Limestone is a sedimentary rock composed primarily of calcium carbonate (CaCO3) in the form of the mineral calcite. Most limestones form in shallow, calm, warm marine waters. That type of environment is where organisms capable of forming calcium carbonate shells and skeletons can easily extract the needed ingredients from ocean water. When these animals die their shell and skeletal debris accumulate as a sediment that might be lithified into limestone. Their waste products can also contribute to the sediment mass. Limestones formed from this type of sediment are biological sedimentary rocks. Their biological origin is often revealed in the rock by the presence of fossils. Some limestones can form by direct precipitation of calcium carbonate from marine or fresh water. Limestones formed this way are chemical sedimentary rocks. They are thought to be less abundant than biological limestones.
Caves form in limestone when water containing dissolved carbon dioxide (carbonic acid) seeps into rock crevices and joints. The carbon dioxide comes from decaying organic matter in soil, and also directly from the atmosphere. This slightly acidic water dissolves the rock, forming cavities which can enlarge and join up to make larger cave systems of interconnected chambers. An underground water flow can develop when many rain-fed subsurface drainages join up, or a river can be captured by an open cavity collapse structure (sink hole or doline) and flow underground. This leads to further enlarging and sculpturing of the caves by chemical and physical weathering.
Getting there: The Big Hole 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; 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 walking 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. Then follow the 1.8km track as it climbs up the gully, on to the ridge and then around the back of the hill to a large wooden platform. The platform looks down into the impressive limestone sinkhole which is over 100m deep.
This is what author John Blay said about Big Hole in his 1987 book Trek Through The Back Country: "I walked round the hole, marvelling and trying to work out how to photograph it; it was about 50 metres across. The sides fell in a sheer drop of some hundred metres. Plants clung to the sides in places and grew in a sloping band of earth at the bottom. Swallows flittered in clear arcs."
Others visitors have reported lyrebirds scuffling about at the bottom of the Big Hole. Who knows what you'll see when you visit?
Further information can be found at: http://www.nationalparks.nsw.gov.au/deua-national-park/the-big-hole-walking-track/walking
Logging Requirements
1. Send the CO a message with answers to the following questions:
a) How was the Big Hole formed?
Include the type of rocks and when they were laid down and what formation was created before the Big Hole formed.
b) What types of plants are predominant at the bottom of the hole (common name).
2. Photo task: Take a photo of yourself or your group in front of the Big Hole with your gps or card/note with your caching name. Logs without a photo will be deleted.
Please do not include spoilers for the two questions above.
Good luck and enjoy your visit to the area. You may like to continue along the trail and visit the Marble Arch (GC596ZA).