Arthur's Head EarthCache
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This Earthcache is about the creamy white limestone cliffs which are present here and which are a conspicuous feature of much of the South-west's coastline and the islands near to Perth.
Please log this cache then send answers to the cache owner. We will delete any logs that do not meet requirements. Do not include answers in your log. We would love photographs of your visit, but these are not compulsory.
Limestones are a group of related rocks all composed of CaCO3 and reacting with dilute HCl to form bubbles of gas. Limestone is also chemically related to dolomite (CaMg(CO3)2). Because all these rocks have the carbonate ion in common, they are called the carbonates. Most carbonates are derived from a combination of biological and chemical components.
Two carbonate classification systems in common use today are
R J Dunham(1962) (visit link) and
RL Folk (1952-62) (visit link)
as well as a couple of other variants of these two.
All classifications are based on the distinction of :
GRAINS (skeletal fragments or fossils e.g. of corals or shells; ooids i.e. small pearl-like spheres: pellets i.e. faecal pellets produced by invertebrate animals. They look like ooids but are dull and not pearl-like; intraclasts i.e. chunks of eroded limestone deposited into new limestone as a conglomerate; and other non-carbonate detritus.)
MATRIX (lime mud, carbonate mud or micrite which is greyish or brownish particles less than 5 microns or sparry calcite which is clear coarsely crystalline calcite )
PORES (These may be open spaces within the limestone, or filled with a different looking substance. )
The original Dunham classification is based on depositional texture and can easily be applied in the field without major equipment (try using a magnifying glass). Folk's classification may be more detailed but generally, with exceptions, requires a lab with thin sections or peels.
Dunham originally included five textural classes:
Mudstone: lime mud supported and contains less than 10% grains
Wackestone: the sediment is mud supported and contains more than 10% grains
Packstone:the carbonate is grain supported i.e. mostly grains but contains a small percentage of mud
Grainstone the grains are touching one another and contain no mud
Boundstone.:the original components have been bound together after deposition. Can be bound with or without mud
MORE INFORMATION
This limestone contains a high proportion of lime sand derived from seashells and minute marine organisms. It is known as the Tamala Limestone. The Tamala is an aeolian calcarenite. “Aeolian” refers to the wind-blown origin of the sediment. “Calcarenite” is a type of limestone that is composed predominantly, more than 50%, of detrital (transported) sand-size (0.0625 to 2mm in diameter) carbonate grains,mostly shell fragments or calcareous algae. It was formed as sand dunes and beach deposits during the last one million years. This was during the Pleistocene when the sea level changed dramatically in response to the expansion and contraction of the polar icecaps. The lime sand was slowly dissolved by rainwater, then deposited again when the water evaporated, cementing the sand grains together and turning the dunes into limestone.
In many places here you can see obvious cross bedding, which is evidence of its origin as wind-blown sand dunes. The thin strata are inclined at various oblique angles, often set abruptly against other strata, reflecting the original pattern of sand accumulation and changes in wind direction.
Other notable features are rhizoliths, or fossilised roots of the plants and trees which once grew on the dunes. They form when the roots decay and the space left is filled with lime sand and cement from the dissolved sand. When the surrounding limestone is weathered, their shape is preserved.
To log this Earth cache you will need to answer the following questions. You will find the answers to some of the questions on the many informative signs close by, also from the text in the Earth cache notes,but you might need to do a little research for others.
1.Enter the tunnel from the Western end. On your right is an area where the old lining was removed from the wall in 2000, so we can see the original limestone of the cliff. Look closely at it. Using a magnifying glass and torch would be ideal. Can you see any “grains” as described above? If so how big are they? (sand-size, shell-size, or pebble-size) Are there pores?
2.If there are “grains, are they more or less than 10% of the limestone?
3.Therefore, using the brief description of Dunham's classification of carbonates above, say which of the five textural classes you think this limestone is.
4.Can you see cross bedding? Are the layers horizontal, vertical, oblique or a mixture of these?
5.During the early years of the Swan River Colony, this limestone was quarried to supply the colony's early building needs and can be seen in many of the old buildings near by, including the Roundhouse which was built in 1830 and opened in 1831. It was the first permanent building in the Swan River Colony and is the oldest building still standing in Western Australia. When the tunnel was dug out, much of the limestone was used to build something that was used for the same industry as the tunnel. What was this?
6.Exit the tunnel at the eastern end. Turn right and look up. Do you see masses of tubular structures? What are these?
7.Outside the tunnel at the western end and a bit to the north is a sign with the heading THE OLD WATERFRONT, how high were the original cliffs at this point?
REFERENCES: Iain Copp,2001 Geology & Landforms of the South-West (visit link) (visit link) (visit link) (visit link) (visit link) (visit link)
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