A piece of rock found along the beach between Castlecliff and Mowhanau is not millions of years old as earlier thought.
Whanganui Regional Museum natural history curator Mike Dickison said the chunk of earth found by a Castlecliff man more likely dated back about 400,000 years.
Potonga Neilson found what he thought was a piece of mud flat from when New Zealand was part of Gondwanaland, one of two super-continents on Earth about 600 million years ago. The continent began breaking up about 180 million years ago. While a "fair chunk" of New Zealand is made from Gondwanan rocks, Wanganui is not, Mr Dickison said.
"From where he found it and how far up the cliff it was, it dates to the Shakespeare Group of the Castlecliffian Stage, about 400,000 years ago," he said.
"All the cliffs around here are very young, less than two million years old.