Silcrete

[Hairyman]

Other Names: pseudo-quartzite, billy, shin-breaker, gibber, gannister etc.

Silcrete is a silica-rich (greater than 85%) duricrust that forms as a result of low temperature silification of a weathered regolith, bedrock and/or
sediments. It forms at or near the surface and often contains plant stem and root casts.
Generally silcrete forms in hot arid environments where infrequent waterlogging causes silica to dissolve and be redeposited to cement soil grains together.
It can also be associated with basaltic vulcanism.
It often becomes a resistant caprock up to several meters thick on the top of mesas, buttes and ridges, having first formed in low areas, the surrounding land
is weathered away and lowered. This is termed an 'inverted landscape', the rocks of low valleys becoming the high areas.
Although chemically and physically similar to chert, meta-quartzite and some other rocks silcrete has a different origin.
Silcrete weathers spheroidally to produce boulders and angular fragments.

Distribution in Australia: Found extensively throughout inland Australia (ground water and pedogenic silcrete) and east of the Great Dividing Range (sub-basaltic silcrete)

Uses: Fine grained silcrete was used by Aboriginal people in the manufacture of stone tools. It is difficult to work into a desired shape and was often heat treated
by prolonged baking in a high temperature fire to improve its workability. Silcrete was a tradable commodity.

Sources: Encyc Britanica, (http://www.britannica.com/EBchecked/topic/544043/silcrete)
Wikipedia, (http://en.wikipedia.org/wiki/Silcrete)
Australia, A Land Before Time, (http://austhrutime.com/silcrete.htm)

 

[Hairyman]

Silcrete boulder and pebbles.

 

[Hairyman]

A closeup of freshly exposed silcrete.

Possible heat treated silcrete (natural or artificial?)
Note weathered outer surface, this is called cortex.

Photos by Hairyman 2012

 

[koalaboi]

When I first started knapping I got in touch with an archaeologist Tessa Corkill who has done quite a bit of published research into Aboriginal uses of silcrete. She very generously gave me a lot of samples of silcrete she had collected from around NSW. It is incredibly variable. Some of it pretty useless for knapping but some much better.

Another geologist I contacted has this site on silcrete and it's worth a look:

http://dl.dropbox.com/u/5737284/silcrete.htm

KB

 

[Hairyman]

Thanks KB, I didnt see that page last time I looked at his site.
Lots to digest there!

 

[darren]

Out around Sturt there is heaps of it and the due to its fine grain and uniform structure it was sort after by Aboriginals. It was traded from Victoria to Ahrnem land. The mpws office in Tibboburra has heap of examples and speaking to the rangers there they have found knives 12'' long and very fine in design.
When you walk around on it out there it sounds like your on a pile of bricks

 

[Hairyman]

It would be good to get some pics of silcrete tools for this thread.
A 12" blade is impressive work as silcrete seems to fracture where it wants to.
I wonder if the knives were heat treated before being knapped?
As for sound, I notice large thin spalls when struck give a resonating ring. Primative Rock music???

 

[darren]

the ranger was very helpful and had quite a bit of knowledge on the subject. We actually went there specifically to ask about silcrete as earlier in the day i had been showing the girls about knapping and they had some questions i couldnt answer so i made them ask the ranger.
I have some pics of the stone only i will find when i get home

 

[koalaboi]

Here's an interesting site on the Lake Mungo area. It includes lots of pictures of stone tools.

http://donsmaps.com/mungotools.html

Camped there with my family about 10 years ago I mentioned to one of the Aboriginal rangers there that I had an interest in stone tools and he gave me the directions to a place he called the "rock shop" a large quarry and manufacturing site. It was pretty impressive.

Last year I went to a parcel of Aboriginal land adjoining the NW perimeter of Lake Mungo NP. We went to an area somewhat like a very large amphitheatre. The floor of this area was literally covered in stone artefacts. Thousands of them. Old fireplaces etc etc. Interestingly, the archaeological survey for the area done for a mining company said that there was only a couple of artefact scatters!

KB

 

[Hairyman]

Another great link KB, thanks.
I had never heard of 'ulpirra' and the 'Mungo excursions' of the magnetic field is also
a suprise to me.
Love the caption to one of the photos....
"Mussel shell weathering out of the sediments. When Mungo was last full (about 15 000 years ago), Sydney Harbour was a wooded valley 30km inland and Tasmania was at best the Apple Peninsula. Europe was shivering in the Ice Age, Lascaux was the latest contemporary art gallery and people had just started migrating to the Americas. Meanwhile, the long-established Lake Mungo folk were busy cooking Murray cod and golden perch in campfires, traces of which remain as dark smudges in the lunettes."

 

[Aussie Forager CQ]

I came across some large silcrete boulders that have been placed ornamentally. They have come from the local area of where they lay. Silcrete was used by the Tradiional Owners of the area. They show a really good example of something to look out for when identifying potential knapping material.

They are covered in Hertzian cones. Note all the circular fractures on the surface of the rock. They are the result of many impacts by smaller objects, potentially in a creek or river. The circle is the crack from the Hertzian cone penetrating into the material. Some of them even show a little of the sides of the cone like in the centre of the bottom photo. These can be an indicator that the material may be worth experimenting with.

I have posted this in a couple of places that it may apply.