Marble is a type of metamorphic rock that forms from limestone, a common type of sedimentary rock. How limestone changes into marble is one of the many extraordinary earth processes in geology. This change occurs over millions of years, but it completely transforms the rock from one type into another – sedimentary to metamorphic.
Sedimentary rock is one of the three main rock types. These rocks form by sediment layers. Limestone is a sedimentary rock that forms in clear, warm shallow marine waters. However, what sets limestone apart is the inclusion of calcium carbonate minerals. This mineral is mainly found in the bones of organisms. This calcium carbonate mineral is also what makes limestone so porous because there is space around these mineral fragments.
Metamorphic rock is another of the three main rock types. These stones require an original rock that can be sedimentary, igneous, or metamorphic. Through geologic forces such as heat and pressure, the original rock undergoes significant changes. The new rock that emerges is strikingly different from the parent rock, even down to the mineral level.
To start the change into marble, limestone must undergo the metamorphic process. It could be that the limestone is driven so far underground that the pressure becomes immense. This pressure starts heating up the rock, which leads to the transformation. Another possibility is that a fault line causes the limestone bed to shift. If this shift buries the limestone under tons of new rock, the resulting pressure also creates high heat levels. Again, this starts the metamorphic processes by causing the minerals to re-crystallize.
After it morphs into marble, the rock completely changes. Marble is still somewhat porous, but it is far less porous than limestone. This is because the heat and pressure was so immense that it re-crystalized the minerals within the stone. Gone are any fossil remains, as these minerals are broken up into their basic structures and then crystalized into calcite. These minerals exist as fine grains that are visible on marble's surface. The change of limestone into marble is one of the geological wonders of the Earth. Marble is a prized material but it only occurs under special conditions. These conditions rest with the limestone, which must become heated under high pressure in order to start the metamorphic process. The limestone only turns to marble if it is not exposed to other rocks.
Getting there: The Big Hole/Marble Arch walking track starts at Berlang Campground in the northwest precinct of Deua National Park. From Braidwood, drive south along the Cooma Road for 30 minutes; turn left at the sign to Berlang/The Big Hole/Marble Arch; and drive 700 m to Berlang campground carpark (parking coordinates provided below). The Campground road is an unsealed road but should be accessible by 2WD vehicles in all weather. The campground provides bbq pits and tables and there is a pit toilet too. This can be a busy place on long weekends or school holidays, so parking might be limited.
Follow the track that leaves to the east, down to the upper reaches of the Shoalhaven River. Depending on the level of the river, you may be able to cross on stones, but otherwise a wade is required. This can be quite pleasant in summer but very cold to the feet in winter. Strong currents may be present following rain so take care in the water and supervise children at all times.
Continue along the track to the Big Hole, a massive limestone sinkhole over 100m deep. Then continue on the track as it descends steeply to the valley to the south-east, crossing over the Bettowynd Fire Trail, and continuing through pleasant open forest. The track is infrequently marked, and while it is mostly fairly obvous where it goes, some care must be taken. Continue on to the trailhead (coordinates below) where you will find an information sign. The track descends steeply down a series of 353 steps. A bit of easy scrambling is required to get to the start of the arch. In total it is a 13km return walk. A location map can be found at http://www.environment.nsw.gov.au/nationalparks/parkWalking.aspx?id=N0047
At the entrance, a fallen boulder the size of a 4WD is lodged in the mouth of the cave. The limestone cave system is approximately 4m wide, 25m deep and over a kilometre in length. Wide bands of marble can be seen in the walls of the canyon and the roof of the cave. In the chasm you will be greeted by stalactites and thick veins of marble on the roof.
Besides being a spectacular geological formation, the Marble Arch is of important scientific value because of its resident owl population. The Powerful Owl has left pellet deposits through the subterranean corridor. These deposits are the regurgitated bones and fur of its prey. In some parts of the Marble Arch, the remains of the small mammals and birds are thought to be thousands of years old.
Downstream from Marble Arch is an impressive slot canyon. When wet, the marble rock becomes very slippery and it is tricky to negoiate the pools without getting wet feet. A large pool requiring wading or a possible swim ends the fun for most parties, though the adventurous may like to press on.
Further information can be found at: http://www.nationalparks.nsw.gov.au/deua-national-park/the-big-hole-walking-track/walking
Logging Requirements:
1. Email me the answers to the following questions:
a) How far back does the sign at the trailhead say the geology dates back to?
b) What colour are the rocks in the first cave?
c) What do the caves provide for native animals?
2. Photo task: Include a photo of yourself with a gps or a card/note with your caching name at the trailhead.
Please do not include the information on the sign.
Good luck and enjoy your visit to the area. You may like to check out the earth cache at the start of the walk too, Big Hole (GC596H9).