Adkin. — Porirua Harbour 153 



The uplift of 1855 raised the shingle beach-ridges and spits, together 

 with the other coastal features in this vicinity, a farther 3 ft. The uplift 

 also raised the shallow shingly sea-bed between the inner and outer shingle- 

 beaches at Paremata Point slightly above sea-level ; and subsequently the 

 formation of the younger series of beach-ridges added two more shingle- 

 ridges to the outer margin of the raised Paremata Point spit. 



(4.) Deltaic Flats. 



The cutting-off from the open sea of some of the lateral indentations 

 of Porirua Harbour in its initial form (fig. 4) by the formation of detached 

 shingle-beaches across their entrances, and the conversion of these indenta- 

 tions into lagoons (see fig. 5), has led to the production of a considerable 

 area of fertile flat land. This newly formed land comprises the flats of 

 deltaic origin in the lower portions of the valleys that open on to the 

 eastern shore of Porirua Harbour at its entrance. 



In the valley of Taupo Creek, where the largest tract of deltaic flat 

 occurs (see fig. 3), the sea formerly penetrated inland at least three-quarters 

 of a mile farther than it does now, as is shown by the presence of old sea- 

 cliffs of small size at that distance from the present sea-beach. The space 

 between these inland cliffs and the raised beach-ridge upon which the 

 village of Plimmerton has been built has since been reclaimed by the 

 outward growth of the deltaic deposits of Taupo Creek and its tributaries. 

 Other deltaic flats occur in the neighbouring valleys of Spring Creek and 

 of the Motuhara Stream (see fig. 5). Flats of a somewhat similar deltaic 

 character also occur in the lower valley of the Kahao Stream, as well as at 

 the heads of the main arms of Porirua Harbour ; but it is unlikely that 

 deltas would have been formed,, in the absence of the detached beach-ridges, 

 in the more exposed lateral valleys nearer the entrance of the harbour. 



(5.) The Sandy Beaches. 



The sandy beaches, of which Taupo Beach at Plimmerton is the largest, 

 are of very recent origin. The source of the sand appears to be the sedi- 

 ments of Porirua Harbour. After the formation of the tidal flats in the 

 upper reaches of the harbour a bar was formed just within its entrance. 

 On the bar the maximum depth of water is only 5 ft., while on either side 

 of it the depth is 9 fathoms (Admiralty chart). The bar appears to have 

 been formed by the checking of the sediment-laden ebb tide drawing out 

 of the harbour by the waves raised by the prevalent north-westerly winds. 

 Part of the deposited sediment would then be cast back upon either of the 

 adjacent shores, and would accumulate at the heads of embayments to 

 form sandy beaches. Dunes of blown sand derived from the sandy beaches 

 cover small areas at several places along the shore of the harbour. 



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



The deposition of the Trias-Jurassic sediments and their subsequent 

 folding and uplift followed by peneplanation appear to have been the earliest 

 diastrophic and physiographic events in the area under notice. These 

 conditions held until the advent of a second period of diastrophism (the 

 Kaikoura deformation). The peneplaned land-surface was first uplifted 

 and then block-faulted on a large scale, the squeezing of the earth-blocks 

 by the compressive forces within the earth's crust being in all probability 



