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1. Geology
The physical nature of the rock surface in the intertidal region has a certain selective value regarding the presence or absence of a given community—a feature amply illustrated by zonation patterns in different geological regions of
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Text-fig. 2. The Hauraki Gulf: Contours of the sea bed in fathoms. (Map by R. M. Cassie.)
the Gulf. In fact, the area divides itself up conveniently into a number of sectors according to the general nature of the substrate. The principal rock types present are greywackes and argillites, andesitic conglomerates and breccias, sandstone and grit, and finally a few local areas of volcanic basalt. Historically the area is an interesting one. Details of the sequence in past ages are beyond the scope of this work, and have been largely made known through the careful work of local geologists. For further information the reader is referred to Fraser and Adams (1907), Bartrum (1921), Bartrum and Turner (1929), Ferrar (1934), Hamilton (1937), and Searle (1948).
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Briefly, the highlights of local geological history are these. The northern part of New Zealand underwent three submergences: the first in Mesozoic times, flood waters depositing argillites and greywackes. With the next flooding came limestone beds in the Cretaceous and early Tertiary. In the final Mid-Tertiary submergence sandstones and mudstones were formed.
A series of blocks breaks up the mainland as it stands to-day. The Coromandel Peninsula is an elevated block composed mainly of the ancient greywackes that underly the whole area. On the other side of the Gulf the Hunua block consists of sandstones, limestones and coals, as well as greywackes. These two blocks are separated by the low-lying Hauraki Plains, south of the Firth of Thames. The Auckland block comprises a series of terraces formed by the old Waitemata River as far back as the Waitakere Ranges. Limestone outcrops in it are rare.
The deeply indented coastline and many islands of the Hauraki Gulf came into being as a result of the most recent (Post-Pleistocene) downward movement of the of the land-mass and consequent flooding of low-lying valleys and river systems by the sea (Searle, 1948, p. 34). Rangitoto Island was formed even later by volcanic activity along with other basaltic lava districts about Auckland (e.g., Milford, North Head). Over the past few thousand years there have been no marked changes in land-sea level, any modifying effects having been caused by erosion. Soft sandstone cliffs have retreated at a rapid rate under the continual impact of direct, though gentle waves, and by the action of rain and wind. Thus there remain wide shore platforms as far as these cliffs extend, in contrast with the more steeply shelving coastlines of the firmer greywacke landmasses.