Sandstone is sedimentary rock that forms over a huge amount of time at the bottom of the ocean or some other body of water.
If formed at the bottom of an ocean, the Sandstone can be made up of all types of sediments. Fish and plants that died and made their way to the bottom of the ocean, sand and dirt that flowed into the ocean from the rivers and other debris that sunk to the ocean floor would be compact over thousands or millions of years by the huge pressure from the water above.
Just like you’d find right along the Great Ocean Road on the southern coast line of Australia, after some continental drift and the ocean line retreating, the Sandstone that once at the bottom of the ocean becomes the land. The ocean smashes against the stone, from this and the wind that constantly tears into it, the stone breaks away and erodes.
Although Sandstone can be made up of a huge amount of different sediments, it tends to erode at different rates and usually particles the size of grain of sand will be what breaks off and drifts away in the water or blows off in the wind.
Along the Great Ocean Road and the Surf Coast, the Sandstone cliffs usually erodes away at a rate of about 1.5-2cms per year.
Port Campbell National Park
The Sandstone Lifecycle can be a pretty long one
Sandstone can have a full circle lifecycle and pretty cool to think about.
A big chunk of stone could be on the top of a mountain that once formed at the bottom of the ocean. The rock could roll down the hill and into a river. Over thousands of years, the rock could erode and the sediments flow through the river into the ocean. The sediments of the rock could eventually drift to the bottom of the ocean, compact under its pressure and turn to stone.
If the Sandstone has 50% or more of its sediments being chalk, then it’s called Limestone.
Typically, along the Great Ocean Road is Sandstone with some Limestone, then once you make it to South Australia, it’s mainly Limestone and that’s where the name the ‘Limestone Coast‘ comes from.
As Limestone and Sandstone erode at different rates, some amazing natural wonders are formed. The Shipwreck Coast and specifically the Port Campbell National Park show case a bunch of massive and incredible natural formations that took thousands of years to take shape.
In the case of this coastline, the powerful Southern Ocean has been smashing against the cliffs and pulling it apart, grain by grain. At first, tunnels are formed in the weaker part of the stone. If the tunnels or caves have the inner part collapse, an arch way can be left, just like the London Arch. If the centre of the arch eventually collapses due to the columns experiencing a bit more erosion, then large ‘stacks’ are formed, just like the 12 Apostles. In the rare case that water from rain has created a sink hole while the ocean erodes from the sides, unique formations like the Grotto appear.