Andesites of the Beesons Island Volcanics, erupted locally in the mid to late Pliocene, outcrop over the western flank of the Waitekauri Valley, and are overlaid by minor Plio—Pleistocene dacites, rhyodacites, and rhyolites, of the Whitianga Group. Minor younger quartz-hornblende andesites overlie these acid volcanics in the south of the area. An ignimbrite sequence exposed in the lower part of the valley is significantly younger than the Whitianga Group rocks, with which it has previously been included. It is renamed the Owharoa Ignimbrite and is probably of mid Pleistocene age. Hydrothermal activity in conjunction with active faulting at the end of the main phase of acidic volcanism (Whitianga Group), resulted in widespread alteration of pre-existing rocks. The hydrothermal alteration was characterised over most of the area by high carbon dioxide activity, resulting in development of carbonate minerals in rocks moderately affected, with silicification, and, in the dacites and rhyodacites, potash metasomatism, where alteration was intense; a minor local epidotesodic oligoclase propylitic assemblage also occurs. Where structural conditions were favourable in zones of intense alteration, weakly metalliferous quartz veins were formed; the largest vein, the Waitekauri, has minor base-metal mineralisation in its deeper levels, mainly pyrite, chalcopyrite, galena and sphalerite.