Don’t judge a rock by its cover
One thing that will never cease to amaze me is how much rocks tell us about our earth’s past.
When I grew up, I didn’t pay much attention to rocks and I didn’t really think anything of them. This all changed after I took my first geology class some 4 years ago. Almost instantly, I fell in love with it. Not just because rocks are pretty and you get to go hiking in amazing places, but I was in awe at the breadth of information something so unassuming could give us.
Nearly everything we know about our planet comes from rocks – how the earth formed, the evolution of plants and animals (including humans), the evolution of the atmosphere and climate, just to name a few.
They are like a history book for geologists… a pretty complicated, confusing, and not very well written history book, mind you, but an insight into ancient history nonetheless.
Every rock has a story to tell, and it’s a geologist’s job to translate that story. Some stories are simple, and some we feel we’ll never figure out. But that’s one of the best parts of being a geologist – it’s like detective work, you’re trying to solve a mystery that’s millions of years old. Investigating the rocks always involves a large range of techniques from observations in the field and geological mapping, to chemical and geophysical tests, and petrography (aka microscope stuff).
When you bring all these techniques together, the story starts to click into place and a whole world opens up before you.
Looking at an ancient landscape
One of the most amazing things, is that we can fairly accurately reconstruct what the landscape would’ve looked like millions of years ago. This is possible because all the ancient and obscure environments our earth has cycled through are preserved (to some extent) in rocks.
By looking at the different types of rocks, how they are oriented next to each other, their composition and the geometric structures within them, we can determine where mountains, rivers, volcanoes, rainforests, and beaches once were. This is called ‘paleogeography.’
This is usually easier said than done. However, one very good example of paleogeography is the image below.
I saw this outcrop recently when I was demonstrating on a first-year field trip to Phillip Island and San Remo. We visited this outcrop to teach the students about paleogeography and more specifically, the key parts of identifying a terrestrial river system.
The image shows a cross-section of an ancient, 110 million-year-old braided river system. These rivers were flowing over the super-continent Gondwana which broke up around 80 million years ago. When this supercontinent existed, Australia was still connected to Antarctica. We have since rifted apart, and the southern coast of Victoria now represents the fracture line along which Antarctica used to be connected to.
At a young age of 110 million-year-old (that’s not sarcasm), these rocks are still in good condition. They are a type of rock called a ‘sedimentary rock’ which forms when sediments, such as sand, clay, and pebbles, are deposited and solidified.
How do we know it was a river?
There are two ways we can tell this is a braided river system: 1 – the curved, u-shaped layers, and 2 – the presence of pebbles.
These curving layers shown in yellow in the image below represent where rivers have eroded down into the ground, and cut out a section of rock – this is what we see happening in modern rivers.
As the river continues flowing, it brings in new sediment, such as pebbles and sand, from upstream and drops it along the base of the channel. Gradually, the channel starts to fill up with this sediment until the river is forced to change direction. This infilling of sediment is what creates the curved, u-shaped or, as we call it, ‘lensoidal’ sandstone layers.
The second piece of evidence that this is a river system is pebbles. Pebbles form when a river picks up a piece of rock, carries it and weathers it down into a smooth, rounded shape. The more rounded a pebble is, the longer distance it’s travelled. From this we can start to estimate how long a river is and therefore, how far away the water source or mountain range might be.
Secondly, the composition of the pebbles tells us about what rock types surrounded the river. For the image above there were basalt pebbles, granite pebbles, and mudstone pebbles. This means we had these rocks underlying the river, and since different rock types form different landscape, we can start to reconstruct what the land looked like – for example, the basalt indicates a volcano is nearby.
The fact that we see so many cross-cutting, u-shaped layers indicates that this is a ‘braided river system’ which basically means the river is constantly changing direction as it follows the path of least resistance.
This outcrop makes it very easy to imagine that water was once flowing through these channels 110 million-years-ago.
Rocks are simply extraordinary.