Geology Question

G

grahams

Well-known member
I don't know about your sample but there are fossil trees in limestone near Lulworth and Weardale.
 
A

Andy Farrant

Active member
Hard to tell from a photograph, but its probably a chert nodule. Chip a bit off and see if the fresh face (which will probably be a smooth black fracture) fizzes in weak acid. Fossil trees do occur in limestone, but only in certain unusual environment such as lagoonal or fresh water limestones, for example at Lulworth, see http://www.southampton.ac.uk/~imw/Fossil-Forest.htm
 
A

Andy Farrant

Active member
If it were a fossilised tree, I would expect it to be calcareous like the limestone and thus prone to dissolution. That the concretion has not dissolved like the bit of limestone suggests it is chert - should be a dark grey or black splintery hard siliceous rock. Chert does occur in the Clifton Down Limestone which is probably present in Vurley at depth.
 
The Old Ruminator

The Old Ruminator

Well-known member
Not sure about the chert. This " thing " came from quite high up in the cave. We have passed through the prominent chert cobble layer at about 110m depth. Its about 2 metres wide there and forms part of the penultimate pitch below the 20m pitch. We still are worrying about the air down there. We have %2 CO2 but more worring %17 02. ( will address that in another thread. )
 
M

Martin Laverty

Member
Looks to me like the silicified cast of a piece of drifted wood with a bit of the once-enveloping limestone still adhering to it - rather than sticking out - as the other end may show?

I'm not clear why Andy would have expected a fossil to be calcareous as silicified (usually originally calcareous) fossils retaining their original structures are not uncommon in limestones: if any structure (cells, rings, cracks) were to show up on polishing, that would indicate direct replacement rather than filling of a hole following decay.
 
A

Andy Farrant

Active member
Fossil trees are very rare in the Carboniferous limestone, as the shoreline was some distance to the north at the time (well north of Bristol). The sample doesn't have any internal structure that would be expected from a fossil tree (I have a nice lump in my garden from the Vale of Wardour - albeit of Jurassic age and you can see the tree rings). The most common Carboniferous tree is Lepidodendron, which has a very characteristic bark pattern - you can see a nice example in the roof of Siambre Ddu, in the Twrch sandstone. This sample doesn't appear to have any of these. It is more likely to be some form of large burrow that then was the focus for silicification - in the same way as flints in the Chalk are formed.
 
Pitlamp

Pitlamp

Well-known member
I agree with AR - that looks just like the sort of chert which frequently shreds wetsuits in our beloved Northern Dales caves.
 
pwhole

pwhole

Well-known member
Aren't there some 'fossil tree' chert nodules somewhere in the Peak District that look just like this? I saw them last year - are they near the bottom of Waterways Swallet? I seem to remember something similar near the SRT pitches in Winnats Head Cave too.

The first photo below is of an actual tree fossil, which had fallen from the roof of a ganister mine (thinly-bedded flaggy sandstone host rock), and the next photo shows its impression in the roof. There appears to be another branch still stuck in the roof in the first photo, aligned at 90? to impression of the larger piece. The third photo is what I presume is a piece of Lepidodendron fossil that I found in a stream in Sheffield a couple of years ago.
 

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