If you make your way down to almost the bottom of New Zealand’s South Island, you can walk among the stumps of a petrified Jurassic forest (that’s about 170 million years old). It’s a gem of New Zealand’s fossil plant history, and because of its complete preservation -pine tree stumps and tree-fern stumps, it’s practically unique.
Get to Curio Bay at low tide because the forest is right at sea -level. At high tide or stormy weather the waves slosh around the forest and it’s this wave activity that has uncovered the fossil forest. There is a look-out above the forest and at first you may have to get your eye in to what is below you. You’ll see a rocky shore platform, with pools of water everywhere. But poking up above the water are lots of mounds – these mark the positions of fossil tree stumps. Look more closely and you will see the long lines of fallen tree trunks in between the stumps.
When you get down to the forest, you can see that dark sandstone of the projections surrounds pale, yellowish material – this is the actual petrified wood, and you see the bark as well. On the better specimens you will be able to make out all the growth rings, exactly the same as if someone had cut down a living tree. Petrified wood is very hard, and some of that petrifaction has seeped in to the surrounding sands – which is why the mounds with their stumps resist the waves.
How did the forest get petrified? It was covered by a flood of loose volcanic material – perhaps this washed off the side of a distant volcano some days after it erupted, rushed towards the sea in a raging torrent of a river, and then spread out to overwhelm the trees. The fresh volcanic material would have contained lots of silica (the material that makes quartz, or glass) and this is surprisingly soluble in ground water. Liquid full of dissolved silica would have permeated the buried wood, then solidified within the wood cells. Some time later, the wood itself would have decayed away, and silica would have solidified in those spaces. The end result is a replacement of the wood, often right down to cell-level detail.
My interest in Curio Bay goes back to high school days, when, after first seeing it, I wondered if I could study it in the same way botanists look at living forests – by mapping the position and size of each trunk. This led, over several visits and a few years, to doing just that. First attempts just triangulated with a tape measure, then I tried a flexible canvas square (sewn together by my Mum), but when that kept snagging on the fossil tree stumps, I tried a semi-rigid square of bamboo.
I eventually mapped 118 trees that had a diameter over 100 mm in an area just over 2100 m2, as well as numerous smaller ones. Most stumps are less than about 200 mm in diameter, but there are a few that get up to about 640 mm. This is not very big as forests go. In fact the oldest trees, based on the fossil growth rings, may only have been some 50-90 years old when they died. These ones were probably about 30 m high, emergent pines over a low, 10 m canopy of younger trees. The Curio Bay forest, it seems, was ‘cut down’ by the flood in its youth.
But it gets better still. If you spend some time, you will find the fossil trunks of tree ferns. You can spot them by the surrounding mass of roots. There are also about half a dozen small, but very peculiar stumps. Instead of a trunk containing one set of concentric growth rings, these have several. The Curio Bay fossil forest is the only place in the world where these now extinct plants can be seen in growth position. Cycad-like plants were also around, but no stumps have been found at Curio Bay.
Did dinosaurs walk around this forest? You bet. No remains have been found here, not even footprints (yet!) – but dinosaurs were in New Zealand at the time. It’s certain that they would have been here.
It may seem weirdly coincidental that this forest ended up right at sea-level so it could be (and still is) gradually uncovered by wave action. To some degree it is, but the Curio Bay petrified forest is the lowest of ten successive forests, that can be seen in the cliffs, each buried by a flood. There are actually two very closely spaced forest levels on the shore platform, and there will be more below sea level. So no matter what level the sea was at – the waves would start to uncover one of those forests.
Had enough of fossil trees? To see a real forest with pines and tree ferns – just take the forest path opposite the car park. The big difference between this forest and the fossil forest are the broad-leaved plants. These all have flowers, and there were none of those in the Jurassic.
Oh, if you want to see the forest, best don’t go towards the end of the day during summer. That’s the time to move off the forest and leave it to the Yellow-Eyed penguins. In late afternoon they start popping out of the sea and heading across the fossil forest to their nests.
Like penguins? Check out these crochet penguins that are going to Antarctica. Each one of them represents a woman scientist.
*Clicking on these links will take you to my Academia.edu site, where you can download a pdf of the paper.