Pretty much, Limestone is Sandstone that has at least 50% Calcium Carbonate, a.k.a. ‘chalk’.
Most of the time, Limestone is made up of fossils. The fossils are either from super tiny organisms or bone fragments and particles that have made their way to the oceans floor.
Limestone technically is still Sandstone and for sure, it’s sedimentary rock that’s usually formed at the bottom of an ocean or some other body of water. As 50% or more has to be chalk, the other 50% can be just about anything else. Plants that have died and sunk to the bottom of the ocean, sand from the rivers and other debris all compacts over a really long period of time from the huge amount of pressure from the water above. Eventually, the sediments form one large piece of stone. Seeing as Sandstone can be just about anything, Limestone is more commonly used as a building material as it’s more reliable.
About 10% of all sedimentary stone is thought to be Limestone and whole lot of it on the Limestone Coast in South Australia.
Most cave systems in the world are formed in Limestone areas, Mount Gambier and Limestone Coast one of the best places to explore caves on the planet.
Natures natural art
At first, the ocean and in this case, the treacherous Southern Ocean will form caves and tunnels in the weaker parts of the cliffs and coastline. After sometime, the inner part of the cave will drop out and a huge arch like the London Bridge or a gorge like Loch Ard Gorge will be formed. With a bit more erosion from the wind and ocean, more tunnels can be created or the pillars of the arch can become too weak and the arch will collapse, just leaving what are called ‘stacks’. The 12 Apostles are all large Limestone and Sandstone stacks. Eventually, they’ll all erode so much that they’ll collapse into the ocean, but not to worry, more formations are slowly being created all of the time.
The ocean erodes the stone along this coastline at a rate of about 1.5-2cms per year.
There are loads more amazing formations to see near the Apostles in the Port Campbell National Park on the Shipwreck Coast, such as Gog & Magog, the Grotto, the Razorback, Thunder Cave and the Bay of Islands.