Inangahua lies at the northern end of the 60-mile-long Grey-Inangahua Depression, a complex structural feature between predominantly-granite ranges to the east and west. It contains up to 10,000ft of Tertiary and conformably overlying Lower Quaternary sediments, but, because of a general southerly plunge, the preserved thickness diminishes rapidly northwards from 10 miles south of Inangahua, In summary, the sediments consist of basal coal measures, siltstone grading up to limestone, followed by muddy sandstone, further coal measures and thick gravel. The Late Quaternary succession of glacials and interglacials has left a suite of gravel terraces in the valleys.