144 Transactions. 



Art. XVII. — Porirua Harbour : a Study of its Shore-line and other 



Physiographic Features. 



By G. Leslie Adkin. 



[Read before the Wellington Philosophical Society, 13th October, 1920 ; received by Editor, 

 Slh December, 1920 ; issued separately. 4th July, 1921.] 



Plate XXXV. 

 Contents. 



Introductory. 

 The Land. 



(1.) Topography and Drainage. 

 (2.) Influence of Deformation on the Relief. 

 The Coast-line. 



(1.) The Cliffs. 



(2.) The Raised Shore-platform. 

 (a.) On the Mainland. 

 (b.) The Reef. 



(c.) Potholes formed by Wave-action. 

 (3.) The Raised Beach-ridges. 

 (4.) Deltaic Flats. 

 (5.) The Sandy Beaches. 

 The Origin of the Harbour and the Evolution of its Shore-line. 



Introductory. 



The inlet known as Porirua Harbour, a landlocked arm of the sea, is a 

 unique geographical feature of the western coast of south-western Wel- 

 lington (fig. 1). Along this coast all other indentations are the result of 

 marine abrasion acting more effectively than elsewhere on the weaker 

 sections of the coast, the more resistant portions, which are receding less 

 rapidly under wave-attack, being left to form the intervening promontories 

 and headlands. Marine abrasion has played only a minor part in the 

 shaping of Porirua Harbour — a part, however, that was important in 

 connection with the evolution of the shore-line of that inlet. 



The outline of Porirua Harbour is characteristic of a drowned area 

 where the sea has penetrated a branching valley-system of somewhat mature 

 topographic development. Two of the principal branches of this valley- 

 system are now occupied by tide-water and constitute the present harbour 

 or inlet, while other former indentations have been reclaimed from the sea 

 by the infilling accomplished by local streams. 



One of the first to touch on the physiograph}' of the Porirua area and 

 to correlate it with that of Port Nicholson was Dr. J. M. Bell (1910), who 

 expressed the opinion (loc. cit. } p. 539) that the surface of a tilted earth- 

 block dips from near the crest of the scarp of the Wellington fault in the 

 direction of Porirua and forms the slope which originally determined the 

 course of the Porirua as a consequent stream. The validity of the first 

 part of this statement is borne out by the existence of a peneplane surface 

 — -evidently referable to the Kaukau cycle of Cotton (1912) — which sur- 

 mounts the valley of the Porirua Stream and slopes towards sea-level in 

 a northerly direction. In the same paper Bell also referred to certain 

 historical proof that the small uplift which affected the district round 

 Wellington City during the 1855 earthquake extended into the Porirua area, 

 inasmuch as the Pahautanui Stream became noticeablv less navigable than 

 formerly. 



