Large bolders of obsidian found in Chilterns hills south uk

[Jimbob86]

Found in the high wycombe area of the Chilterns hills my understanding is that the hills or rather the valleys were carved out in the ice age so I presume they must be pre-historic as I don’t believe there has been volcanic activity here for millions of years If anyone knows anymore would love to hear. Also found other volcanic rocks the lightweight black rock( forgive me can’t remember what it’s called aswell as the greeny coloured obsidian. thanks for any info
Also a large number of sponge fossils I believe the hollow balls of chalk and flint and a few shell fossils. The area it mainly chalk

 

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[Jimbob86]

Sorry wasn’t clear in my message I just wanted a general overview of obsidian is it found widespread or is it localised to areas but if anyone has specific info about this area would love to hear it. I know it is formed by rapidly cooling lava touching water but the site is about 100 miles inland so must have formed before Britain was Britain maybe when the continents were doing their big shuffle any info on obsidian is much appreciated

 

[smokeythecat]

I am not sure it is obsidian. Around it you found numerous sedimentary rocks. What it may be instead is something called "jet" which is a type of black petrified wood which has been used in jewelry for centuries. Those are nice big pieces. Obsidian is made from relatively "new" lava flows. If it were to be obsidian it would have been brought down from the way north by the last glaciers. Jet would be better as is has a lot of jewelry uses.

 

[Jimbob86]

I am not sure it is obsidian. Around it you found numerous sedimentary rocks. What it may be instead is something called "jet" which is a type of black petrified wood which has been used in jewelry for centuries. Those are nice big pieces. Obsidian is made from relatively "new" lava flows. If it were to be obsidian it would have been brought down from the way north by the last glaciers. Jet would be better as is has a lot of jewelry uses.

Ah thank you interesting the only reason I said obsidian is because or the honey comb effect on the underside of some of the pieces and the flow effect some seem to have !! and the other type of igneous rock! this is all coming from the side of a hill also it is fractured throughout the bolder. Also there were perfect tiny metal balls in it guessing iron as they were ferrous

 

[smokeythecat]

Jet will be softer than obsidian, which is essentially glass. Try a scratch test on it. With the additional photo of the bubbles on the bottom, it could be a manmade melt, maybe even from Roman times. Are there other artifacts near it? It could have been brought down from the glaciers.

 

[fuss]

I am wondering if it isn't some type of slag.

 

[Jimbob86]

That is why I thought it was obsidian it is a very hard material an shatters like glass it is all coming out of a wooded hillside at various levels on the hill it appears to be coming out as the hill is subsiding over time. Thr balls of metal are a mystery they are the size of a bb bullet and a perfect Sphere. There is a large lump rectangular lump of limestone with man made groves in it will take a pic later!! so could possibly be from some kind of workings but what process would produce such a by product it looks and behaves exactly like obsidian . Thank you for your help I find this fascinating and forgive my lack of knowledge

 

[Clay Diggins]

It's slag. A man made byproduct of metal refining and glass making. Often glass making slag has round balls included but I've never seen black glass slag. Doesn't mean it couldn't happen though. Is there a history of glass making in the chalk hills?

You won't find intact obsidian in chalk formations. Glass-like Obsidian is not a long term mineral as it sheds and decomposes through a process called devitrification into rock or perlite and eventually clay as it ages. It might be intact for a long period of time in human terms but it doesn't survive ice ages, millions of years or transport by glaciers. You might find some chert nodules in the chalk but chert doesn't fit your pictures or description.

There is a reasonable explanation for what you have found. Limestone (chalk) is used as a flux in the refining of iron ore to produce pig iron. The iron ore is heated in a furnace and limestone is added to the mix to lower the melting point of other included metals and quartz. This removes most of the impurities and leaves behind clean pig iron and slag which can be different colors and textures depending on how much quartz and other metals were included in the melt. The limestone slag floats on top of the pig iron and is scraped out of the furnace and discarded while still semi liquid.

A small local blacksmith could have been refining iron ore and using the abundant local chalk as a flux. It was not uncommon in the middle ages for rich iron ore to be traded among metal workers guilds. A little local research might reveal the real treasure to be the discovery of a long lost blacksmith's forge.

 

[Jimbob86]

Wow that makes sense there are some fairly large pits dig out up there thought it may have been for flint as we have some flint houses round here but could be for the chalk. Thanks for this something to ponder and investigate. There is a large lump of limestone rectangular shaped with grooves and slots cut into it so def been messed with my man will take a picture and post thanks again that is a brilliant theory that fits so far

 

[smokeythecat]

That settles it then. With the pits, the old timeres were smelting with the limestone. These are manmade slag items as Clay Diggins stated.

 

[Jimbob86]

More finds from area

Large worked piece of limestone Found near all slag finds

 

[Jimbob86]

Also clay digging does slag eventually decompose to rock or Perilite like obsidian as sone of the finds do appear to be turning to rock