At White Cliffs opal is found in Lower Cretaceous age claystone belonging to the Rolling Downs Group. The claystone is of marine origin and opalised shells and fossils of fish and marine reptiles such as ichthyosaurs and plesiosaurs have been found by opal miners. The opal is not concentrated in any one horizon, but occurs spasmodically throughout the Lower Cretaceous claystone. It occurs as thin horizontal and vertical veins, as irregular nodules, as wood opal, replacing shells and as crusts on boulders.
White Cliffs is the only place where "pineapples" occur; they are a pseudomorph of Glauberite or Ikaite crystal clusters, where crystals are substituted by precious opal. This results in an object where the appearance and dimensions look like crystals, but the original mineral has been replaced by opal. They are extremely rare and valuable.
Opal is formed from a solution of silicon dioxide and water. As water runs down through the earth, it picks up silica from sandstone and carries this silica-rich solution into cracks and voids or decomposing fossils. As the water evaporates, it leaves behind a silica deposit. This cycle repeats over very long periods of time and eventually opal is formed.
White Cliffs was Australia's first commercial opal field with intensive opal mining beginning in 1889. Most work in the field was done in the first 20 years. The opal from White Cliffs is known as "white opal" in contrast to the "black opal" of Lightning Ridge.
Some of the best locations were: The Blocks Area where there was no hard silcrete cover to dig through to get to the opal bearing zone and quality opal was found down to a level of 24m; Sullivans Hill to the east of The Blocks and on the margin of outcrops of the Cretaceous, most of the activity took place near the scarp where silcrete and ironstone gravels were not prominent and levels were worked down to 12m; Other areas were Turleys Hill, Smiths Hill, Moffatts Hill and many others.
Extract from http://www.geomaps.com.au/scripts/whitecliffsopal.php
Discovery of Opal Mining Techniques
Potch or Precious The Tribute System
Taking the Visitors Trail you will see the many aspects of White Cliffs will bring to a Look Out and GZ.
Question Time:
Please email us the answers to the following with the cache name. We will contact you if more information is required. You can log this cache prior to sending us the answers, but your log may be deleted if the answers are not received within 10 days.
1. How long ago was the Opal Bearing Sediments laid down on an ancient seabed at White Cliffs?
2. What two (2) prehistoric animals are mentioned and what are they? Also what was discovered at Sullivans hill in 1976
3. Explain the difference between common potch and precious opal.
4. What is the main difference between Lighting Ridge Opal and White Cliffs Opal? Also what sort of Opal is White Cliffs famous for?
5. Standing at GZ and facing NNE, describe the object you can see! (This is to stop armchair loggers logging the Earthcache.)
Optional: Post a picture of you in White Cliffs.