Quarrying & Mining Magazine

An expert dissertation on aggregate

To celebrate Contrafed’s 40th birthday we republish this gem from The Contractor, Vol 1 Issue 1, October 1976. 
By David Yerex.

Being an expert on aggregate I thought I’d better impart some of my knowledge. Or, to put it another way, since I don’t know the first thing about aggregate I thought I’d better tell you about something I read recently. About aggregate.
The ‘expert’ was describing how aggregate was formed and while you may prefer just to be thankful it’s there and get stuck into mining the mother lode, it wouldn’t do any harm to know the background.
Well now, once upon a time… oh, well, if you want to be bloody awkward and ask for precise dates I can tell you it was about quarter past 280 million years ago and continued on till about 150 million B.C. Give or take a minute or so either way.
At this time there was this dirty big trench in the ocean bed with a sign sticking up above the waves saying “New Zealand. Under Construction. Completion Date 60 Million B.C. Contractor: Mother Nature Construction Ltd.”
You also need to know that off to the western side of this trench was a dirty big area of land which might have been Australia. Anyhow, over a few hundred million years the sea and wind and rivers tore hell out of this land and the sludge carried out to sea and down into this trench. Some of the sediment wasn’t packed too hard and that got labelled greywacke. But as the sediment built up – this bloke reckons it might have got to being 90,000 feet deep but if anyone who was there at the time measured it he wouldn’t argue too much. Anyhow that was a fair old thickness and it meant the early stuff got pushed down into the bowels (excuse the word) of the earth.
Now, the point is this sludge that settled in the trench had a fair bit of water in it, but as it got pushed down into the innards of the old world the heat built up and drove the water off, so the stuff started to compact into rock. F’instance, about a mile deep you’d get rock which rippers, bulldozers and carryalls could handle, but anything buried two and a half miles or more would need blasting. For all that it’s only fair to middling road metal.
The really good hard rock was the stuff that got buried to depths of seven and a half miles or so; that’s your really first-class concrete aggregate and road metal.
Of course I’m not suggesting you start digging to those sorts of depths. Fortunately Mother Nature Ltd., having sent the Aussie-run-off down into the depths to harden it up, also arranged to bring it back. It seems that as the water was driven out the rock got to be lighter (I know it sounds odd, but that’s the way this boffin explains it), and started to ‘float’ back up. That’s how New Zealand got to be here. This trench filled up and then the bottom stuff pushed back up and there you have it… Land of the Long White Cloud.
Complete with lots of lovely aggregate. What worries me is that if the land mass that spewed all its topsoil into the trench eventually became Australia, what do we do if they ever claim their dirt back?

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