GEOLOGICAL EVOLUTION
The Sydney Basin formed in an extensive depression on older Palaeozoic sediments hardened and deformed in deep burial and extensively intruded by granites. Subsequent uplift and erosion exposed these rocks in highlands generally westerly of the present coastline, to provide sources for the simple succession of distinctive sedimentary layers deposited in the depression. Tilting of the region during this deposition changed source areas of the sediments, the Narrabeen Group being derived from the north and the overlying Hawkesbury Sandstone from southerly sources.
PERMIAN
The oldest (lowest) sediments are of Permian (290-240 million years) age. Ferns had just become abundant on earth, and accumulated into thick peat-like layers which with burial were progressively transformed into black coal.
TRIASSIC
The Permian sequence is well below the surface at Sydney, underneath the overlying Triassic (240-170m.y) sediments deposited to form three distinct layers (see cross- section map). At Balmain Permian black coal is 900m below the surface.
Narrabeen Group.
This lowest Triassic member is interbedded sandstone-shaley sandstone-shale and forms the prominent coastal cliffs from Narrabeen to Palm Beach, and major cliffs in the Blue Mountains (which are not Hawkesbury Sandstone). A narrow strip of uppermost Narrabeen Group forms the base of the western slopes of Pittwater.
Hawkesbury Sandstone.
This prominent sandstone directly overlies the Narrabeen Group and forms the bedrock of the area. The dominant relatively pure quartzose sandstone produces the sandy soils, especially on the flat ridge tops. When deeply buried, crystallisation and re-crystallisation of the constituent quartz has produced sandstone with a range of hardness.
Wianamatta Group.
These shales in turn overlie the Hawkesbury Sandstone. In our northern suburbs these have been eroded away except on flat ridge tops where this rock type formed superior soils, once forested and subsequently the site of orchards early settlers. The boundary between Hawkesbury Sandstone and overlying Wianamatta Group steadily decreases in height south-west from the Pacific Highway to below sea level. The subdued topography south of the Parramatta River thus preserves Wianamatta (Ashfield) shale at the surface with Hawkesbury sandstone in depth, whilst rising easterly and southerly to the surface to form the prominent coastal cliffs along a narrow strip from Drummoyne to South Head and southwards along the coast.
This pool was first constructed in the late 1950’s or early 1960’s and there was some rather dangerous rock falls early in the pool’s life. This caused supposedly safeguarding action by blasting the overhangs, however the blast went way wrong and thus caused the pool to be closed during a winter as the now very loose rock was slowly excavated and removed. This did make the pool a much safer place, however there were still some occasional slips which could have had devastating effects and the wire mesh was installed after some drainage works were completed above, this taking away all the run off water and making the whole area much safer.
To log this cache, please answer these questions and forward to me, I will only respond if there are errors in the answers. Feel free to log and claim your smiley and please any photos of the beautiful beach or the wonderful rocks and the surf.
1. What are the main features and type of rocks you note at GZ?
2. Once you have identified the origin of the main rocks, what is the other "Famous icon" located in N.S.W of the same rock?
3. What is the major reason for the man made, other than the pool, structure found at the site?