London Arch is a natural arch in the Port Campbell National
Park, Australia. The arch is one of the tourist attractions along
the Great Ocean Road near Port Campbell in Victoria. This stack was
formed by a gradual process of erosion, and until 1990 formed a
complete double-span natural bridge. The arch closest to the
shoreline collapsed unexpectedly on 15 January 1990, leaving two
tourists stranded on the outer part: they were rescued by
helicopter. No one was injured in the event. Prior to the collapse,
the formation was known as London Bridge because of its similarity
to its namesake. 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. The arch that you see today, was once
joined to the mainland in a spectacular natural bridge,called
London Bridge. It did have two arches just like its namesake, but
in 1990, one arch collapsed into the sea. Eventually, the remaining
arch will erode sufficiently that the overlying rock is not able to
be supported causing it to collapse into the sea. In time this
formation will become a single sea stack. Just like the Twelve
Apostles, along this coast line. A sea arch is a natural arch or
bridge made of stone that has been created through the process of
land, wind, or water erosion. Of course, a natural arch is often
made due to a combination of types of erosion. A natural arch or
natural bridge is a formation of rock that includes a passage way
in its lower half. Very often, arches are made by the meeting of
two types of rock. The rock on at the top of the arch, which forms
the bridge is made of a harder type of rock than what is on the
bottom. If the bottom rock is a softer type of stone, it will erode
away more quickly, thus leaving behind an arch. Natural bridges
commonly form where cliffs meet the sea. In this instance, the
formations are called “sea arches.” A sea arch is created when the
natural erosive forces of the water break through a slice of rock
and leave behind an arch.
To log this cache walk to the viewing platform and:
1.Estimate the height of the arch above sea level.
2.How many distinct layers of rock make up the headland around the
arch?
3.What would cause the reddish appearance of the uppermost rock
layer?
4.Take a photo of yourself in front of the sign showing London
Bridge in the background.
Sources include:
www.australianexplorer.com/great_ocean_road.htm
www.fettes.com/shetland/arches.htm
www.wisegeek.com/what-is-a-sea-arch.htm www.britannica.com
Wikipedia Parks Victoria