Pleistocene and Recent Formations.
Subaërial Pleistocene Sands. Near Cornwallis on the south-east side of Puponga Point, Manukau Harbour, there is a bed exposed for a height of 10 feet of soft partially consolidated sands rich in oxidised titaniferous magnetite. As shown by his section, Hochstetter (1864, pp. 17, 18; 1867) placed these sands in the same series as Waitemata beds exposed beneath them; they differ in character, however, from any known Waitemata strata, and in addition exemplify the large scale false bedding that is characteristic of dune sands. Their exact counterparts appear in the sands of the Pleistocene range between Manukau South Head and Waikato River (Gilbert, 1921). Hochstetter (loc. cit.) placed these latter in his Pleistocene “Lignite Formation”; Ferrar (1934) more recently has referred them to his Kaihu Sands (Pleistocene). They attain an elevation of over 600 feet in higher parts of the plateau-like range mentioned above and have wide distribution north of the present area beginning at Anawhata, 9 miles north of Manukau North Head, where they occur above the sea cliffs in remnants of a dissected plateau, roughly 500 feet in average height, which broadens rapidly beyond Te Henga to form a conspicuous feature of the landscape.
The occurrence of these wind-bedded sands at Puponga Point is of interest in demonstrating that when they were deposited the entrance to Manukau Harbour was not in its present location but further south.
Pleistocene Clays, Silts and Lignites. Cox (1884) early examined clays with seams of impure lignite near Whau Creek, Waitemata Harbour, and correlated them with Hochstetter's Lignite Formation of the Manukau lowlands. Turner and Bartrum (1929) have dealt with them more recently and note that the 40 feet to 60 feet erosion level that is so prominent around the shores of Waitemata Harbour is underlain in part by these beds as well as by Waitemata strata and occasional tuffs that resulted from early eruptions of the basaltic volcanoes of the Auckland isthmus.
Beds of this Pleistocene formation cover about 4 square miles of the present area, with their greatest development near Avondale and there is good reason to believe that they once continued thence to Hobsonville, 7 miles north, as a broad sheet which has subsequently been removed by erosion.
In the main they consist of white or grey plastic clays with not infrequent pumice and other sands and silts and many thin lensoid beds of lignitic mudstone which are especially freely developed at Point Chevalier Beach. Gardner's clay pit, near New Lynn Railway Station, affords an excellent section of the formation; in several of the beds there are woody stems in the position of growth with roots attached. Pumice pebbles as much as 3 inches in diameter occur in some of the layers, and at Hobsonville there are thin beds of small pebbles of greywacke and other rocks (Turner and Bartrum, 1929).
The contact of this Pleistocene clay formation with underlying Tertiary strata is seldom visible, but Mr. C. W. Firth states that excavations for a sewage tunnel between New Lynn and Karaka Bay, south on the Manukau coast, showed clearly that the younger beds occupy steep-walled trenches eroded from Waitemata strata.* Similar relations are suggested near Point Chevalier, while the fact that at Whau Creek, near Avondale, excavations for the foundations of a bridge reached 50 feet below sea level without passing below the Pleistocene clays shows that the trenches occupied by these latter were carved deep below modern sea level; this trenching indicates that sharp uplift (or lowering of sea level) preceded the deposition of the beds concerned. The observations just recorded also give information as to the depth of this Pleistocene series, for, while descending to more than 50 feet below sea level at Whau Bridge, its beds are found at 100 feet above that level south of New Lynn. Although generally bedded horizontally, as noted by Turner and Bartrum (loc. cit.) they may locally be gently inclined; south of New Lynn their dip in one clay pit is 30°, so that they shared in the later of the compressional movements that have affected the underlying Waitemata sediments.
The writer agrees with the conclusion of Turner and Bartrum (loc. cit.) that this series of Pleistocene beds was deposited “… in lakes or on the swampy floors of the valleys of sluggish streams during progress of very slow subsidence.”
Recent depoits of the present area include marine sands and muds which occupy drowned valleys in the submerged regions of Waitemata and Manukau harbours; extensive mudflats dotted here and there by groves of mangroves are ubiquitous around the shores and extend as tongues up most of the numerous tidal creeks. In addition, there are occasional floodplain deposits and, as already recorded, on the Tasman coast there has been recent progradation of the shore line by sand beaches followed by a relatively narrow belt of windblown sand which extends to the base of the former sea cliffs. It is only at Muriwai, well north of the present area, where the resistant rocks of the Manukau Breccia Series turn obliquely inland, that the
[Footnote] * Personal communication.
Fig. 1.—Intraformational drag folding due to delta slumping; tuffs of Manukau Biecias, 1 mile east of Big Muddy Creek, Manukau llarbour. Fig. 2.—Deformation of Waitemata Series, shora at east end of Blockhouse Bay, Manukau Ilarbour Fig. 3.—Fault along crest of anticline; tuffs of Manukau Breccias near Pararaha Stream, Tasman Coast. Fig 4.—Short platform about 4 feet above high water level and about 40 feet in width, Manukau Brecias, Manukau North Head
Fig. 5.—Clastie dykes A (Y-shaped) and B of Taylor (1930) in Manukau Bieccias three-quarters of a mil north of Manukau North Head Fig. 6.—Matenal of pumee in elastic dykes of Fig 5, the phenocryst is hypersthene. Fig. 7.—Clastie dyke filled by volcame ejecta, Windy Point, two miles north of Manukau North Head. Note the bedding of adjacent bieccias Fig 8.—Differential weathering in 200 feet bluff of Manukau Bieccias near Parataha Stream, Tasman Coast.
belt of modern wind-blown sands materially increases its width and threatens danger to adjacent farm lands.