146 Transactions. 



to form one arm of the present inlet is ascribed by Cotton (loc. cit.' 

 p. 257) to a downward movement of 30 ft. or 40 ft. subsequent to the 

 general movement of elevation of the Wellington Peninsula, but no precise 

 cause of the subsidence is proffered. Cotton also states that " at Porirua 

 there appears to have been little or no movement either up or down 

 in 1855. Raised rock platforms similar to those at Wellington are not 

 found." I shall be able to show, however, that raised shore-platforms of 

 wave-planed rock do occur along a very considerable part of the Porirua 

 shore-line, and form one of its most conspicuous features. 



The present writer had occasion to refer to the Porirua area in con- 

 nection with an apparent deformation of the southern end of the Horo- 

 whenua coastal plain (Adkin, 1919, pp. 110-12). The deformation of the 

 coastal plain was ascribed to its intersection by the subsiding, or downward 

 tilting, of the earth -block, bounded east and west probably by flexures, 

 which extends from Port Nicholson to Porirua Harbour, and thence north- 

 ward past Pukerua inside the Island of Kapiti After a detailed examina- 

 tion of a large portion of the Porirua area the writer sees no reason for 

 any modification of this solution of the problems involved. 



The Land. 



Since the present paper has for its main theme the description and 

 interpretation of shore-line features, only the relevant elements of the 

 land-surface of the Porirua district will be discussed, under two headings, 

 as follow: (1.) Topography and Drainage; (2) Influence of Deformation 

 on the Relief. 



(1.) Topography and Drainage. 



The countrv surrounding; Porirua Harbour is one of moderate elevation 

 but of high relief. This moderately elevated tract rises to a greater height 

 inland, especially in a north-easterly direction, and consists of a series of 

 fairly even-crested hill-ridges, which for the most part have a N.E. by 

 N.-S.W. bv S. trend, thought a few of them are orientated more nearlv 

 north and south. The ridge-tops are commonly broad and undulating, 

 and the ridges themselves are flanked on either hand by long branching 

 lateral spurs that taper off as they descend to the bottoms of the intervening 

 longitudinal valleys. The principal valleys have flood-plains in their lower 

 portions, and graded bottoms extend practically to their heads. In their 

 upper reaches, however, overlapping spurs are still a prominent feature. 

 The valley-sides are well dissected by the numerous lateral gullies, but 

 this dissection has not everywhere extended to the main ridge-crests, 

 where what seems to be a more mature relict topography still prevails. 



There is some evidence, in the form of a high-level bench, notably at 

 the head of Taupo Creek and in the valley of the Kahao Stream, of an 

 intermediate partial erosion-cycle, probably corresponding to the Tongue 

 Point cycle of Wellington Peninsula, and in addition to this there are areas 

 of rejuvenation due to coastal recession and other causes. A full con- 

 sideration of these matters is beyond the scope of this paper, but it may 

 be remarked that, while the topography is undoubtedly composite, indica 

 tions of the intermediate erosion cycle or cycles have been practically 

 obliterated except in the instances cited above. Broadly speaking, there- 

 fore, the topography of the Porirua area may be described as being just 

 past early maturity — that is, in the stage when maximum relief is giving 

 place to more subdued forms. . 



