The Great North Head Calamity

Just a note: you can get this story and much more in a dead-tree book called The Nature of North Head. The price depends on whether you want monochrome or full colour, but $3 gets you a full-colour PDF, optimised for mobile phones.

A view of the fall from an area now off-limits.

Philosophers who argue about trees falling in a forest where nobody hears them fall, now have a new conundrum, this one involving a rock falling and nobody hearing it.

At some point, one Wednesday in August 2016, some rock came down off the cliff, between the Hole in the Wall track and Fairfax Lookout. Perhaps somebody heard a bang, or two bangs, but that was it. Nobody seems to be sure about anything, and I don't report rumours, even if I react to them.

I picked up a rumour on the web, and hurried off to gather photographs. I was just in time, because the panic-merchants were already reacting wildly, fearing that Armageddon was upon us, we were all doomed, all of those things that flailing mismanagers love to shout to make sure that everybody else starts to panic. (This is a cunning ploy to hide the fact that they started to panic first.)

Quite a few weeks later, the best access points were still blocked off. The shots above came from those two points, because I beat the authorities to it, assessed the safety, and went in to record an unusual event.

The panic was based on the squeal that “the whole cliff might come down”. It will, one day, but not right now, and they blocked off unrelated bits of coast in any case.

I gave up a promising career as a management consultant in 1990 to avoid dealing with flailing knee-jerk managers like these. To manage risks, you need to understand the facts and the principles.

Rocks are peculiar solids, filled with flaws, planes of weakness called joints, and geologists have a bit of trouble accounting for them. The best explanation is that when the sediment becoming rock is buried deep enough to become rock, it is under pressure, and later, as it rises to the surface when erosion uncovers it, the rock expands and planes of weakness develop.

All rocks have joints in them, so there is something missing in that explanation. Anyhow, joints are there, and rock falls off when a joint is sufficiently undermined. The joints shape our cliffs, keeping them vertical.

Hawkesbury sandstone usually has two sets of joints, more or less at right angles to each other. (You could write a book about them, and I have done one for youngsters, Australian Backyard Earth Scientist and I'm doing another one for adults, to be called Mistaken for Granite).

Some of the sandstone beds are less resistant to weathering, the way that rocks “rot”, some of the beds in the sandstone are more like shale, and erode out, undercutting the beds above. 

Inner North Head has two clear undercuts, as you can see more clearly in the composite shot below. When the undercutting goes right under a joint, the situation is right for a block to fall, and that is what happened.

It wasn’t the whole cliff, just a block weighing perhaps 600 tons (my first, and wildly inaccurate  guesstimate): not nice to have land on you, but not Armageddon, either.

My neighbour Geoff Lambert suspected that it was bigger, and he did the research, using aerial photos, and came up with this:

"It was much bigger than I imagined. The surface area of the rock that fell was about 950m² and the height (if no overhang), was an average of 33m. Thus a volume of 31000m³ and, at an assumed specific gravity of 2.5, a mass of about 75,000 tonnes."

That's a bit more impressive, but still not a record. The last time we saw a fall like that was in January 1931, and it was called a landslide. The process was slower and better observed, beginning with a fissure or cleft near Dog Face Rock.

This opening went from 2 metres to 4.5 metres over a couple of days, and already, “hundreds of tons” had fallen by 27 January — comparable to the whole fall at North Head. Within 24 hours, an alleged 100,000 tonnes had fallen. That puts our fall in perspective, just a bit.

Sir Edgeworth David knew what was what: this process had shaped the valleys of the Blue Mountains, and it had been going on for millions of years, he told the Sydney Morning Herald in May 1931. (See http://trove.nla.gov.au/newspaper/article/16774630, if you want the full story.)

These events are rare, but inevitable, and for the past few years, I have been photographing likely future fall areas, in the hope of getting a before and after. In geological time scales, they are frequent, but on our scale, such falls are rare. The sky is not falling, Chicken Little!

And seven years later, the area is still off-limits. I note that July 6 is International Fried Chicken Day...