Adkin. — The Horoivhenua Coastal Plain. 



Ill 



Te Horo the sandstone beds are, however, traceable at the foot of the hills 

 up to a height of 320 ft., leaving a discrepancy of 120 ft. 



Plotted diagrammatically to scale (fig. 1), the available data show that 

 the inner margin of the coastal plain had a southward slope of only 10 ft. 

 per mile, so that the southern end of the plain originally extended beyond 

 Paekakariki some miles south of the present entrance of Porirua Harbour. 

 This being so, it is evident that some great change has taken place, a 

 change involving the destruction, or at any rate the disappearance, of the 

 former southern portion of the coastal plain, since south of Paekakariki a 

 bold steep coast, and not a lowland fringe, borders the sea. I shall now 

 endeavour to demonstrate the cause and features of this change, and shall 

 also present certain corroborative phenomena. 



The south-western portion of the Wellington Province may be regarded 

 as consisting of a series of earth-blocks, both large and small. The largest 

 block, comprising the Tararua and Rimutaka Ranges, is bounded on the east 

 by the Wairarapa fault, and is rising, as evidenced by its relation to the 

 Wairarapa depression, situated at the base of its steep eastward-facing 

 scarp, and by the emergence of the Horowhenua coastal plain, which lies 



Fig. 1. — Longitudinal section of the Horowhenua coastal plain, showing the sagging 

 (warping) of its southern end due to the subsidence that drowned the 

 IPorirua-Horokiwi valley-system to form Porirua Harbour. 



on its tilted back slope. Another large block, comprising what is known 

 as the Wellington Peninsula, is also rising, seemingly rather uniformly, as 

 shown by the platforms and benches of the Kaukau and Tongue Point 

 cycles of Cotton.* Between these two lies the subsiding Port Nicholson- 

 Porirua Harbour block, now cut in two by the Wellington fault. On the 

 south-east, or downthrow, side of the fault the subsiding tendency became 

 a pronounced fact, and this part of the block has been depressed below 

 the present sea-level to form the harbour of Wellington ; on the other 

 hand, the portion north-west of the fault was differentially tilted north- 

 ward, drowning the stream-valleys incised upon its seaward border to form 

 the branching Porirua Harbour. 



* C. A. Cotton, Notes on Wellington Physiography, Trans. N.Z. Inst., vol. 44, 

 pp. 246-65 (ref. to pp. 248-51), 1912. 



