School geology notes part 1

I work as a volunteer 'visiting scientist' at a local K-6 school, and the last two years have been turmoil as it was rebuilt to meet climbing population figures. In the process, some beautiful nature was destroyed, but the place is bouncing back, and I saw that there was now some exciting sedimentary geology to play with.

Today was the last day of the year, and I was there to show the teachers what is available to use, and I wrote these notes before I went.

Stories

Rocks tell stories, and some of these stories are at a simple THE CAT SAT ON THE MAT level. Other stories that the rocks have to tell are more like Virginia Woolf on a bad day.

Getting kids started on reading is a bit like edging them onto a slippery slope on a bicycle and keeping them on an even keel as they get up to speed, carried along by natural forces. OK, that’s a stretched analogy, but slippery slopes are one of my long-term temporary obsessions. All you need, in order to play is a sand dune, but a Vegemite jar half full of sand will do as well.

Once seen, never forgotten: I had to learn this sort of stuff for myself, but each bit, once I acquired it, became part of my ongoing observations. The linking theme here is that sand that is piled up collapses to form a fixed angle (for that sort of sand), and that angle shows up in dunes, rocks, sandbanks, pits and holes in dry sand and more.

When we try to pick out anything by itself, we find it hitched to everything else in the universe.
—John Muir, My First Summer in the Sierra, Boston: Houghton Mifflin, 1911, chapter 6.

A slippery slope in a jar

The thinking for this began when I had an idea to try something. I was getting ready to work on the track with Stage 2, I think. Our topic was flowers and stuff, but I spotted the rock on the right, and saw one of my favourite simple stories: current bedding or cross bedding.

I will explain what that is shortly, but let’s walk before we gallop down the slope, OK? Just watch how all of the bits come together, in the end.

Advancing dune, Sahara

Ant lion pit, coin for scale

Current bedding, as I prefer to call it, tells us something odd about sand. If you tip dry sand out of a funnel in the sandpit, it will

form a conical pile, and I can predict to within a couple of degrees the angle that the sand will lie at. We call this angle the angle of rest, and that’s the last bit of jargon.

Angle of rest in a jar. Use a cylindrical
and clean, very dry sand.

Ant lions (I will talk about them later) make pits in dry sand, like the one on the left, and the sides also lie at the angle of rest, as does the front of an advancing dune in the Sahara.

So there I was, clutching a Vegemite jar, half full of dry sand. I always carry this in case somebody happened to need a jar of dry sand, and a full jar is too heavy… I bounded onto the rock, declared that I was sitting on a 205 million-year-old fossil, then moved among the kids and showed them the jar, rolling it.

I explained that when sand is pushed along by wind or water, it gets pushed over the front, and this creates (drum roll please!):

Current bedding

Once you know what you are looking for, it’s everywhere, and any kid who has been shown the secret will be able to share it with others. Any cutting, any cliff is likely to reveal beds of sandstone, laid down as sand banks: the picture geologists have of Sydney in the Triassic (~205 million years ago) is a giant sandy river delta, a bit like Bangladesh today.

Three examples of current bedding: (top) Old Man’s Hat, Inner North Head; (centre) Sydney Road Fairlight, Manly side of the shops; and Malabar.

The shots above are easy to spot: the ones around the school need a trained eye. Let the training begin!

(A note to my teachers, less relevant to others: In all of these shots, I have left background in place, so you can come back and look at them.)

 

 

 

 

 

As you can see, the layering is more subtle when it comes to bush rock. The right-hand shot éabove is on the cycle track, and every cyclist going along there is running over 205 million-year-old fossils.

But how do we know the age? We don’t, not really, but the rock is Triassic, making it between 180 million and 220 million, and the sandstone is early to middle Triassic, so 205 million years is near enough for government work. Talk to me if you want more, because explanations require lots of hand waving..

 

On the western side of the school, the sawn stone has nice banding, and this is a Virginia Woolf sort of story, so here’s the Classics Illustrated/Cliff Notes version/. Iron occurs in compounds in two forms that old chemists called ferrous, which is soluble and ferric which is insoluble.

Banded iron in sandstone.

The ferrous form is now called Fe²⁺, and the ferric form is Fe³⁺. The key thing is that one can change into the other, Fe²⁺ seeps away, but when it changes to Fe³⁺, it stops where it is. Chemists say that iron II is oxidised to iron III and iron III is reduced to iron II.

Iron banding and Liesegang patterns

I didn’t find any Liesegang patterning around the school, but this is the same iron II/iron III story.

     

The same explanation applies to the iron banding we can see, and some of that is spectacular:

 

Joints

I was tricked when I thought I had found a joint in the sandstone. Joints are planes of weakness that are seen in most rocks, but they are particular important in the shaping of Sydney, because when sea levels fell, streams and rivers were directed along the jointing patterns, which is why the city has so many east-west and north-south valleys.

When the sea rose at the end of the last Ice Age, it flowed into those valleys, and we say that the fern leaf pattern of the harbour is a drowned river valley. Anyhow, those ‘joints’ were cut with a saw, but they give you some fresh rock to look at. There are no joints that I can see in the school grounds now, but below are some local joints:

  

Above: three examples of joints: (top) Old Man’s Hat, Inner North Head; (centre) Sydney Road Fairlight, north side; and on the way to Fairy Bower.

There appears to be a limit to the number of images I can insert, so this is continued in part 2.