Werribee Gorge

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Werribee Gorge is the perfect place to spend a hot summers day swimming at Miekle’s Point Picnic Area, to rock climb (Falcon’s Lookout), hike through some of Melbourne’s most ancient rocks or… go fold hunting! The geology at Werribee records almost 500 million years (Ma) of Victoria’s geological history.

The Ordovician turbidites shown in the figure below were deposited in a deep marine setting off the eastern margin of Victoria – most of which was under water at this time. Almost 100 Ma later the interbedded shales, slates, sandstones and mudstones were folded and uplifted as a result of east-west compression (like if you push two ends of a carpet together).

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Click for larger image.

Walking along the red track in the yellow area above you should get your geology goggles on and keep a look out for folds – not just the big ones either! Folds, like most things in nature, are fractal, so a fold as big as a house probably contains a fold smaller than your finger-nail. See the images below!

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Photo: Can you see the fold here? You can see the cracks, and the vein.. there’s some nice bushes… How about in the middle of the image? No?

From a distance it looks like a boring wall of rocks, but up close… a fold! This is what makes the job of a geologist so hard – you have to get up close and personal with a rock to really understand it. Sometimes you have to cut the rock up and look at it under a microscope to see what’s going on.

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Photo: There they are. The folds!

Also keep a look out for graptolites, extinct marine creatures that formed net-like colonies which floated freely on the sea during  the Cambrian and Ordovician. These fossils are what help geologists to date the rocks – they ONLY occur during this time period, and specific types occur in even shorter time periods.

The large granite intrusion to the north-west of the park (350-320 Ma) cannot be reached via any of the tracks, but it is important in the geological history of the area (pink in the figure above). Granites intrude bedrock (the turbidites) much like chewing-gum intrudes the bottom of your shoe when you step on it. It takes advantages of gaps, it’s stickier and runnier on a hot day and the more pressure you put on it the more it will spread! This is why granites are usually syn- (same time) or post- big periods of deformation.

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Photo: Permian sandstone with tillite deposits. You can see the powdered rock in between the orange sandstone. It contains different sized, irregular stones of granite, quartz, shale and chert all transported there by a glacier or ice sheet. Look for scrapes on the rocks which formed by sliding over other rocks whilst stuck in the ice.

Rocks overlying the folded turbidites are Permian tillites, or a big pile of different sized rocks collected, transported and dumped by the movement of ice sheets. If you have a look at the geological time scale at the top, you’ll notice that there is almost a 100 Ma period of time missing! This is called an unconformity. You’ll see the tillites towards the end of the walk on both sides of the Werribee River (green in the image above)

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Photo: The volcanics on top of this hill are younger than the Permian tillites below, which is younger than the Ordovician turbidites on the path where I was standing. This is a good example of the law of superposition and is another way geologists can date rocks! The sandstone had to be there for the turbidites to be deposited on, which also had to be there for the basalt to flow on to.

The other significant rocks in the area belong to the Newer Volcanics – lava flows which erupted a few million years ago from nearby volcanoes such as Mt Blackwood. The lava flows would have filled the valleys and blanketed the land. There are various signs along the walk which point out the places to see the Newer Volcanics (purple in the figure above).

Links:

State Park Notes
Rock climbing at Falcon’s Look Out

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