I observed a grey discoloration on a ceiling in a low cave ( you have to crawl to get in) which I do not recognize. It could be a natural phenomenon but it strikes me that the grey colour just ends at a certain height (see photos). Some grey parts show imprints of plant material but no actual plant remains (see photos). The floor of the cave was in a stage of cracking or armouring.The cave is situated at a low hill and the top of the hill has a fissure with (probably) gypsum needles (photos 161-179). I have no idea if these 2 phenomena are related to eachother. The site is on the north flank of the flaming mountains so it has a very arid contintental climate. It is close to some historical remains so I would like to be able to explain its presence. All suggestions are appreciated! Thank you!
Dear Mrs. Barbaix,
color effects in natural sites like that can have various origins and a remote diagnosis like that you wish to be done by the readers is very much fraught with uncertainties and may very easily put you on the wrong scent. Therefore my suggestion, the tiniest bit taken from some parts of the ceiling and investigated with one of the common mineralogical or chemical analytical devices is more appropriate than pocking around in the fog.
Based upon this additional information you can much better design your question and I am sure that you will get an answer which comes closer to the truth than at this state of data collection.
With kind regards H.G.Dill
Dear Mr. Hill,
Thank you for your answer. I am very much aware that this type of question will not lead to a convincing determination. I am only trying to collect some suggestions before blindly analysing the subject any further. This way, I could have an idea what to look for.
Thank you for your advise and with kind regards,
S. Barbaix
OK for the gypsum.
This looks very much like secondary gypsum, i.e. crystals which formed recently, because of water percolation /evaporation.
The cave seems to be dug into porous colluvium /palaeosoil/consolidated scree; right?
On one photograph (inside) there is apparently a layer of solid rock layer just below the "grey ceiling level"; correct?
On the ceiling chip the grey colour seems to cover the outer surface, the interior remaining pale buff; true ?
If this is OK, then two hypotheses : local accumulation of various oxyds and/or organic matter on rock surface by evaporation of a percolating solution ; or periodical development of a bacterial film on the rock surface. The two hypotheses are not exclusive.
Thank you very much for your answer, Marc Philippe! I was thinking the same about the gypsum. Your observations are correcr. The grey is only on the surface. The soil 'behind' it seems unaffected.
I will certainly take your hypotheses into account.
Thank you very much!
S.Barbaix
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