The Acton Peninsula Limestone Outcrops are a significant group which extends 80-100m along the eastern shore of Acton Peninsula. The age of the limestone is Late Silurian (approximately 425 million years) and contains sparsely distributed fragmentary macro fossils of corals and shells. Very little limestone is now exposed in the area which makes this outcrop a special place to visit. The outcrop found here, and similar ones found along the Queanbeyan and Molonglo rivers, prompted British explorers to name the locality 'the limestone plains'.
Canberra's limestone was laid down 420 million years ago during the Silurian period when the region resembled a tropical, volcanic and shallow sea environment. These limestone formations became exposed when the ocean floor was raised by major volcanic activity in the Devonian period, which formed much of the east coast of Australia. There are interesting records of Canberra's limestone caves and it's underlying karst landscape. A karst refers to a landscape underlain by limestone, which has been eroded producing caves, underground streams, fissures and sink holes. Limestone is dissolved by acidic rain and groundwater, creating caves as well as many smaller limestone cavities.
The outcrop provides evidence of the underlying limestone - you are standing on a fossil coral reef! Early Canberra settlers' records details of the only mapped and formerly accessible cave, located nearby, below the Acton foreshore of Lake Burley Griffin. The Lennox House Cave, is around eight metres deep and was once used as a rubbish tip. The cave was submerged when the lake was created by damming the Molonglo River in 1963.
To earn credit for this earthcache, send answers to questions 1&2 to me via e-mail or messenger to the contact details on my geocaching profile. The photograph needs to be attached to your log. You can log the cache immediately but need to follow up with answers within ten days or so. Your Found log may be deleted if you do not send me the answers or attach a photo to your log. I am happy to accept group answers as long as your message clearly sets out individual caching/team names.
Please email me information about the limestone outcrop you can see looking west at GZ:
1. Describe the colours you can see in this outcrop of limestone.
2. Estimate the height of the rock outcrop in front of GZ.
Photo Requirement. Take a photograph at the location and include yourself and/or your caching name on a piece of paper/card and include it with your Found log. Please make sure your photo does not provide answers for the above questions. Note: as per newly published earthcache guidelines, the photo is required to claim the find.