Geology with a surprise ending

A prior note: conglomerate, which I refer to here, is a sedimentary rock which contains large pebbles of older rocks, mixed in with sand, mud and clay. It makes a resistant rock which feels odd to walk on, or horrible to walk on in thin-soled shoes. It only forms when torrents of water carry the pebbles along.

Vertical conglomerate surface, at the "up and over".

A bit of background. I am an obsessive writer coming towards the end of his useful writing days, so I am trying to get all the stuff that I can out there, where people can access it when I go gaga or pop my clogs.

I was trained as a scientist, so I look at these things realistically.  If there is an afterlife, it will make me quite furious if I find I can look down, and see my efforts wasted.

As a result, I am putting freebies out there, like some of the stuff in this blog but especially my Many Voices project, a collection of 1.5 million words of original-source Australian history, all in machine-readable form, much of it unavailable as text anywhere else, and this is available as a PDF with no DRM. It is available free to anybody and any organisation expect the Charles Sturt Memorial Museum in Adelaide, who trampled over my copyright and refused to apologise or admit their guilt.

Some of my students once bought me a mug as a Christmas present: it said "Piss Me Off And Pay The Price". This is the lesson I now hammer home to the incompetent swindlers at the Charles Sturt Memorial Museum, who probably don't realise how that particular blog entry is used as a teaching resource in schools and tertiary institutions around the world.  My ban on them, my habit of reminding people about them and the blog entry itself, will cease to be if, and only if, they admit that they helped themselves to my work without permission before putting their own copyright claim on it — and they have to apologise for doing so.  They did the crime, so it wouldn't be hard.

Here begins the geology

I am also churning out commercial books like there aren't too many more tomorrows, and one of these, drawing on a long habit of bothering the rocks, has the working title Not Your Usual Rocks. But while I have a large collection of rock pictures, there are a few gaps, and one of these is good pictures of Sydney's bottom.

What's that?

It's the base of the Sydney Basin, the lump of geology that I live on top of. Centred on Sydney, Australia, the Sydney Basin reaches out west over the Blue Mountains, south past Ulladulla and north past Newcastle.

"Siccar Point" by Anne Burgess. Licensed under
CC BY-SA 2.0 via Commons -
https://commons.wikimedia.org/wiki/File:Siccar_
Point.jpg#/media/File:Siccar_Point.jpg

It is made up of Permian and Triassic rocks, lying unconformably on top of tilted and eroded Devonian metamorphic rocks.  There are scattered dykes, the occasional flow. and at least three volcanic necks that I can recall.

On the right above, you can see an image of a place that was pivotal in modern geology, a view of the rocks at Siccar Point in Berwickshire, south-west Scotland. This is one of the gaps in my collection that I plan to fill in a year or two, but for now, I will use this one. If the trolls from the Charles Sturt Memorial Museum who peruse my stuff would like to take note, the caption to the picture on the left is a handy example of how you acknowledge other people's work.

Well I wanted to capture the best unconformity that my local area has to offer. First up, an unconformity is a gap in the geological record, and this one adds up to 150 million years, or thereabouts. One location, appearing just below this, left and right, is at Myrtle Beach on the south coast of New South Wales.


Once upon a time, just after the Carboniferous, rocks from the Devonian era which had been cooked out of recognition and squished way down, had become what we call metamorphic rocks. They were also folded or tilted, but then any Carboniferous rocks that lay on the Devonian stuff were eroded away and the Devonian rocks came to the surface.

The thing is, from either folding or tilting, the old rocks were all on an extreme angle, and they got eroded back to a jagged landscape that sank down in the Permian, far beneath the sea. This happens in geology, so just live with it.

At Myrtle Beach, the sediments that covered the rocks were fairly fine, but in what is now the Budawang Ranges, inland from the Nowra-Ulladulla strip, a big flood must have pushed a huge supply of boulders, mud and sand out over the newly submerged old rocks. In either case, this covering of tilted rocks is what we call an unconformity.The rocks underneath slope up, the ones on top are more or less horizontal.

But remember that the original surface of the eroded Devonian rocks was a jagged one. I forgot that, and that was the seed of the surprise ending.

As an old bushwalker, I knew there were exposed portions of the Devonian rocks in the Budawangs, and that the back wall of one cave in particular was very obviously below the cut-off, so in February 2014, accompanied by my two sons, I set out on an ambitious one-day raid, involving about 30 km of walking (in and out) from the Wog Wog entrance, past Corang Peak and up to the caves fronting Burrumbeet Brook.

This is obviously based on the CMW map: to the best of my knowledge, this is public domain.
I will make amends if anybody can advise me on the current whereabouts of the Budawangs Committee.

There had been massive fires four months earlier (just when I was photographing Myrtle Beach with my wife's help), so the track was obscured: usually, the track is defined by a lack of plants.
On top of that, there was heavy fog as you can see in these photos from the 2014 trip, so we made poor time. We never saw any rock ribs (large circle), and at the middle red circle, we decided to turn back, so as to get out during daylight.  Our target was the camping caves marked CC in the right-hand circle on the map, so we were close, but not close enough. (By the way, the rock ribs aren't visible from the conglomerate slope.)

Now I have been walking the area since 1970, and we started taking our kids in there in the 1980s, so many parts are well-known to us, like the "bent tree", which we have been watching for something over 30 years.

Tinderry Lookout, which features in a surprise ending.

We have our favourite stopping-and-resting points, and one of these was, and is, Tinderry Lookout (left), the first bit of serious conglomerate that you see when walking in: before that, you are on Devonian metamorphics. So being a bit disappointed by the fact that we had been forced to do the sensible thing and retreat, I started thinking about the geology as we walked back out, heading home. I realised that we were missing something.

Somewhere to the west of Tinderry Lookout, we must have crossed over the boundary, and by walking slowly down the hill, we just about pinpointed it, but it was buried, and I would never get a satisfactory photo of the sort people get at Siccar Point. No matter, we got this shot (right) of what we had available.

In the picture, Duncan's right foot is on Permian rock, his left is on the Devonian. The log was already there, making me suspect that somebody else had been chasing the same goal. But we knew we would be coming back to try again for the real thing.

With busy lives, it took us a while, 20 months, in fact, before we got back there. We walked in past Tinderry Lookout, where we stopped because there is a phone signal from the top of the rock. We passed a kangaroo that thought we couldn't see it (left), and assorted small dragons (right).

We also saw wombat holes and wombat scats (droppings) in large numbers, and on the Sunday, one of us saw a black snake. There was also a big skink, but more of that later, because it is important at the very end.

The "up and over" pass.

Termite mound.

Then we went past a number of termite mounds like the one on the left. Termites or white ants get bad PR, but they are marvellous for recycling dead timber to return nutrients to the ecosystem, over the up-and-over, a narrow defile between two stacks of conglomerate, and onwards.

Soon we were into the area that was burned, just on two years ago, places where we had seen just touches of green in February last year, but now the regrowth was flourishing, albeit beneath a forest of dead sticks.

Lightning-blasted sandstone

Then we pushed on past Christmas Bells (left) (Blandfordia sp. — but "Christmas Bells" in October?? tell me again why climate change is all a myth!), past lightning blasts in the sandstone (right) and cross-bedded sandstone (left).

 Before long, we had our first glimpse of Pigeonhouse (right). This is a mountain with a remarkable resemblance to a human breast. Oddly, when Lieutenant (as he was then) James Cook saw it in April 1770, he thought it resembled a pigeon house. You will need to look very hard at the left hand one to see the tip of the mountain. No, its not a volcanic plug: it's hard Nowra Sandstone.

The Aboriginal name, Didthul means 'breast', so how did Cook miss it? Perhaps, being fresh from Tahiti, he was satiated?  I doubt it, but there it is. Prudishness? There are mountains called paps elsewhere, and the Grand Tetons...

Anyhow, we've been up Pigeonhouse, and soon two other old friends came in view: Mt. Owen and the Castle,  both of which we visited as a family in fitter days. Mt Owen is closest, with the Castle behind it.

But now we were close to our target, able to look out over Canowie Brook, with only a huge tumbledown of conglomerate. In this shot, you can see the track going up the hill on the far side: it veers right, over the ridge, but then swings left into the middle valley.

This was our camping cave, and behind me as I stood taking this, is the section of wall where I took these next four pictures: look for conglomerate above and metamorphics sloping to the left underneath.

Duncan's hand there, like my wife's hand at the start, is spanning 150 million years of missing geological history.

So, mission accomplished, we settled in to beer, port, curry and dark chocolate, slept, and headed out the next morning. Up the conglomerate slope we went (left), much easier going up than down, and then finally found Corang Arch, which you can see on the right.If, like me, you have been misreading the CMW map, the arch is in close to the cliff on the western side. Walk along, one body length from the edge of the cliff, look down and you will see it.

Well, after that, it was plane sailing (and if you think that is wrong, you are in error), all the way to Tinderry Lookout, where we broke for lunch. There, we were amused by as lizard that may or may not be Egernia sp., which was clearly used to filching humans' food. It decided it didn't like the cucumber after a bit of a nibble, it wandered off, and so did I — and now comes the surprise ending.

Earlier, I explained how on the previous trip, we had found a Permian/Devonian boundary down the hill. that was perhaps 150 metres walking and 10 to 20 metres lower. So the boundary wasn't far away, but it was closer than I thought.

You see, after I got bored with the lizard, I walked away from the conglomerate a bit to see what else was living there (I am mainly a biologist, after all). That was when I  noticed that the rock I was walking on was all metamorphic — and it was bedrock. A quick look, and all was clear: Tinderry Lookout is conglomerate, sure enough, but it is sitting on a high ridge of really tough metamorphic rock.

Take a look at this picture above and the one below: most of the rocks with white lichen growing on them are firmly attached to the bedrock, and they aren't conglomerate, though that wall of rock further over is conglomerate! 

When the floods came, back in the Permian, the rubble and junk that later became conglomerate flowed into the deepest parts, levelling the surface and then growing upwards. I had mapped the boundary, some little distance west, down the slope, but the uneven boundary, directed by the shape of the old rocks, was also up here, where we always take a break, and I had NEVER seen it!

So, we had walked seven hours in and six hours out (Angus and Duncan carried most of my gear out, so we went faster on the way out), but I could get to the boundary, about two hours in! Mind you, it wasn't as convincing ... and I gave my legs a good Last Hurrah. I will still keep walking for years, but my legs aren't up to this level of activity, they are wearing out. We found some good joints, though:

My thanks to Angus and Duncan, because without them, I probably would not have got in there, and I certainly wouldn't have got out. When they were young, I carried the heavy pack, and now I am getting old, they did the same for me, but I am properly grateful.

I do have to say, though, that the idea was to get me in there before I got too old to do it.

In retrospect, I think I already was too old.