Early this morning I was doing some tramroad and quarry photography in the frost and saw distinct near-continuous wisps of warm air vapour rising from a small cave entrance at NGR SO 2506 / 1150.

This a prominent entrance about five feet wide and three feet high on the side of the tramroad at the east end of the cutting immediately south of the main Pwll Du quarry.

I approached it by walking from the west along the base of the cutting. The entrance is immediately east of a north-south transverse wall across the cutting. The wisps of vapour were so prominent and near-continuous,  that I thought  there might be a smouldering fire lit in the lee of the wall.

Can anyone provide any information about this intriguing entrance?

Many thanks! 

This could be Siambr Ddu. A short but very interesting cave. All in gritstone and some fine fossil trees to spot in the magnificent chamber. I understand it has SSSI status.

http://www.ogof.org.uk/siambr-ddu.html

There is no shortage of draughting holes in this area. It is like a Swiss cheese. The land is owned by PDC Ltd and the dead hand of PDCMG lies heavy over it.

Your grid ref suggests Siambre Ddu.

Not very far away is its son....

https://www.flickr.com/photos/11409438@N06/38641347650/in/album-72157679947221496/https://www.flickr.com/photos/11409438@N06/40407917402/in/album-72157679947221496/

Who is the bird?

https://www.flickr.com/photos/11409438@N06/38075623144/in/album-72157679947221496/

Stolen from a book:
Simbri Ddu
Discovered by the Cwmbran CC in October 1967 in Pwll Du Quarry.  A 6 ft wide entrance leads on the left to a small unstable chamber.  On the right the passage narrows to a flat out crawl and then after 120 ft opens out into a large chamber roughly circular in plan and 30 ft  high in the middle.  The cave is unusual, it is in millstone grit and the floor of the chamber is covered with a thick black peat deposit, hence its name.

Much information here. (Just zoom in.)
http://www.ogof.org.uk/Clydach-Gorge-Cave-Location-Map.html

jbaer said:

Who is the bird?

https://www.flickr.com/photos/11409438@N06/38075623144/in/album-72157679947221496/

Click to expand...

No connection to Siambre Ddu. You should be able to see a picture of her in her school uniform if you search on Google using the term:
  Caver 1970

Wayland Smith said:

Much information here. (Just zoom in.)
http://www.ogof.org.uk/Clydach-Gorge-Cave-Location-Map.html

Click to expand...

An excellent resource, with in the case of Siambre Ddu and a few other caves, photos and a more detailed description. It does not cover every site of interest near Ogof Draenen. For those sites shown that do not have a detailed description, calling up the Cambrian Cave Registry entry from this map provides a link to the CCR map with geological and hydrological information available. Some sites are on the CCR map but not on the Clydach Gorge cave location map.

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Many thanks everyone for your informative replies. Siambr ddu sounds a geologically intriguing cave despite its short size.