HISTORY
Approximately 450 million years ago, most of the east coast of Australia including the Yass, NSW area lay beneath a deep ocean. About 440 million years ago tectonic activity caused the east coast of Australia move eastward. This caused the Yass area as we know it today then became a warn, shallow tropical sea that was teaming with life.
During that time, there were periods of violent volcanic activity, interspersed with quieter periods when silt and sand were deposited to form sandstone layers. Coral reefs formed in the shallow waters. These became siltstone and sandstone beds, interspersed with some fossiliferous (containing many fossils).
The Volcanic activity resulted in the formation of volcanic rock such as dacite, rhyolite and tuff. Subsequent tectonic activity resulted in the beds being folded and faulted which is now observed as exposed dipping beds. These different formations can be observed in and around the Yass area.
EARTHCACHE LOCATION
At this Earthcache you can observe spectacular Fossiliferous Hume Limestone that has form layered cliffs that can be seen from Hatton’s Corner on the Yass River, NSW. The Limestone is particularly rich in fossils, including tabulate and rugose corals. The site is a perfect example for Late Silurian sedimentary sequences. Along the river near the Limestone Cliffs there are exposed volcanic outcrops from an ancient volcanic lave flows.
PHOTO: Fossiliferous Hume Limestone Cliffs at Yass, NSW (Photo by Sui_001)
GEOLOGY LESSON
Limestone is a sedimentary rock composed primarily of (at least 50%) Calcium Carbonate (CaCO3) that usually forms in clear, calm, warm, shallow marine waters. However, limestone can also be formed by limewater evaporation during cave formation. The following are all varieties of Limestone: Travertine, Tufa, Caliche, Chalk, Sparite, and Micrite are all varieties of limestone.
Limestone is usually a biological sedimentary rock, forming from the accumulation of sediment, shell, coral, algal, faecal matter, and other organic debris. As layers of material and mud built up, the lower layers slowly hardened under pressure into limestone.
PHOTO: Example photo of Fossils that can be found at the Hume Limestone Cliffs at Yass, NSW
Limestone may contain considerable amounts of magnesium carbonate (dolomite) as well. However, minor constituents of clay, iron carbonate, feldspar, pyrite, and quartz are also commonly present. The size of Limestone grains is variable and can consist of clasts of all sizes, However the average grains in Limestone range in size from 0.001 mm 0.001016 mm.
Limestone is available in virtually any colour based on the elements combined with the calcium carbonate Because limestone is often formed from shells and bones, it is a light in colour including white, tan or gray. Limestone can even sometime be blue, beige, and cream in colour.
The formation Limestone typically happens within less than a million years of deposition. Some cementing occurs while the sediments are still under water, forming hardgrounds.
Because limestone contains the remains of dead organisms, it is considered an organic sedimentary rock. There are rare chemical sedimentary rocks that form from the precipitation of calcium carbonate from ocean water. Lithographic limestone is a type of limestone that contains fossils.
QUESTIONS
Question One: Why is Limestone usually classed as a Biological Sedimentary Rock and How is Limestone formed?
Question Two: What is “Fossiliferous Limestone”?
Question Three: The Limestone at this Earthcache location is particularly rich in fossils, What are two types of Fossils that can be found?
Question Four: Explain the process of how the Limestone Outcrop at this Earthcache become visible?
Additional Task: You must include a Photo of either yourself or something with your Geocaching Name in front of the Limestone Cliffs with your “Found It” log. Bonus points for anyone that photographs themself next to a Fossil also.
Email or Message your answers to the questions above. You do not need to wait for a response to Log your Find. I will contact you if I require any more information. Any “Found It” log without answering the questions and the required photo will be deleted.