These are the collapsed remains of the silver mines or mica mines on Penyard Hill. These were not actually silver mines but were levels driven in horizontally into a layer of dark coloured clay or marl. Below this is a layer of crumbly sandstone rich in mica giving it a very sparkly, and highly prized, appearance.
The workings were started in the 1920's by and my grandfather remembered ponies working a tram system that was used to move the sand and spoil away from the site.
The entrance to the mica or silver mine (click for a larger image)
The inside of the mine (click for a larger image)
The line of the workings is marked by a collapsed tunnel running up the centre of the photo (click for a larger image)
The workings start out as a trench at least twenty metres long and about a metre deep running east-west along the contour of the hill. This then goes underground forming a tunnel, which now resembles a badger set from a distance, which is around another ten metres long and has collapsed in places. The tunnel then forms a trench where it has collapsed completely. This ends with what looks to be a shaft or steep incline, which is blocked up with corrugated iron with debris collapsed in on top of this.
I am told that in the early 1950's it was possible to crawl from the entrance along the line of the level for about five metres, which matches up with a point where the tunnel has collapsed in. The tunnel has almost completely collapsed now so it is not easy to judge.
In The Mines of Newent and Ross, by David Bick, there is a description of this site that ties in reasonably well with my observations although more recent subsidence may have revealed more of the workings. Mr. Bick also states that there were bellows for forcing air around the workings but there are now no signs of any buildings or other works except the tench and tunnel.
A point in the middle where the tunnel has collapsed (click for a larger image)