of its valley which still shows the mature 

 development. 



The topography of the district at once gives a clue to the 

 nature of this disturbing influence. The accompanying map. 

 (PL XXXVI) shows that the upper part of the Leith Valley is a 

 direct continuation of the Kaikorai Valley. There is a well-defined 

 mature valley behind Maori Hill ; and a continuation of this can 

 be traced over the reservoir-site to the Leith Valley, where indi- 

 stinct terraces carry it forward to the Upper Leith, in which 

 region the valley-form is mature. Throughout this distance, river- 

 gravels are to be found in many places. This was apparently the 

 bed of a stream that rose where the Leith now rises, the course of 

 which coincided with that of the Leith as far as the reservoir, 

 whence it flowed behind Maori Hill, and along the present Kaikorai 

 Valley (which is now almost a dry valley) to the sea. 



This stream appears to have been beheaded by a tributary of the 

 North-East Valley-stream, which, rising at a source of the same 

 altitude as that of the Leith, had a much shorter and therefore 

 steeper course, and consequently eroded its valley more rapidly. 

 The present gorge of the lower portion of the Leith is the valley of 

 the intersecting tributary deepened and enlarged by the Leith 

 waters. The beheading has occurred so recently, that the deepening 

 of the valley resulting from its sudden shortening has not yet 

 affected the upper portions of the old, more mature valley. 



