Zigzag Quarry
This is an Earthcache, so there is no physical container to find.
Logging This Earthcache:
To claim a find, you will need to physically visit the quarry at GZ, read the information contained in the Earth Science Lesson below then complete the following logging tasks and send me your responses. Include the earthcache title in your correspondence. You can log your find immediately, however, I ask that your responses to the logging tasks be sent to me as soon as you are able to. I read all messages/emails to verify the responses and will send a confirmation message/email back to you. If your correspondence is not received within a timely manner of your online log, then your log may be deleted. There is no need to send me photos. Just upload any photos with your log.
Logging Tasks:
1. Standing at GZ, describe the overall shape of the quarry and the appearance of the rock face.
2. To the right of the quarry, as you walk in, near where there is some black graffiti on the wall, you can get close to the rock face. How would you describe the texture and grain size of the metamorphised sandstone here?
3. In your own words, describe how metamorphic rocks are formed.
4. What makes quartzite a hard, tough durable rock?
Optional - Take a photo of you and/or your geocaching name and/or your GPS device with the Historical Engineering Marker located just off the path near the way in to the quarry. Brownie Points if you have all four items in the one photo lol.
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Thornleigh's Quarry and Zigzag Railway
Tucked away in the leafy suburb of Thornleigh lies the remnants of a quarry and a zigzag railway, one of three that has operated in NSW over the years.
The Lithgow Zigzag in the Blue Mountains is probably the most well known of the three zigzag railway lines built in the Sydney region. The third one was at Lapstone.
A zigzag railway (called a switchback in the UK), is a method of climbing steep gradients with minimal need for tunnels and heavy earthworks. For a short distance in the middle section of the zigzag, trains will travel backwards before resuming the original direction of travel.
The Thornleigh Zigzag was constructed in 1883 by railway contractors, Amos & Co, as a branch line coming off the main Northern Railway Line just north of what is now Thornleigh Station and descending to a quarry some 35 metres below. The quarry supplied "white metal" to be used as ballast to support railway sleepers for a section of the Homebush to Waratah railway line. The zigzag facilitated the steep ascent from the quarry to the main rail line for the railway trucks carrying the stone. Unfortunately, over time, the white metal proved unsuitable for the job as it tended to deteriorated more quickly with heavy trains passing over the tracks. The quarry stopped production and the zigzag branch line closed just prior to 1900.
While much of the route of the zigzag railway has today been taken over by residential development, you can still see evidence of the line, including a cutting near Thornleigh station and embankments in the bush near Tillock Street, as well as follow the descent from the top point across Pritchard and Wells Streets to the bottom point near Janet Avenue.
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Earth Science Lesson
The rock that was quarried from this site was metamorphised sandstone. Let's start at the beginning.
What are Metamorphic Rocks?
Metamorphic rocks started out as some other type of rock, but have been substantially changed from their original igneous, sedimentary, or earlier metamorphic form. Metamorphic rocks form when rocks are subjected to high heat, high pressure, hot mineral-rich fluids or, more commonly, some combination of these factors. Conditions like these are found deep within the Earth or where tectonic plates meet. The word "metamorphic" comes from the Greek and means "to change form".
Process of Metamorphism:
The process of metamorphism does not melt the rocks, but instead transforms them into denser, more compact rocks. New minerals are created either by rearrangement of mineral components or by reactions with fluids that enter the rocks. Pressure or temperature can even change previously metamorphosed rocks into new types. Metamorphic rocks are often squished, smeared out, and folded. Despite these uncomfortable conditions, metamorphic rocks do not get hot enough to melt, or they would become igneous rocks!
Common Metamorphic Rocks:
Common metamorphic rocks include phyllite, schist, gneiss, quartzite and marble.
Foliated Metamorphic Rocks:
Some kinds of metamorphic rocks -- granite gneiss and biotite schist are two examples -- are strongly banded or foliated. (Foliated means the parallel arrangement of certain mineral grains that gives the rock a striped appearance.) Foliation forms when pressure squeezes the flat or elongate minerals within a rock so they become aligned. These rocks develop a platy or sheet-like structure that reflects the direction that pressure was applied.
Non-Foliated Metamorphic Rocks:
Non-foliated metamorphic rocks do not have a platy or sheet-like structure. There are several ways that non-foliated rocks can be produced. Some rocks, such as limestone are made of minerals that are not flat or elongate. No matter how much pressure you apply, the grains will not align! Another type of metamorphism, contact metamorphism, occurs when hot igneous rock intrudes into some pre-existing rock. The pre-existing rock is essentially baked by the heat, changing the mineral structure of the rock without addition of pressure.
Quartzite:
Quartzite is a nonfoliated metamorphic rock composed almost entirely of quartz. It forms when a quartz-rich sandstone is altered by the heat, pressure and chemical activity of metamorphism. These conditions recrystallize the sand grains and the silica cement that binds them together. The result is a network of interlocking quartz grains of incredible strength.
The interlocking crystalline structure of quartzite makes it a hard, tough, durable rock. It is so tough that it breaks through the quartz grains rather than breaking along the boundaries between them. This is a characteristic that separates true quartzite from sandstone.
Quartzite is usually white to gray in color. Some rock units that are stained by iron can be pink, red, or purple. Other impurities can cause quartzite to be yellow, orange, brown, green, or blue.
The quartz content of quartzite gives it a hardness of about seven on the Mohs Hardness Scale. Its extreme toughness made it a favorite rock for use as an impact tool by early people. Its conchoidal fracture allowed it to be shaped into large cutting tools such as axe heads and scrapers. Its coarse texture made it less suitable for producing tools with fine edges such as knife blades and spear points.
Quartzite can have a grainy, glassy, sandpaper-like surface.
Pink Quartzite
Click on the following link to see a YouTube video on Metamorphic Rocks:
Metamorphic Rocks
I hope you really enjoy your visit to this great location and learning about the geology you see before you.
References: The Institution Of Engineers Australia; Hornsby Shire Council; Department Of Environment And Conservation (NSW) 2006; zigzagrailway.com.au; historyservices.com.au - Blog; geology.com - Quartzite; Wikipedia - Quartzite; Pocket Oz Travel and Information Guide Sydney - Lost Railways Thornleigh Zigzag Railway
FTF Honours
Gatherer Hade
Earthy Earthcacher Extraordinaire
Did you know that NSW has a geocaching association? Geocaching NSW aims to enhance and improve the activity of geocaching and holds regular events where geocachers meet to enjoy their common interests. Visit the association website here.