Along a distance of 85 kilometres between Whanganui and Hāwera, the coastal cliffs show almost continuous exposure of layers of sediments ranging in age from 3.5 million to 700,000 years old. Because it is one of the most complete sequences of rocks of this age anywhere in the world, it has been comprehensively studied and sampled.

The aerial photograph shows the cliffs near Hāwera, where the older part of the sequence is exposed. The rocks, named the Tangahoe Formation, consist of thick alternating beds of siltstone and sandstone, and contain fossils.

The flat plateau above the cliffs is a marine terrace – a remnant of an old shore platform cut by waves about 125,000 years ago. It has been preserved by uplift during many past earthquakes