The geology of Wellington is complex, and that of the harbour no less so. The western half of the harbour is a continuation of the Hutt Valley and is a fault angle depression in the downthrown eastern side of the Wellington Fault. The eastern side of the harbour appears to be the continuation of a buried north-south valley system with Somes Island as a remnant of the ridge between the fault angle depression and the valley. The outlet of the shallower water-bearing beds of the Lower Hutt Valley Artesian System, lies in the area between Somes and Ward Islands. The deeper beds of the Artesian System, which has a total thickness of 1,000 ft at the western Petone foreshore, in all likelihood continue seawards under the harbour and may well lie at depth in the Rongotai-Kilbirnie isthmus. The very straight line of the western side of the harbour has long been recognised as having been formed by movement on the Wellington Fault which probably lies not far off the rocky points dividing the shallow embayments of the line of the Hutt Road. In the Lambton Quay area the Lambton Fault, a splinter of the Wellington Fault, formed the cliffs which lay behind what was a narrow shingle beach when the first settlers arrived in the area. The whole of the land seawards of Lambton Quay and Wakefield Street has been reclaimed from the harbour.
The large movement on the Wairarapa Fault which occurred in 1855 lifted the Wellington Harbour area some 6 ft and the small town of Wellington was severely damaged. Plans for the construction of a shipping basin in the saltmarsh estuary of the Basin Reserve were abandoned; the area between Kilbirnie, Lyall Bay, Rongotai, and Miramar, which contained numerous lagoons and salt marshes in a sand-dune area, was largely drained and the beach on which the Hutt Road is built became much wider. Petone was uplifted, and flooding of the Hutt Valley decreased.
On the northern shores of Wellington Harbour and on the site of the present borough of Petone was Britannia, the first European settlement in this district. East of the Hutt River, in the Gracefield area of the City of Lower Hutt, are extensive areas of reclaimed land. This work has been done largely by the Wellington Harbour Board which was given sole reclamation rights over an area of 140 acres of harbour by an Act of Parliament.
by Thomas Ludovic Grant-Taylor, M.SC., New Zealand Geological Survey, Lower Hutt.
- The Great Harbour of Tara, Adkin, G. L. (1959).