an autodidact meets a dilettante…

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Posts Tagged ‘ediacaran

Kangaroo Island – return to Emu Bay

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Canto: I wanted to find out more about Emu Bay’s famous fossils so I decided to return and take the trek round the bay to the eastern extremity, photographing any rocky scenario I could find. The good thing was that, again, the weather was perfect for a long walk, and my thoroughly salubrious saunter helped me to break the record for most daily steps recorded on my iphone since I bought one eighteen months ago. The bad thing was that I really had no idea what I was looking for – Emu Bay shale, Burgess shale, WTF is shale? Is it a kind of rock? What colour and texture does it have and how is it formed? I should have researched the matter before proceeding, perhaps.

No matter, I took plenty of photos and now it’s a matter of mapping what I’ve found onto the descriptions in the literature.

shale – typically exhibits varying degrees of fissility, breaking into thin layers, often splintery and usually parallel to the otherwise indistinguishable bedding plane because of the parallel orientation of clay mineral flakes.[1] Non-fissile rocks of similar composition but made of particles smaller than 0.06 mm are described as mudstones (1/3 to 2/3 silt particles) or claystones (less than 1/3 silt). Rocks with similar particle sizes but with less clay (greater than 2/3 silt) and therefore grittier are siltstones.[1] Shale is the most common sedimentary rock. (Wikipedia)

This description doesn’t really help me. Fissility means the tendency of rocks to splinter along lines of weakness, which doesn’t help me either. I tried google images, but the variety of shale presented, and the near-complete lack of any connecting factors, didn’t help me either.

However, when I tried images for Emu Bay shale in particular, I felt some definite progress. Some of the shale was copper-brown, some was slate-grey, some dull yellow, some penicillin green. But I recognised some of the colours and textures in the rocks I photographed.

A trilobite in Emu Bay shale. Trilobites are the most long-lived class of complex creatures – in this case arthropods – of all time, I think, having inhabited the planet for about 270 million years: more than a thousand times longer than Homo sapiens (so far). Some 50 species of trilobite have been found at Emu Bay

Like many, of my generation at least, I learned at school that there were three kinds of rock – igneous, metamorphic and sedimentary. As Wikipedia informs us, shale is sedimentary, and that makes sense as it’s in sediment that fossils are found. Volcanic rock is extruded igneous rock, and there appears to be quite a bit of it at Emu Bay.

volcanic (extruded igneous) rock – I think – at Emu Bay. Lots of it about. Spongy and low density

A lot of this rock sits in gigantic chunks on the sand, but elsewhere they form craggy, small cliff-like structures.

One small area had what I suppose were sedimentary rocks with a coloration completely different from the rest – and of course the more you pay attention to rocks (and everything else) the more variety you find. They were a sulphurous yellow…

an anomalous bunch of yellowish rocks at the eastern end of Emu Bay

Other rocks looked like granite – intrusive igneous rocks – but another prevalent type I saw, forming ridges high above me, was a rock type I can’t easily identify, though no doubt it’s common enough.

Slate-like rock forming small cliffs, visible along much of the eastern side of Emu Bay

But it’s only through reading that I’ve found the type of rocks I’m after, and the more precise location of the fossil-rich shale. The colour is a dark coffee brown, as shown in the photo of the trilobite above, and these rocks only crop up (or crop out, to write technically) at the easternmost tip of the bay. In fact the onshore location of the fossil-rich shale is further still, a few hundred metres east of the bay proper, with a further site a few hundred metres inland. I would’ve had to clamber over the rocks photographed below, and get a bit wet, to find myself on-site.

the end of the road for me – but these are precisely the kind of rocks I was looking for, without knowing it. They’re only at the easternmost tip of the bay, and beyond

No matter. Hopefully the sites themselves are well-protected. I’m mindful of the concern about looters and trophy hunters, and the preciousness of such places as minefields of info about the extraordinary variety of the first highly successful complex life forms after the initial experiments of the Ediacaran biota a few million years before.

More about that in my next post.

Written by stewart henderson

April 5, 2018 at 12:53 pm