Riddolls, B.W. 1974 Geology of southern approaches, Poro-o-tarao tunnel duplication. Lower Hutt: New Zealand Geological Survey. New Zealand Geological Survey report EG 211 15 p.
Abstract: The slopes near the proposed south portal of the new Poro-o-tarao Tunnel require excavation to construct the approaches. The hummocky ground surface indicates the existence of landslides which except for superficial slumping, appear to be inactive. Detailed subsurface investigations show that the landslides at the southern approaches are debris slides. These generally consist of saturated silty clays, with variable amounts of rock fragments derived from the Mahoenui Mudstone which underlies the entire area. There is no conclusive evidence to support the suggestion that the apparently intact Mahoenui Mudstone has itself been involved large-scale in landsliding. (auth)