Adkin. — Porirua Harbour. 151 



(c.) Potholes formed by Wave-action. 



An interesting minor phase in the destruction of the raised shore- 

 platform is the formation of potholes by wave-action. I have nowhere 

 come across a reference to the formation, in any part of the world, of 

 potholes by wave-action, and, though this phenomenon may have been 

 previously noted, its occurrence is probably very rare. 



The conditions requisite for the formation of such potholes are : 

 (1) The presence on the shore-line of a fairly level surface of relatively 

 soft rock ; (2) waves of sufficient power to perform the work required ; 

 (3) cobbles of relatively hard rock to act as the cutting instruments. 



The necessary combination of conditions and factors as above occurs 

 at Porirua Harbour near Plimmerton (see Plate XXXV and fig. 1). At 

 Plimmerton, because of the low altitude of the hills and their mature 

 topography, the shore-platform is composed of comparatively soft weathered 

 rock. Other important factors are the thin-bedded character and the 

 vertical attitude of the rocks forming the platform, and the strike of the 

 strata, the trend of which is here approximately at right angles to the 

 shore. Being lines of weakness, the bedding-planes of the vertical strata 

 have been hollowed out by the waves. Upon being washed into one of these 

 grooves a travelling cobble or boulder of the extremely hard greywacke is 

 propelled landwards until it becomes immovably wedged in a fissure or 

 meets an obstruction preventing its farther progress. In the latter case its 

 forward motion is changed to a circular one, and a pothole is initiated. 

 This may grow to a considerable size unless interrupted by the breaking- 

 away of one of the walls of the hollow. The largest pothole noted 

 measured 3 ft. 6 in. by 2 ft. in diameter and 2 ft. in depth. 



(3.) The Raised Beach-ridges. 



The raised beach-ridges of Porirua Harbour have a narrower range than 

 the uplifted shore-platform, being confined to the eastern coast at the 

 harbour's entrance from near Te Kewarewa Head to the vicinity of the 

 Paremata railway-bridge, and to the outer coast from Te Re ware wa Head 

 northward to Pukerua Bay. Along these stretches of coast-line two con- 

 terminous series of shingle and boulder beaches may be distinguished : 

 (1.) An older series which was in existence prior to the 1855 earthquake and 

 was uplifted during that event, so that it is now beyond the reach of the 

 waves and is not in the course of accumulation. This series of beaches 

 is referred to herein as the raised beach-ridges. (2.) A younger series still 

 being deposited along portions of the present shore-line within the limits 

 defined. This is undoubtedly the post-earthquake equivalent of the older 

 series. 



The most northerly point of origin of the material of both series of 

 beach-ridges is the coast at Pukerua and Wairaka Points, where great 

 outcrops of intensely hard greywacke form headlands abutting on this verv 

 exposed portion of the coast. Some of the detritus derived from the 

 abrasion of these greywacke headlands is carried eastward into Pukerua 

 Bay, but the greater part is swept southwards down the coast-line by 

 powerful waves acting under the influence of the prevailing north-westerly 

 wind . 



Immediately south of Wairaka Point (see fig. 1) there is a great embay- 

 ment, which on account of its general appearance and for convenience of 

 description I have named Desolate Bay. Along its entire length the 



