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Redhead: Into the Permian Age

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Hidden : 02/07/2020
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Geocache Description:

This is a spectacular walk along one of the best coastlines in the region, with some great sandstone cliffs, coal seams, fossils, volcanic intrusions and a picture perfect rock pool! The EC is pretty geology heavy, however hopefully will provide lots of information to help you think a little further into the ‘how’ not just what is in front of you!

Welcome to the Redhead coastline

The walk should be easily completed in half a day, best in the morning on a low to moderate swell. Take some food, the walk is family friendly, however children may need some help in some places. From the parking location, head down the obvious track for 5-10mins to a picnic table. Continue on the trail that heads down the hill. It will shortly come to an exposed and steep track down a break in the cliff line. This leads to the first platform and waypoint.

WP1 Into the Permian Age

Having walked down the cliff and onto the beach, you will have gotten up close to the rock. All of the geology in this EC is from the Permian age, sedimentary rock that formed here 250 million years ago. For sedimentary rocks, there are three important processes:

1. Deposition; sediments are laid down and accumulate on top of previously deposited material. (more about this later)
2. Lithification; When there is enough sediment that has built up that its own weight compresses the material underneath turning it into solid rock.
3. Uplift and erosion; Deeper tectonic forces push up land exposing material that was once underground. Erosion from the ocean and wind (here) sculpts the rock.

Observe the cliff and beach, describe some current erosive forces that you see on on your visit to Redhead

WP2 Rock pools and basalt dyke

At this point you will find a beautiful pool to swim in. It is quite safe from moderate swell, but you may notice a thin line of dark rock extending back toward the cliff. This is a small basalt dyke, an igneous (volcanic origin) intrusion (meaning it has forced its way to the surface after the surrounding sandstone was deposited). Basalt it a heavy, dark mafic rock meaning it has a low quartz content (less than 50%) and high amount of metals including iron and magnesium. There are basalt dykes all over the Sydney-Newcastle region many small but some are very large. One of my favourites actually doesn’t exist anymore; having long since eroded, essentially pounded by the ocean and ‘rusting away’ as the salt from the ocean reacts with the iron leaving a 40m deep chasm!

Using the description, observation and your knowledge, what are some of the differences in terms of minerals between the basalt dyke and the surrounding sandstone?

As you head to the next waypoint measure (roughly) the length of the dyke. Also consider why this dyke is relatively narrow in comparison with others in the area.

WP3 Mystery fossils

Here are the first of many fossils you will spot along this walk, although interestingly this one is particularly unique. Firstly find this rock:

Near the pointed end of the rock you will find:

I am a little uncertain as to the species of this fossil however knowing what time period it is from helps narrow this down considerably. From this period, terrestrial vertebrates (land dwelling creatures with a spine) are quite rare, leaving most fossils to marine, hard shelled invertebrates or corals.

There are also a handful of terrestrial plant species common in this time period, usually associated with coal deposits (which you will see clearly a little later). Looking at the rest of the large rock there are other fossils to be found, some which may be a little easier to recognise on your return journey once you have seen a few more.

What time period are these fossils most likely from?

Do you think the fossils in question are terrestrial plants or invertebrate sea creatures, remember to think about their surroundings and the other fossils in the rock, what do the others look like, can that help you understand where the mystery fossils might come from?


WP4 Iron-rock formations

(If there is a large swell, you can still answer this from a safe distance)

You will notice some odd formations on the rock platform:

These sections of raised sandstone are still here due to a much harder, iron rich sandstone (the brown part) on top, somewhat sheltering the softer rock underneath from erosion. However, look at the cracks around on the platform. These are the initial way that the pounding waves begin to enter what would have once been a (reasonably) flat platform. These points considered; the hard capping is also eroding too.

What element/mineral differentiates this rock from the softer sandstone underneath?

Apart from the obvious pounding of the waves, what is another form of erosion happening here (think back to WP2? What happens to the basalt dykes because of the iron component?)

WP5 Tessellated pavement.

This formation is quite unique. Although there are many tessellated pavements in the Sydney/Central Coast area, these are a little different from a few aspects. Firstly the rock here is older than the Triassic Sydney (Hawkesbury) sandstone, meaning in this case a different chemical composition, much less iron and bright orange/red clays that are usually associated with its warm appearance. Also, Hawkesbury tessellated pavements are usually almost square/rectangular and a similar size, and importantly worn flat from the waves. So the multitude of raised, bizarre shapes is somewhat of a mystery here. However there are some things we can still understand about these remarkable features:

Firstly, the bizarre shapes are actually quite simple, imagine it not as individual shapes, however a flat surface with cracks. Like with the square formations, there are really two sets of parallel cracks, ones travelling (from your perspective) horizontally, and the others vertically. Usually the two sets of lines are perpendicular, meaning at 90° with each other. But here, imagine that you have turned the vertical set slightly and create a wider, obtuse angle.

Approximately, what is the obtuse angle formed here?

Usually these cracks form in the lithification (from when this was sand then turned to rock from heat and pressure) process, why they are on this angle? Probably from uneven pressure applied in the lithification, however, there are some other more complex geological concepts that I am happy to discuss further in response to your EC answers!


WP6 Coal Seam

Stay up high on the ledge and begin to walk around to the point. On the way you will notice some black, somewhat shiny, brittle rock on the ground, pick some up hand walk toward the coordinates. What you have is some coal! In front of you is a thick black layer of ‘rock’ which is the coal seam. This seam further north in Glenrock/Dudley was the very foundations of Newcastle’s existence as a coal mining and export town, but back in the Permian time, Newcastle would have looked quite different. Coal is formed in warm swampy environments where plants fall over into mud where they are protected from oxidation (the process that causes organics to break down). Here this would have accumulated before covered by sediment, then compressed by increasing material put on top, heat and pressure from moving rocks etc.

Describe the coal, consider the difference of this carbon ‘rock’ with a silica (sand) based rock, ie comparative weight, strength, texture.

Now drop down off the ledge just near the point and follow the lower rock platform around,

What do you notice about the size of the coal seam? How thick (in meters) is it and does this change (thinner, thicker or the same). Is this what you expected remembering coal is sedimentary.

WP7 Opera House Rocks

Continue along the shelf before an easy scramble up onto the next ledge with these impressive rocks:

This is a large boulder broken into three, that somehow stopped just short of the plummet over the edge. If you have the dare, there is a great rock jump just here into a deep sandy spot. Also some beautiful snorkelling on a calm, clear day. What makes this such a nice snorkelling location are all the large boulders that have fallen in; some from where the Opera House Rocks came from.

Under the sea, the movement of waves back and forth is the primary form of erosion. On land however there is another prominent form, what else is it that also helped erode the Opera House Rocks?

WP8 Exercise Rock

Well a break from the Geology lesson, here is to put it simply ‘’a cool rock’’ and a great photo opportunity! Bonus points for creativity!

On the walk to the next waypoint find something interesting, take a photo of it and post it with your log, maybe describe it and feel free to do some research/message me about it!

WP9 Petrified trees

Scramble through the boulder jam to the coordinates that are two pieces of petrified wood, one of which looks like this:

In a similar process to the formation of coal, trees fall into a mud or waterlogged sediment where they are protected from oxidation/decay. Although the difference lies in a much fine balance of conditions where a mineral rich fluid saturated the timber, filling every cell, leaving the cell wall to decay later. Depending on the timing, of the filling/decay, sometimes a perfect match (as you will later see) can happen. However, this piece has been exposed to the weather for quite some time but across the way, you will spot a very large, newly exposed tree that is (currently) in near perfect condition:

Climbing up close can be quite difficult, so I have done this for you (not needed for the answers), but its astonishing how well it has been preserved, considering that its millions of years old.

To the north there is an opening with a number of petrified trees overhead, now behind you low on the ground are two samples that have broken off from somewhere, the smoother one possibly is very close to where it came from with an exposed tree just behind it.

How large is the biggest of the fossils here? Describe its condition of its preservation in comparison with the other ones, why is this so? (I have hinted at the answer but you can elaborate).

Think about your answer in relation to other fossils locally and globally, it makes sense why there are so few fossils to study in comparison to how many organisms have existed!

WP10 Conglomerate Outlook

On the way to this waypoint, keep an eye out for more preserved trees, there are a few in the roof of the caves. Walk through the opening at WP9 then scramble onto the higher ledge when possible, staying high on the grassy footpad past the caves. Please observe the overhang at WP10 on approach and keep well back from the edge as this rock can be quite unstable!

Conglomerate rock, which is very distinctive around Newcastle is formed from two components: clasts, the ‘pebbles’ and the matrix (no not with Keanu Reeves) like the cement sand that binds every thing together. When there is less cement (matrix) than pebbles (clasts) together, this is called ‘clast supported’ generally quite weak as there is not enough matrix to hold it together! Further south at Caves beach to Frazer Park, there is very little matrix meaning weak and easily erodible rock, hence the amount of really big caves along the Frazer Park/Catherine Hill Bay coast. Along this coast, there is a protective wave barrier of stronger, fine grained sandstone platform before the weaker cliff. However from WP9 onward, large conglomerate boulders dominate the coastline.

Looking closely at the conglomerate, its possible to get an idea of what conditions were like when the rock was formed long before the boulders had fallen off the cliff behind. Before the cliff was solid rock, it was layers of deposited sediment. Every clast and grain of sand had to have been moved from somewhere to get to here, and geologist are able to speculate how far this may have been by looking at the roundness of the clasts. Firstly, the shape of the clasts here in Redhead are referred to as subangular, meaning that they aren’t sharp, nor are they spheres, but somewhere in between. This tells how much movement each clast has had to get to this point as the diagram explains:

Transportation in this case could have been a river moving the pebble along, rounding it as it travels along the riverbed. Also waves at the beach move pebbles back and forth rounding them too. Colour, for the purpose of this EC differentiate different rock types/origins. Clasts can come from different ‘parent (original) rocks’ then transported together and end up at the same destination; here.

Is the rock mostly matrix or clast supported?
Have the clasts travelled a short, medium or long distance (refer to diagram)

(Roughly) how many types of rocks are there in here?

If you are feeling keen, there is so much more to talk about with conglomerate rocks and how they can tell us about what has happened here, just ask me when you are emailing your EC answers!

WP11 Sculpture Park Challenge

Heading back to the first WP, you will probably notice many more fossilised trees along the way. On approach, you will find a rather unusual looking rock on the beach:

For your final task, describe how this rock got here. You can make this answer as complex or simple as you feel comfortable with, perhaps with some basic erosion principles that are current or maybe some of the earlier processes (like what was discussed at WP10).

*You may log this when you have found it, however please email me your answers shortly after (within a couple of weeks) or the log will be removed. All answers can be found on your walk with the help of description, however please email me if you are unsure about how to approach the questions and I will help you to understand!*

Additional Hints (No hints available.)

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