playwiths and fun: Sci: Cross bedding

You know how they say, “don’t leave home without it”? Well, to see this piece of geology, you will need to leave home, because cross bedding is only found in the outdoors, and it is the result of the angle of rest acting, many years ago.

A lesson in geological showmanship: I spent quite a few years as a visiting scientist, working with kids in K-6. I got my young audience interested when I sat on an outcrop of cross bedded sandstone and said, “I’m sitting on a 250 million-year-old fossil.” Of course, they wanted to know why and how I knew this, and I had them in the palm of my hand.

Cross bedding in a cutting in Hickson Road. The horizontal bed below tells us these are not tilted strata.

I produced my Vegemite jar angle of rest measurer (you can see it here), and explained that my seat was a fossil sand bank. Because we were all in Sydney, where the dominant feature is Triassic Hawkesbury sandstone, I knew the approximate age. The end result was that they started to see the cross bedding that was visible in their school grounds.

The cross bedding is set up when sand is pushed sideways, like the Sahara dune, seen here. That dune was pushed along by wind, but most cross bedding is formed by the pushing of water. As sand reaches the front of a sand bank, and tumbles down the slope, the sand starts to pile up at the bottom, and a new thin layer forms. Once this layer reaches the top, a new layer starts to form, on top of and in front of the last one.

Cross bedding in a cliff at Malabar, Sydney (inset: close-up), and in a road cutting at Fairlight, Sydney.

Some cross bedding lasts a long time. This picture below shows quartzite, seen in Norway. Quartzite is a metamorphic rock, formed when sandstone is buried under huge amounts of other rock, heated and squashed—yet the cross bedding still persists. That takes a long time!

Once you know what you are looking for, cross bedding can turn up anywhere. Always look for horizontal beds, above and below a series of sloping beds.

Cross bedding in quartzite, Norway.

Tilted and folded beds like these, seen on Mt Pilatus in Switzerland do not show cross bedding.

Science is like that: just when you think you have it all worked out, something changes. Then again, many animals seem to have their own environments worked out.