Flat tops

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This is a transcript of a QuickTime movie (6.9 MB) recorded in the Pilbara in 2005. Dr Martin Van Kranendonk, a geologist with the Geological Survey of Western Australia, explains why the hills around Marble Bar all have flat tops.


One thing you notice is Marble Bar ridge then the top of this one and the other ones, there’s sort of a very flat top to all of the hills even though they’re very jagged in profile. That’s an old land surface from I’m not sure how long ago but somewhere in the tertiary and stuff. So this landscape has been uplifted and the tops of those hills were all, of course, once connected in a flat line just weathering zone. And this whole area from here down to the Hammersley Range and further has slowly been uplifted and so these rivers are all cutting down through it. And that makes the Pilbara pretty good because you are actually exposing pretty fresh rock all the time. Otherwise you might see that as very deeply weathered and you wouldn’t get the nice fresh exposures that you do. So that’s a nice surface here. But you can see it as you’re driving around, really, really obvious in a lot of places that all the hills have flat tops and many of them have a laterite cap. But what’s really amazing is that these chert ridges in places where chert ridges at 2.7 billion years ago because we have the basal Fortescue group, which is an overlying sequence pretty flat layering, actually filling in valleys between chert ridges at 2.7 billion years. So this land surface, even though it’s now been uplifted a bit, has been pretty much at the same level through an extraordinary period of time. It was covered by the Fortescue group but just before the Fortescue group was deposited it was at this level because you see the basal flows filling up the valleys, which are valleys today again. So that surface, yeah, it’s gone up and down, it’s been buried by volcanics and then that’s uplifted and been eroded again. But we’re pretty much back at the place where we were, it’s just starting all over again, two and a half billion years ago. It’s amazing. It’s a long period of history.




Early Life



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