Adkix. — Porirua Harbour. 155 



a factor in the elevation of the higher blocks. In what is now the southern 

 part of the Wellington district the main earth-blocks have a N.N.E.-S.S.W. 

 direction, this being the general trend of the principal fractures. The 

 present high-standing blocks were in some cases uplifted uniformly, but 

 more commonly tilted, the usual direction of the tilt being towards the 

 west. The compression that accompanied the block-faulting and contri- 

 buted to the uplift of the high-standing blocks was also responsible for the 

 warping of the upper surfaces of these. The warping took the form of a 

 series of broad anticlinal and synclinal flexures disposed longitudinally 

 with respect to the N.N.E.-S.S.W. elongation of the earth-blocks. These 

 longitudinal flexures, which determined the lines of the present longitudinal 

 drainage, in some places coincide with the strike of the ancient folding, 

 and in others intersect it at a considerable angle. The latter fact is 

 accepted by the writer as unequivocal evidence that the secondary folding 

 or warping, where operative, was the predominant factor in the deter- 

 mination of the present longitudinal drainage. 



The degree of secondary folding sustained by the surfaces of the 

 high-standing blocks was doubtless variable : in some instances it was 

 pronounced ; in others, again, perhaps insufficient to shape the initial 

 drainage-pattern. In the latter cases the longitudinal drainage is possibly 

 the result of adjustment to the ancient structure. 



In the highest-standing block — viz., that from which the Tararua Range 

 has been carved — the compressive forces acted in two directions at right 

 angles to each other, with the result that the principal longitudinal folds 

 were accompanied by transverse folds, or perhaps the latter were represented 

 in part by dips in the main axes of folding. This accounts for the occur- 

 rence of the zigzag and trellis drainage-pattern solely characteristic of the 

 Tararua Range in the area under notice. On some of the high-standing 

 blocks of lesser elevation only longitudinal flexures were developed, and 

 in others the warping appears to have been practically absent. 



The last diastrophic event prior to the historical period was the 

 inbreak of the Wellington fault and the resultant northerly tilt of the Port 

 Nicholson - Porirua Harbour earth-block. This subsidence was the cause 

 of the drowning of the maturely developed Pahautanui - Porirua valley- 

 system, and of the creation of Porirua Harbour. In its initial stage the 

 shore-line of the newlv formed inlet had all the characteristics of infancv. 

 such as are present when a drowned land-surface of the mature stage of 

 topographic development is involved : the hill-slopes descended to the 

 water's edge and continued without interruption below the level of the 

 water-plane ; in plan the outline of the shore was one of great irregularity, 

 the projecting spurs forming prominent salients, and the drowned lateral 

 valleys acute tapering indentations. Near the entrance of the inlet a few 

 hilltop islands — the higher parts of a nearly submerged spur — lifted their 

 rounded summits above the new sea-level. The general configuration and 

 the outstanding topographic features of Porirua Harbour at this stage of 

 its development are shown in fig. 4. 



The initial stage of the Porirua shore-line was a very transient one. 

 Under wave-attack, even in sheltered positions well within the harbour's 

 entrance, the cliffing of the partially submerged hill-slopes was rapidly 

 effected, and in addition detached beaches or shingle-spits were thrown 

 across the mouths of certain of the indentations on the eastern shore, 

 converting them first into tidal and later into brackish or fresh- water 

 lagoons. 



