Jetty Mine EarthCache

Warning: The path to the cache is exposed and demands an experienced hiker.

Historically, the area of Little Bulli was originally inhabited by the Wadi Wadi Aboriginal group. In 1796 it was first traversed by 3 shipwrecked sailors in an epic journey of survival along hundreds of miles of coastline until rescued at Wattamolla, north of Stanwell Park. Two of their companions were unable to negotiate the Coal Cliffs where the Sea Cliff Bridge is today, and their remains were found by explorer George Bass, who also reported on the rich coal seam apparent in the cliffs. Because of the inaccessibility coal was not mined before 1878. A jetty 150m from the shoreline was constructed to access the mine, becoming known as the Jetty Mine. The last shipment of coal from the jetty was in 1912.

Today, you see the remainders of the wharf to which the jetty was attached. It is a tremendeous spot to sit in the morning sun and watch the seas, the waves breaking or a fisherman in the vicinity. Facing the cliffs, there is the eye-catching black seam of coal, which penetrates to the surface. The two entrances to the mine have been carefully sealed, but you can walk along the coal lode and experience its texture, brittleness and surrounding geology. Most interesting, how the coal is embedded in the surrounding rock - almost as if someone has placed a layer of bricks between them. This material, however, is weathered mudstone broken into quaders of brick size. Touch the coal - take a photo or two - and post them in your log! But first to the serious part:

Geology - The Earthcache
Quite a few Earthcaches of the area describe the Narrabean Group of sedimentary rocks abundant in the larger Sydney region. A good introduction to the geology of the Illawarra Escarpment Sea Cliffs is given by the earthcache GC2K0TJ. In the lower lying Sydney Basin (GC1YH39) Hawksbury sandstone dominates at sea level, some 200m thick. To the north and to the south, underlying sedimentary layers are inclined slightly and successively raise to the surface. Thus a drive from the Royal National Park into the Illawarra brings you to subsequently older layers of sedimentation. Here in Clifton, the Illwarra Coal Measures make up the ocean shore line. They have been deposited in the Permian age, 286-248 million years ago. At sea level, you stand on Loddon Sandstone, superimposed by the latest coal seam, the Bulli Coal, which is impressively exposed to the surface and which the history of this place is all about. The coal measures have been formed in the late Permian by fluvial deposition in lower delta plain conditions. Their development was apubtly terminated by a marine incursion responsible for the Wombarra Claystone formation at the beginning of the Triassic age. At Clifton, the Wombarra Claystone serrates frequently with Coal Cliff Sandstone and an Otford Sandstone member, altogether superimposed by Scarborough Sandstone, Stanwell Park Claystone, Bulgo Sandstone, Bald Hill Claystone, the Garie Newport Sandstone and finally the massive Hawkesbury Sandstone.

As you stand here in front of the Bulli Coal seam, you recognize the layredlayered and altered deposition. Have a look to the bedding of each layer. Some are cross bedded, other show a laminar bedding. Is the stone homogeneous or are there frequent inclusions?

To qualify for a find of this earthcache
Go down to the sea level - remember, access is for the experineced mountaineer only. There may be other earth caches at the road and bridge level, however, for this cache, you must venture down, far underneath the bridge. You won't find the right answers from the top, and if you are unable to demonstrate you went down, your entry will be deleted. Once at GZ, you will need to answer the following questions:

Q1: Find the concrete seal at GZ. There is considerably high density of different sedimentary layers directly above the concrete seal until the massive sandstone forming the roof. Describe them:
Q1a: kind of stone (sandstone, claystone, mudstone) and color
Q1b: bedding and homogeneity
Q1c: thickness of each layer (we are talking centimeters to decimeters - but not inches nor feet)

Q2: When was the mine sealed

Q3: What is the danger being warned of?

Q4: Who to call on damage?

Q5: There is another big seal north to GZ. With your GPS, measure the distance from here to there.

Q6: What is the thickness of the coal lode appearing at the surface under the bridge?

Q7: Describe the appearence and consistency of the coal, particularly towards the top of the seam.

Q8: Optionally, upload a photo of yourself or your travel bugs at the coal seam.

References
* R.S. Moffitt: "Southern Coalfield", App. A, Mineral Ressources NSW, GHD Geotechnics (2007) (link)
* M. H. Monroe: "Sydney Basin", in "Australia: The Land Where Time Began" (link)
* "Geological Survey: Sydney Basin", NSW Government - Industry & Investment (link)
* Nicholas M. Short, et. al.: "Some basic concepts underlying the science of geology", NASA (link)
* Southern Coalfield Regional 1:100000 Geology Map, Minerals and Petroleum (link)
* Wollongong – Port Hacking 1:100 000 Geological Map, Minerals and Petroleum, Code 9029-9129 (link)

Access: Best you start at the nearby earthcache GC2K0TJ, from where you take the almost overseen bush path beside the bridge, leading down to the water. Be warned, it is very exposed and may be very slippery and muddy, when the clay is wet. Best you bring some mountaineering experience. This is a warning for the city or bridge cacher - although for the experienced bush or adventure cacher, it's not a problem at all. Last not least, as you see from the track, the path is frequently used by locals and fishermen. Once at the bottom - easy - it's all yours to enjoy!

GPS signal: The satellite signals can be misleading at the cliffs. You city-cachers should know! Settle the signal 20-30m off the cliff and then approach - watch how it jumps. Upon approaching, extrapolate where Ground Zero could be. If necessary, repeat the procedure from 2-3 different directions.