The importance of Tahanga basalt declined after about 1600 AD when adzes made of stone such as greywacke began to dominate the scene. This material is geologically of a more widespread nature, present in many areas of New Zealand.
Several reasons have been put forward for the decline of Tahanga basalt including a working out of the high quality stone suitable for adzes, increasing territorial awareness with the development of warfare restricting access to the stone source, and the loss of the skills needed to flake and shape the stone. The answer may be a combination of factors. Another possible explanation is that flaked adzes went out of favour (or fashion) as a trading item and were replaced by other products which have left no archaeological record (perhaps fibre work or food). Trade networks were maintained after warfare and territorial conflict developed as obsidian and chert continued to be moved over long distances. It is interesting to note that a parallel situation occurred with the metasomatised argillites from Nelson, the other important and widely distributed stone in the first few centuries of settlement. Its influence also declined until by about 1600–1700 AD it was only being used in the local area (Challis 1991:120). This suggests the answer lies in a widespread cultural shift in attitude towards the flaked adzes, and a shift in technology from flaking to shaping stone by hammerdressing and grinding, to produce adzes of a quadrangular section known as the Type 2B (Fig. 15). The new technology was less risky in terms of an adze breaking during manufacture compared to flaked adzes which had a high failure rate due to breakage and flakes chipping erratically (Turner and Bonica 1994).
The Tahanga basalt source continued to be used locally. Skippers Ridge 11 (N40/73), a site at Opito occupied during the 18th century, had roughly made adzes and stone flakes and N40/16 at Opito also had flaked adzes but is clearly of a later date to the main use of the quarry on the hill above (Law 1982:58).
Tahanga basalt adzes were not discarded because the quarry ceased to be a major source of new adzes. The stone was valuable. Broken adzes were worked and reshaped (often into irregular shapes), the length reducing over time until they were too small to be used any further. At Oruarangi, occupied from approximately 1500 AD to 1820 AD, the majority of the very small adzes (between 40–100 mm) are of basalt (Furey 1996).
The decline in use of the Tahanga quarry must be looked at in conjunction with the use of Motutapu greywacke. This stone was also flaked and used in the same time period as Tahanga, and may have been in competition with the basalt in the Auckland and inner Hauraki area. The distribution of Motutapu greywacke adzes also seems to have retracted in later times (after 1600 AD), being used mainly on the island and the Tamaki area.
There is reputedly a basalt quarry at Tryphena on Great Barrier Island but it is likely to have been of local importance only. Similarly, an outcrop of basalt at New Chums Beach near Whangapoua on the Coromandel Peninsula may have been only for local use. Other small exposures of the basalt may yet be found but Tahanga will remain the major archaeological source of stone.
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