A fossilized (petrified) forest in the ground

There is a place in Tasmania where stunning fossil remains of an ancient forest can be found by just about anyone. This is Lune River, located near the south tip of the island (~ 2 hours drive south of Hobart). Fossilized tree branches, ferns and beautiful agates are buried within the soft gravel ground, and there are dedicated blocks of the forest where public people can go and find some of these neat specimens. In this blog post I’ll briefly describe how the fossilized forest formed, my venture down there, and how to get (i.e., fossick) for them yourself!

Formation of a petrified forest

Hidden in the clay-rich, gravely ground of Lune River there is a petrified forest. Petrified is a term used when an organic material is replaced by mineral. In the Jurassic time period, specially 182 Ma (Bromfield, 2004; Bromfield et al., 2007) the area near Lune River was a lush forest covered with tree-ferns and conifer woods. With time this organic plant material was buried by sediment and thus protected from decay by oxygen and biological activity. The sediment was further buried by basaltic to andesitic lava. While buried, groundwater that was volcanic-heated and full of dissolved silica flowed through the sediment and replaced the original organic plant material (i.e. ferns leaves, tree branches, etc.). In some cases this replacement was cell by cell, thus beautifully preserving the more fine structure and detail of the plants.

If you know where to look you can you find these rare tree-ferns, as well as agates, leaf impressions in mudstone, and silicified conifer wood! First step is to check the Mineral Resources of Tasmania website http://www.mrt.tas.gov.au/portal/lune-river on fossicking in Lune River. Here there are designated areas the public can go to.

I would also suggest stopping by Lunaris Gemstone shop in Lune River. There is a lovely lady named Christine who runs the place, and you can get information exactly where to go, plus check out all the awesome specimens in the shop.

What you need for fossicking is a shovel and spade. The ground is relatively soft and composed of clay and gravel, but can be fairly consolidated so a trick is to look under the roots of fallen over trees, or dig a hole. It is also important, and should be noted, that you must fill up your holes once you are done (although, as in our case, it was beneficial that the previous fossickers didn’t as we found the pre-dug holes easier to work with). The specimens in the ground are quite dirty and hard to tell, but once you get your eye into the more rectangular- and uniform-like shapes of the fossilized wood you can pick out a fair bit.

Final thoughts

While we didn’t manage to find any of the elusive tree-fern, we found a fair bit of agate and fossilized wood; so that leave us with another reason to return I suppose! As a geologist you always have to remember things as they are today were not as they once were. This petrified forest is a nice example of a ecosystem that was quite different to the current eucalyptus forest that occupies the Lune River area of Tasmania today. Thus, it is pretty neat to be able to go out and collect a little piece of the preserved ancient forest for yourself.

-Stephanie

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