The Grotto (Victoria) EarthCache
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The Grotto is a geological formation known as a sink hole which can
be found on the Great Ocean Road just outside of Port Campbell in
Victoria, Australia. It is a popular tourist attraction. Wooden
steps wind down the cliff face to the bottom where the sea is
visible beyond a quiet pool at low tide.
A sinkhole, also known as a sink, shake hole, swallow hole,
swallet, doline or cenote, is a natural depression or hole in the
surface topography caused by the removal of soil or bedrock, often
both, by water. Sinkholes may vary in size from less than a meter
to several hundred meters both in diameter and depth, and vary in
form from soil-lined bowls to bedrock-edged chasms. They may be
formed gradually or suddenly, and are found worldwide. These terms
are often used interchangeably though many will distinguish between
those features into which a surface stream flows and those which
have no such input. Only the former would be described as sinks,
swallow holes or swallets
Mechanisms of formation may include the gradual removal of slightly
soluble bedrock (such as limestone) by percolating water, the
collapse of a cave roof, or a lowering of the water table.
Occasionally a sinkhole may exhibit a visible opening into a cave
below. In the case of exceptionally large sinkholes, such as Cedar
Sink at Mammoth Cave National Park, USA, a stream or river may be
visible across its bottom flowing from one side to the other.
Sinkholes are common where the rock below the land surface is
limestone, carbonate rock, salt beds, or rocks that can naturally
be dissolved by circulating ground water. As the rock dissolves,
spaces and caverns develop underground. These sinkholes can be
dramatic because the surface land usually stays intact until there
is not enough support. Then a sudden collapse of the land surface
can occur. A special type of sinkhole - formed by rainwater leaking
through the pavement and carrying dirt into a ruptured sewer
pipe.Sinkholes can be human-induced. New sinkholes have been
correlated to land-use practices, especially from ground-water
pumping, construction, and development practices. Sinkholes can
also form when natural water-drainage patterns are changed and new
water-diversion systems are developed. Some sinkholes form when the
land surface is changed, such as when industrial and runoff-storage
ponds are created. The substantial weight of the new material can
trigger an underground collapse of supporting material, thus
causing a sinkhole
The soft limestone rock of these cliffs was formed between 10
and 25 million years ago,when the area was under the sea. The sea
retreated and deep, vertical joints formed in the rock. Rainwater
and spray percolated down the joints, dissolving the rock and
widening the cracks. The sea advanced again at the end of the last
Ice Age, reaching its present level about 6000 years ago. Since
then waves have been attacking and undermining the rock, producing
the cliffs that are present today. Harder rock remains as the
headland. Waves continue to erode the softer underlying rock,
causing caves and arches. This is how The Grotto was
formed.
To log this EarthCache, you must do the following and email us the
results. You can go ahead and log your find, (ie. you don't
need to wait for permission) but if we don’t think you have
honestly completed the required tasks we will delete your log
(after emailing you first of course):
Please note: The photo is most important to prove your
visit
1)Find the Info board near GZ and tell me the age of the
limestone. Email me the answer
2)Measure or estimate the width of the grotto. Email me the
answer
3)Take a photo of yourself with gps at the Grotto,please display
your gps and have the "grotto" in the background. Post it with your
log.
Please email me this info, please DO NOT post it with your
log.
Just post your photo.
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