Tarn Shelf - 13th February 2009

Tarn Shelf - 13th February 2009

Had a good walk to Tarn Shelf on Friday. Haven't done this walk for a while. It's really good in autumn when the fagus is out, but it was pleasant enough yesterday. Tarn Shelf is just that - a shelf, crowded with large and small glacial lakes half way up the side of the Rodway Range in the Mt Field National Park. The range falls steeply then flattens out across the shelf, and then falls steeply again to Lake Seal and Lake Webster. The shelf itself ranges between about 100m and several hundred metres wide, and it gradually falls from south to north. It extends a little over 2 km between the Rodway Hut and ski tow in the south to Lake Newdegate in the north.

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The walk has great views across the eastern section of the park, especially looking very steeply down on Lake Seal. Views also of Lake Webster, Platypus Tarn and the Mt Field East block.

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The tarns themselves are great for sitting beside. Swimming was tempting in the hot sun, but the water was predictably freezing cold.

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The deep green reflection of the slopes of the Rodway Range in Mackenzie Tarn was striking.

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Mackenzie Tarn, with Johnston Tarn beyond.

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This interesting and scarred rock sits beside a very good lookout along the windy track from the lowest ski lodges out onto the open plateau.

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This very obvious faultline sits on the steep slope of the saddle between Tarn Shelf and Mt Bridges, and is obvious from the lookout mention just above, as well as from several easy vantage points along the shelf.

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Remarkable Cave

Remarkable Cave

I tried to find some good information about Remarkable Cave online, as the bits of info from brochures and signs is fairly sketchy. Having failed to find much, I read my books. There I found some good info. I'll put this up for now, but I need to read more, get some better pictures, now I know what I'm looking for, and also get inside the cave.

Most of the solid information I have found about Remarkable Cave comes from the work of David Leaman. This comes from his books The Rock Which Makes Tasmania (Review here) and Walk Into History In Southern Tasmania.

The cave was originally called “Remarkable” because of the way there appeared to be a map of Tasmania if you stood inside and looked through the cave at the right angle. You can see this clearly as you stand on the viewing platform.

Now, to make the cave more interesting, it has two entrances from the sea-end. Two separate caves have joined up. I recall walking through the cave many years ago. I think at that time access to the floor of the cave was less restricted. The viewing platform these days tends to discourage venturing further. I didn’t clamber down, as the sea was washing into the cave rear, and there was little point. However, when you can walk through the cave it is very interesting. I’m trying to find photos from all those years ago.

David Leaman’s view is that the cave is quite unremarkable (“run-of-the-mill”). He’s speaking as a geologist. His view is that the intrusion of the dolerite, the rocks it intruded, and the effects that intrusion has, is what makes the area remarkable. I suppose most tourists would disagree, but the rocks are very interesting. They can be seen clearly from a number of vantage points. I think there is also much to be seen inside the cave, so I’m keen to return when the tide is low enough.

In summary, Leaman says the following:

Most of the rocks here are Triassic sandstones. These were intruded by Jurassic dolerite. The intrusion can be seen clearly around Remarkable Cave. The top of the intrusion and its contact with the sandstone can be seen in the cliffs and shore platforms around the area.

The sandstone in contact with the dolerite as it intruded displays “exquisite and intense folding”. The easiest place to see this is from the lookout below the car-park, looking south to the cliff below. Leaman says that if you can walk through the cave, then there is more folding to be inspected. Dolerite of course forms many major nearby features; Mt Brown, Cape Raoul, Cape Pillar etc

The question is whether the folding occurred at the time of the intrusion, or when the rock was laid down. There is no firm answer. A previous doctoral thesis (Powell, 1967 - Studies in the geometry of folding and its mechanical interpretation) had suggested that the folding was caused by the heating of the rock with hot liquids and gasses related to the intrusion. Leaman thinks this is unlikely, and that the sandstones were folded soon after they were deposited, when they were poorly compacted. They were then overlaid by more sandstones, and then later intruded by the dolerite. The dolerite intrusion has altered (metamorphosed) the sandstone by heat. Leaman notes that the distorted sandstones are also intruded and cut by the dolerite suggesting that the folding existed before the intrusion of the dolerite.

Interesting place anyway.

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Fossil Cove - 7th August 2008

Fossil Cove - 7th August 2008

In the Sunday Tasmanian last weekend, John Cannon wrote about some walks south of Kingston. Firstly, there was the walk to Fossil Cove. A well made track descends to the coast from the end of Fossil Cove Road, which is a little way south of Blackmans Bay. The small cove has a variety of interesting features.

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To the north is this natural arch, cut by the action of the sea into the Permian sedimentary rocks.

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You could wade through the arch when the tide wasn't too high.

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The cliffs here are formed of tilted layers of Permian rocks. The cliffs are somewhat fractured and crumbly, and overhang in places. You can fairly easily move southwards around the wave-cut platforms for a short distance.

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In the rock platforms and cliffs are a great many fossils, hence the name of the cove. At high tide the platform would be awash, at least in places.

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There is also this dyke of igneous rock which has intruded into the sedimentary rocks, cutting across beds, splitting them apart, and climbing the cliff. I'm assuming it's dolerite, as this is in evidence nearby at Tinderbox.

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Mary Ann Bay Rocks

Mary Ann Bay Rocks

At the north end of Mary Ann Bay there is an outcropping of sedimentary rocks in conjunction with the dolerite which predominates along this section of coast. In the photo, the dolerite forms the headland behind, and contrasts quite starkly with the outcrop in front. Looking at the rock, I suspect it is metamorphosed. My Hobart geological map doesn't help. There are Permian deposits in various places; I suspect this is rock which has been overlaid by the dolerite, and is probably from the Malbina Formation which is present elsewhere on South Arm. This rock is fairly gritty, and somewhat bedded, but not like the usual undisturbed sandstones around Hobart.

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At the south end of the bay the Mary Ann Bay Sandstone has overlain the weathered surface of the older dolerite. However, I'm unclear how this outcropped sandstone at the north end relates to the dolerite and Mary Ann Bay Sandstone. I'm pretty sure the northern outcrop is NOT Mary Ann Bay Sandstone; the description is wrong and I'm sure they'd have mentioned this easy-access outcrop on the beach in writing about the incidence of it at the south end. Next time I must find this Mary Ann Bay Sandstone, which has attracted quite a lot of interest over the years.

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Mt Wellington - 26th April 2008

Mt Wellington - 26th April 2008

Nice walk today on Mt Wellington despite the cool breeze, occasional drizzle and showers and general overcast. The mountain is, as ever, changing with the seasons! It was one of those days where you get very warm walking in more than a thermal top, as long as the wind isn't blowing. Then, when you stop, the breeze picks up, and you get very cold within about five minutes. So you put your jacket on, but even walking on the flat makes you sweat in your nice warm jacket. Oh well, never mind. I have some infirmities, which I hoped would be helped by a good gentle walk. The hip may have bursitis, which is inflammation of the bursa. I have visions of a schoolyard, with a small man in a grey suit running round it on fire. Sorry! I suppose I should now actually go to the doctor. Sounds boring, as does the treatment at that link.

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I shall have to investigate the lichen. The lichen on these rocks along Hunters Track were interesting. I am unclear what the circular patterns are or why they might grow like that. I think the books about these things are quite expensive - I suppose it's a quite specialised area of study.

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More rocks at South Cape Bay

More rocks at South Cape Bay

The rocks at South Cape Bay are really quite interesting. This is a small selection of photos of different aspects. More can be seen by looking at all the South Cape Bay posts.

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This interesting dolerite boulder, which sits close to Lion Rock, exhibits quite regular jointing.

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The Triassic sedimentary rocks weather in quite interesting ways. I assume this is caused by the action of salt and seawater over time. Some of these boulders end up looking like enormous sponges.

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The cliffs at the eastern end of the bay carry a warning sign at the top suggesting that people stay away from the edges. When you get a good look at them from beneath, you can see why; they are very badly eroded, and obviously large pieces of cliff have fallen in the past, some look quite recent. I expect to arrive at the bay one day and find these cliffs remodelled significantly.

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