Whilst pottering about in Cornwall (South Coast) I was wandering around some of the small, picturesque villages and harbours ….

In Charlestown (just below St. Austell) I found a “sea cave” that appears to be a “traditional cave” (in the sense it isn’t a cave created by sea erosion).

It is located in the cliff face around 100 yards from the harbour on the right as you look at it from the sea (or it is on the left side of the harbour if you approach the harbour from the road heading towards the sea).

(The entrance on the beach – low tide)


A significant amount of water is coming out of the cave and running across the beach and into the sea. (that’s the giveaway).

At first glance I thought it might be a mine adit but the inside tapers immediately and there are no tool marks.

(Just inside the cave)


30 feet into the cave the roof drops down to a flat crawl of no more that 18-24 inches (more if you scoop the floor out).

However, I did not have a lamp (only a pen torch light from my old fashioned mobile phone) and I could not undertake the flat crawl because I was not properly dressed and the running water would soak me leading to a major bollocking from my wife.

Although the photo is hopeless (because the flash bounces), I could see that the flat crawl was less than 6 feet and then the cave appeared to open up again.

(The flat crawl)


I wondered if anyone know anything about this cave and is it worth “pushing” so I shoved it up on UKc here;

ukcaving.com/board/index.php?topic=20573...264512;topicseen#new



Ian

(The sea easily reaches the cave entrance at high tide)

Looks like a mine drainage adit to me

Even with such a low roof inside ?

Ian wrote:
Yes

It's not really that low but the sea has filled the floor up.
Drainage adits were often cut at low tide level to get the extra few feet of depth and the sea will infill them with sediments over the years. The drainage water keeps them open, but only open enough for the water to flow out.
The whole area has been mined extensively around where you were (a quick look at the OS map shows many shafts very near where you were and and the shape of the Portal, the fact it is along a mineral vein and seems to be "man" height, at least at the start suggest adit to me.
I would expect to see drill/shot holes to confirm it, but that near the sea any evidence may have been eroded away, particularly as the rock looks like Killas, which is much softer than the Granite...

EDIT: Just looked it up on the BGS website and it's not Killas but is the Meadfoot Group - Slate, Siltstone And Sandstone. Sedimentary Bedrock formed approximately 398 to 411 million years ago in the Devonian Period. Still much softer than the granite and unlikely to preserve much evidence of being worked when exposed to the sea like that

The Granite is very near and it's emplacement during the Carboniferous (Variscan/Hercynian Orogeny) is likely to be responsible for all the mineral emplacement and fracturing of the local country rocks during the uplift.

Hum ....

I suspect you are right then ....

The apparent quartz/calcite (whichever) was imbued with black siderite. I was going to fetch some back but I forgot.

Perhaps they were mining that? (I considered it worthless though)



Ian

Ian wrote:
Almost certainly Quartz but very easy to find out. Quartz has a hardness of 7 on the Mohs Scale and is the hardest commonly occurring mineral. You should not be able to easily scratch it with steel (penknife, key, etc)(Mohs Scale of 5.5)

Calcite is only around 3 on the scale so will easily scratch with steel, but not with your fingernail (Mohs scale 2.5)

Given the location, with no limestone anywhere near, and a major Granite intrusion about 1km away it would be very unlikely to be anything but Quartz.
Ian wrote:
It is very unlikely they were mining Siderite, Cornwall is not famous for it's iron mines...
You need large deposits of Iron Ore to make it commercially viable anyway.

Given the location of the portal, and the nearby concentration of shafts it is almost certain it is a drainage adit. Whether they felt that following the Quartz vein might pay off with some ore bodies or perhaps they recognised it from underground in the mine and used it as a guide to make sure the adit went where they wanted to I doubt we will ever know (unless of course it is recorded in some old notes on the mine somewhere.