A sheet of limestone was deposited over much of the South Island in the late Oligocene period, about 25 million years ago. As the land began to rise, about 10 million years ago, much of the limestone was eroded, and only remnants are left.

This aerial photograph shows one of these remnants – a thin, resistant band of limestone that forms Kohaihai Bluff, north of Karamea. It is prominent because the softer rocks above and below have eroded. Originally horizontal, the limestone has been tilted up to 50° by uplift of the granite mountains (right). The same limestone band, lying almost horizontal, is found beneath the sea floor to the left.