Bents Basin is located on the Nepean River 150kms upstream from the ocean in the Bents Basin Conservation Reserve which has a cost of entry if you drive in, you may park at the ranger station and walk in for no charge on the Eastern side, it is about a 2 km return walk from the ranger station or you can access the park on the western side via Bents Basin Rd but there is no optional parking arrangements on this side, you will have to pay to park.
The Basin is what is known as a “scour pool”, has been formed during times of flood and heavy water when the water coming out of Gulguer Gorge moving the rocks on the bottom of the basin causing erosion in the Hawkesbury Sandstone Layer and shale base of the area.
The gorge was formed over many years and is one of the more unique gorges due to the fact that the water course was here before the mountains as around 10 million years ago the surrounding mountains rose at a rate of around 7mm a year and the water continued on its path cutting the gorge as the mountains rose. The water enters the top end of the gorge during flood and high water at around 4-7kms per hour, the shape of the gorge works like a funnel speeding the water up to around 30 kms per hour on the exit creating a vortex in the pool.
This area is known as the Nepean monocline, (further north it becomes the Lapstone monocline) and as it rose the river eroded its course through the Hawkesbury sandstone which consists of mostly quartz particles with very minor amounts rock fragments, clay pellets, feldspar and mica.
The pool its self is oval in shape and has a surface area of 21000 m2 and holds 160840 cubic metres m3 of water, the average depth is 7.7 meters with a maximum depth of 22.5 mts in the middle, as the entrance to the pool is not in the centre the pool has a natural current in one direction but can change direction or stop rotating due to water level.
Bents Basin or 'Gulguer' as it is known by the local Aboriginal and holds great significance to the Gundangara/Dharawal/Darug groups. Gulguer is an Aboriginal word meaning falling, or shooting down, or swilling round resulting in a large round hole.
The Aboriginal people associate Gulguer waterhole with an ancestral creature called Gurungadge or Gurungaty, thought to be a malevolent giant eel or aquatic monster. A prominent figure in the ancestral stories of the area, Gurungadge had a reputation for drowning and eating foreign Aboriginal people who came to drink from the water hole.
The area is also home to some rare plants one being the Camden White Gum (Eucalyptus-benthamii)which only grows here and in the Kedumba Valley in the Blue Mts, Rare fauna species include the Cumberland Plain Land Snail, Eastern Bentwing Bat and the Sooty Owl.
To get a good appreciation of the pool characteristics a climb up to Caleys lookout is recommended, it is a formed path with steel steps with a good view over the basin which will help you answer question 5. (This is an optional climb but recommended)
To log this EarthCache you will need to email me answers to the following questions and upload a photo with your log.
1) On the pool exit where GZ is, put a leaf or stick in the water, as close towards the middle of the outlet as possible and time how fast the stick / leaf takes to go under the bridge - for interesting comparisons post this result in your log. we will measure the water flow.
2) What is not permitted past the sign at S33 55.810 E 150 37.930°
3) Estimate the distance across the pool from GZ to the opposite bank under the power line. (hint you can walk around the pool from the sign)
4) Approximately how far is the water under or over the bridge?
5) What direction do you think the pool spinning on your visit?
6) (Optional) Take a photo showing yourself and/or party holding your GPS at the GZ location facing west toward the pool or from Caleys lookout and post it.
You MUST attend the EarthCache location and upload answers to all of the questions and optionally upload your photo to claim a find. Any logs without correct answers will be deleted, no exceptions!! If you are not contacted about your log assume it is correct
You are encouraged to look at the following park information regarding closures due to fire and flood prior to attempting this cache
http://www.environment.nsw.gov.au/NationalParks/parkHome.aspx?id=N0618
http://www.environment.nsw.gov.au/parksafety
http://www.environment.nsw.gov.au/NationalParks/FireClosure.aspx
Resources used compiling this earth cache
Websites
http://ojs.library.unsw.edu.au/index.php/wetlands/article/view/262
http://www.environment.nsw.gov.au/NationalParks/parkNature.aspx?id=N0618,
http://ojs.library.unsw.edu.au/index.php/wetlands/article/viewFile/262/317.
Pictures
http://www.hawkesbury.nsw.gov.au/__data/assets/pdf_file/0018/21483/Wayne_Erskine.pdf
http://www.traveldownunder.com.au/New_South_Wales/Greater_Sydney/Bents_Basin_State_Conservation_Area.asp
http://www.environment.nsw.gov.au/NationalParks/parkGeoFormation.aspx?id=N0004
Books,
The Blue Mountains A Grand Adventure For All – Peter Stanbury
Bents Basin brochure and park Ranger