This is an Earthcache, so there is no physical container to find.
As you approach the magnificent lookout, you will be amazed by the expansive views of Twofold Bay with its backdrop of forested hills. Take a moment to enjoy this wonderful panorama before embarking on your geological journey of discovery.
Logging This Earthcache:
To claim a find, you will need to physically visit 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 answers to the questions be emailed to me as soon as you are able to. I read all emails to verify the answers and will send a confirmation message/email back to you. There is no need to send me photos. Just upload any photos with your log.
Logging Tasks:
1. Describe, in your own words, the colour, texture, structure and foliage of the cliff face to the left of the lookout between the radio tower and the navigation light.
2. What type of lava flow can be seen in the cross section of cliff face? (See Earth Science Lesson)
3. What was this type of lava flow made up of? (See Earth Science Lesson)
4. Optional - The best way to prove you were actually at GZ is to include a photo or two of you enjoying this beautiful location. Please do not include any photos that give away answers to any of the questions.
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Earth Science Lesson
The rocks in this area span almost a half a billion years of geological time. HALF A BILLION YEARS!! That is a really hard concept for us mortals to comprehend. Over many millions of years, geological forces, including stretching and compression of the earth’s crust and movements of tectonic plates, have created the landscape you see before you.
Looking to the southwest, you will see Balawan (as it is known by the Yuin People). It is more commonly known as Mt Imlay. It looks like a young, active volcano, but it’s not. It is an erosional form, capped by resistant sandstone, seen as a pimple at the summit.
The huge arc of Twofold Bay's southern and western parts is a sea-carved landscape formed in soft sandstone and mudstone since the sea rose to its present level 6,000 years ago. During the last ice age that ended about 20,000 years ago, a great river flowed through a rocky gorge across what is now Twofold Bay. The drowned gorge is now filled with sand and is clearly visible in sonar maps of the sandy bay floor.
Boyd's Tower, built as a lighthouse in the 1840s but never used for that purpose, sits atop the long, sloping southern headland of Twofold Bay. Both headlands of Twofold Bay are formed of ancient river sediment – pink sandstone and red mudstone – deposited during the Devonian period.
The Devonian period is known as the “Age of Fishes” and spanned a period between approximately 420 million years ago and 360 million years ago. It is named after Devon in England where rocks from this period were first studied. The land was criss-crossed with many great rivers carrying large deposits of sand and mud which later became the pink sandstone and red mudstone that can be seen in the region. These rivers were ideal habitats for many different fish to evolve. Most of the coastline of the Sapphire Coast is made up of red, riverine mudstone. If you visit rock platforms along the coast, you may be able to spot the fossil remains of platy-armoured fish that were trapped and buried by sediments carried by the next flood. So far, 9 species of armoured fishes (Placoderms), 4 species of spiny fish (Acanthodians), 2 species of cartilaginous fishes, the early sharks (Chondrichthyans) and 9 species of bony fishes (Osteichthyans) have been listed in the region. Around Eden, a new species of fossil fish has been discovered and has been appropriately named, “Edenopteron keithcrooki”. The name honours local Eden resident, Dr Keith Crook, of the Australian National University (now retired) for his discovery of several fossil sites in NSW. Check out the following link to view a very interesting program on the subject:
ABC Catalyst - Biggest Devonian Fish
Edenopteron keithcrooki
The Devonian Period is also significant because it is the first time back-boned animals crawled out of the water and on to land. Another exceedingly important feature was that the very first forests evolved during the middle of the Devonian period. The Eden region is a highly significant geo heritage place because of its Devonian fossil fauna and flora. The fossils encompass two of the Earth's most significant evolutionary events: the appearance of the first forests, and the appearance of the first land animals.
The rocks exposed around Twofold Bay run approximately northwest-southeast, so the bay cuts a cross-section through them. The oldest rocks surround the inner parts of the bay, and geologically younger rocks form the north and south headlands. Erosion-resistant volcanic rocks forming the high land at the lookout and its mirror image chip mill headland halfway across the bay have divided the bay into two parts and give the bay its name: Twofold Bay.
To the left of the lookout, you can see an inner headland, (Middle Head) that is made up of erosion-resistant volcanic rock.
The cliff is a cross section through two 380 million year old ignimbrite flows, tilted towards the ocean. Ignimbrites are not liquid lava flows, but super hot clouds of sticky ash particles, so charged with gases that they flowed like water. These flows would have originally been laid down flat, covering the underlying landscape. The almost vertical structures in the cliff are fractures separating hexagonal columns that were formed as the rock shrank slightly during cooling.
I hope you really enjoy your visit to this great location and learning about the unique geology you see before you.
**Special thanks to Dr Anne Felton, Research School of Earth Sciences, The Australian National University (Retired), for her assistance in preparing this earthcache and for her kind permission to use information from her article, “Wilderness Coast – time travel and the evolution of a continent.” Also, a big thank you to Bella At Waggy for his expert input.
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FTF Honours
ThePacman and CT_ZA
An Awesome Team Job!!
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.