Synopsis of Physiography.
Fig. 1 shows diagrammatically the major structure of the whole upland region embraced between the Firth of Thames on the east and the lowlands bordering the eastern shores of Manukau Harbour on the west, and illustrates how the Bombay-Happy Valley area
constitutes the southern part of a major earth-block bounded on the east by a N.N.W.-S.S.E. fault (Wairoa Fault of Laws, 1931), with relative downthrow to the west, which follows the line of Wairoa River until it makes its swing back to the north-east, after which this fault continues for some miles along the eastern margin of Happy Valley. On the west this block is defined by the scarp of another major fault (Papakura-Drury Fault of Laws, 1931), subparallel to the first; this rises steep and rectilinear from bordering flats and lowlands except where its regularity is locally impaired by volcanic extrusions.
As on its western margin, so both north and south the block described abuts against plain-like lowlands; in the north against the youthful sediments that fill the fault-angle depression of Papakura Valley (Laws, 1931), and in the south against the lowlands of Pokeno Valley, its southern face being again a steep scarp which is here due to a north-east fracture which may be called Pokeno Fault.
If it be permissible to regard the surface of the resistant grey-wackes which constitute the basement rock of the area as substantially that resurrected by erosion from beneath younger covering beds, it may be concluded that the block as a whole has been tilted to the east against Wairoa Fault, for again and again the summit-levels of greywacke ridges decline eastwards, whilst it appears also to have suffered a further tilt which has caused its southern portion to be upraised into a lofty range of hills well over 1000ft. in height, which is breached more or less medially by two southward-flowing streams, whilst a third further east follows south along the line of Wairoa Fault.
This southern range of hills forms the boldest relief of the area described. Abruptly terminated on its southern flank by Pokeno Fault, northwards, however, it declines in height more gently, and the region becomes one mainly of closely-dissected subdued hills of moderate relief, varied by one or two cup-like hollows and by an occasional higher elevation such as an isolated hill 1046ft. in height, 5 miles north-east of Bombay, which has here been named Dome Hill. One of the cup-like hollows referred to lies about 3 miles east of Bombay and may be called Bombay Basin; it is an area of Tertiary beds now largely removed by the erosion of the head-water tributaries of Ingaia Stream. At Ararimu, north-east from this basin, there is another low-lying depression united with the last by a narrow but deep air-gap (see Map). Its modern drainage is now to the north by Mangawheau Stream, which escapes from the basin-hollow by a narrow gorge carved in a basalt barrier. A second but broad air-gap occurs in a broad meridional aggraded valley about 3 miles east of Paparata.
Still further east there is a remarkable aggraded intermontane hollow, about 420ft. above sea-level, which is called Happy Valley and hugs on its north-east margin the steep scarp due to Wairoa Fault. The chief interest of this paper arises from the problems aroused by Happy Valley and the two air-gaps west of it.
The only other area possessing physiographic distinctiveness, apart from the volcanic mass of Bombay Hill, is one lying north of the lower reaches of Ingaia Stream and constituted by Tertiary rocks
of soft character. It is characterised by gentle, broad slopes on which mammillated forms due to slumping of weak shaly members are not infrequent, but its limits are often poorly defined against the almost equally gentle greywacke slopes sometimes existent.