The most widely accepted theory was written by Dr. N.A. Boutakoff (then Director of Geological Survey, Dept. of Minerals and Energy) in his book-"The Geology and Geomorphology of the Portland area", published in 1963.

The theory is that there was once a forest of trees, probably Moonah (Melaleuca Lanceolata) which was eventually smothered by a large sand dune. Water seeping down through the sand formed a crust of sandstone on the outside of the tree trunks, gradually increasing in diameter. At the same time, the decaying organic matter of the trees produced an acid which dissolved the sandstone crust from the inside, thus pushing out the crust on a chemical front.

So from small trees grew large "petrified trunks". They are not petrified wood, but sandstone and are of no value once removed from the site. Over the years they have suffered considerably at the hands of the so-called collectors, so please respect our fabulous natural attraction, and leave it there for others to appreciate.