112 Transactions. 



Tlie subsidence or downward tilting of the northern part of the Port 

 Nicholson - Porirua block involved the southern end of the Horowhenua 

 coastal plain, which lay athwart its axis, gradually bending and depressing 

 it below sea-level somewhat in the fashion depicted in fig. 1.* North- 

 westward of the main axis of subsidence the surface of the subsiding 

 portion of the coastal plain sloped up to the normal slowly rising surface 

 of that part lying beyond the influence of the Port Nicholson - Porirua 

 block, t 



It is a well-established physiographic principle that sea erosion is seldom, 

 if ever, extensively developed on a rising coast, but that subsidence not 

 only causes sea-advance but also facilitates sea erosion. On the rising 

 and emerging major portion of the Horowhenua coastal plain there is no 

 evidence of its seaward edge having been cut back by the action of the 

 sea ; on the other hand, the down-warping of the narrow southern end of 

 the coastal plain permitted not only a local inroad of the sea, but also 

 .simultaneous marine erosion. This erosion advanced, in the immediate 

 vicinity of the intersecting axis of subsidence, to the cliff ed margin of the 

 old land : this took place a couple of miles north-east of Paekakariki, as 

 pointed out by Cotton. J Farther away, as at Hadfield,§ the newer line of 

 cliffs lies half a mile to seaward of the old-land cliffs, and more remote 

 still they die out and do not reappear. 



The Significance of the Riveb-cut Rock Floors in the Ohau 



Valley. 



The evidence I wish to bring forward in support of my contention! | that 

 the Horowhenua piedmont alluvial plain, formed by the lateral coalition of 

 the larger river-fans, was built up on a stationary surface at the foot of an 

 inland mountain range, and not, like the Canterbury Plain, as conclusively 

 demonstrated by Speight,^ laid down on a subsiding maritime area, is fur- 

 nished by the old river-cut rock floors in the intermont portion of the Ohau 

 valley. Unlike the Canterbury rivers, those of Horowhenua — e.g., the 

 Ohau — as the result of a fixed base-level, reached a state of perfect adjust- 

 ment of load to volume and grade. If the Ohau fan and valley-plain had 

 not reached completion, and if the deposition of detritus by the river had 

 not ceased, the rock floors, by which the area of the valley-plain was 

 considerably extended, could not have been formed. The capping of the 

 shingle beds of the valley-plain with a thick layer of clay alluvium was in 

 itself evidence of the termination of the task of fan-building ; and the 

 cutting of the rock floors by the powerful lateral corrasion which immedi- 

 atety followed conclusively proves the fact. 



* This assumes the very recent advent of the Wellington fault. (See also Cotton 

 loc. cit., p. 258.) 



t The southern end of the Horowhenua coastal plain was intersected obliquely by 

 the axial line of the Port Nicholson - Porirua block, the northern extension of which 

 is regarded as running through the gap at Pukerua railway -station, and thence, inside 

 the island of Kapiti, seawards, its general trend being slightly east of north. The 

 entrance to Porirua Harbour seems to be a submerged water-gap. 



X Loc. cit. (1918), p. 218. 



§ The coastal-plain formation has been truncated for a distance of a mile and a 

 half along the line of railwaj- northward from the Hadfield flag station, and cliffs from 

 .60 ft. to 90 ft. high have been developed. 



II G. L. Adkin, loc. cit.. p. .504. 



If R. Speight, A Preliminary Accoimt of the Geological Features of the Christ- 

 church Artesian Area, Tmns. N.Z. Inst., vol. 43, pp. 421-24, 1911. 



