North Wollongong Beach fossicking

Prospecting Australia

Help Support Prospecting Australia:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.
Joined
Nov 30, 2016
Messages
10
Reaction score
2
Hi all I have been collecting petrified wood pebbles on the beach for many years and polishing them in my tumbler. Of interest also are the tiny agates found in the gravels.
I've been wondering about the origins of them both. I believe the petrified wood is assocaited with the Illawarra Coal Measures, but what about the agates.? Cheers OBH
 
The wood might be associated with the coal, although often it comes from young gravels, often beneath basalt flows. The original wood has gone of course, but each cell in the wood has been perfectly replaced by silica that has crystallized from low temperature water flowing through an aquifer (e.g gravels beneath impermeable overlying basalt).

Agates can have varying origins - again it is silica (chalcedony) precipitated from water but it usually fills an open cavity that is often present already, commonly but not always in a lava. There are both low temperature and high temperature varieties, the latter a bit like thunder eggs and formed from hot water in the cooling volcanic rock (that may be derived from it, rather than travelling through it from elsewhere).
 
😅 paging, geez that seems like a world away from where technology is today. Apologies for the off topic post, showing my age 😃😬
The pager was quite a step up from the morse key - I wonder if it comes from calling your page?

Who thought we would have video communications on a mobile phone - we didn't have TV in Australia until I was 8.
 
The wood might be associated with the coal, although often it comes from young gravels, often beneath basalt flows. The original wood has gone of course, but each cell in the wood has been perfectly replaced by silica that has crystallized from low temperature water flowing through an aquifer (e.g gravels beneath impermeable overlying basalt).

Agates can have varying origins - again it is silica (chalcedony) precipitated from water but it usually fills an open cavity that is often present already, commonly but not always in a lava. There are both low temperature and high temperature varieties, the latter a bit like thunder eggs and formed from hot water in the cooling volcanic rock (that may be derived from it, rather than travelling through it from elsewhere).

Thanks Goldie rocks. The intriguing thing, though, is the agates that you find are consistently pea-sized or slightly smaller - they have been sorted by erosion and deposition over countless years. If they are associated with the basalt flows on the coast you might expect them to be of various sizes.
I believe there are two possibilities to explain their origin:
* the agates are a secondary phenomenon weathered out the conglomerate beds in the Narrabeen group, near the coal measures, so are much older than the basalts,
* or, they are the result of silica replacing original tiny shells contained in the sedimentary rocks (low temperatue precipation as you suggest). Anyway we'll probably never know until some proper geological field work is done. Great to chat.
 
Thanks Goldie rocks. The intriguing thing, though, is the agates that you find are consistently pea-sized or slightly smaller - they have been sorted by erosion and deposition over countless years. If they are associated with the basalt flows on the coast you might expect them to be of various sizes.
I believe there are two possibilities to explain their origin:
* the agates are a secondary phenomenon weathered out the conglomerate beds in the Narrabeen group, near the coal measures, so are much older than the basalts,
* or, they are the result of silica replacing original tiny shells contained in the sedimentary rocks (low temperatue precipation as you suggest). Anyway we'll probably never know until some proper geological field work is done. Great to chat.
They can grow in various ways, but when small spheres are usually fillings of empty gas bubbles in basalt. The bubbles are commonly of similar size.1678446527439.png1678446642431.png
 
I take your point Goldierocks. Are your photos local? There are certainly some basalts present among the coal measures and some volcanic tuff which would be the source of the silica.
I'm in Victoria but rocks like this are all over the place - it is normal for the upper parts of basalt flows to "bubble" and the cavities to be filled (eg with zeolites around Melbourne, agate in Cardinia Creek area).
 
Hi all I have been collecting petrified wood pebbles on the beach for many years and polishing them in my tumbler. Of interest also are the tiny agates found in the gravels.
I've been wondering about the origins of them both. I believe the petrified wood is assocaited with the Illawarra Coal Measures, but what about the agates.? Cheers OBH
As a young fella, I was given a few nice chunks of petrified wood collected from Wollongong beach by a guy from a Sydney gem club. He told me two things of interest - the rocks won't polish well because of the long period in saltwater leaving micro cavities and there is an offshore reef full of petrified wood which delivers some up to the beach in big storms. On a separate note, I think vesicular basalt is pretty common and with the right conditions, chalcedony can form in the bubble cavities
 
As a young fella, I was given a few nice chunks of petrified wood collected from Wollongong beach by a guy from a Sydney gem club. He told me two things of interest - the rocks won't polish well because of the long period in saltwater leaving micro cavities and there is an offshore reef full of petrified wood which delivers some up to the beach in big storms. On a separate note, I think vesicular basalt is pretty common and with the right conditions, chalcedony can form in the bubble cavities
Yep, that is what agate is - chalcedony (see photos above)
 
Yep, that is what agate is - chalcedony (see photos above)
Years ago we went looking for petrified wood on a beach near Wollongong.
We had been told that the petrified wood was so common there that we would fill half a bucket in no time, then we would find a piece so much better that we would tip out what we had found and start again looking for the better pieces. Then, when we had about a jam tin full of better pieces we would find a piece so much better that we would toss out what we had picked up and start again, looking for the good pieces. We would then end up with two or three pieces of extra special quality.
That is exactly what happened. We went home happy with four excellent pieces of petrified wood.
 
New to this fossicking game but am currently working with a dredge in Port Kembla and finding petrified wood in the spoil. Haven’t bothered collecting any as yet but it looks alright to my untrained eye. Would it polish up nice
 

Latest posts

Top