Rocks that Siriol has found on and around the Cwmorthin tips. Originally published in the Aditnow forums

I have uploaded some photos of rocks other than slate, that I have found on and around the Cwmorthin tips in recent years. Any help with identification would be most welcome.

First is a rather nice large slab which has been photographed dry (even better when wet)

Inclusions caught within a volcanic ash flow is a possibility. Below are a couple of close ups of this material.

Possibly a fine ashfall tuff about an inch across (sorry forgot to put reference for scale).
Image on the right shows a well defined inclusion

Outer cortex of the above with small flake removed..

Next is a fragment of what was a rather nondescript looking lump of rock which I think was from the tips. It had a thin yellowish/fawn coloured skin on it, and looked like concrete but I later took a hammer to it as it was annoying me (poor rock!) and it broke into sharp fragments. The fresh material inside has a nice sparkle which is not so obvious here - I’m not certain what this rock is.

The following yellowish fragment was found on Cwmorthin back vein incline around floor B. Unsure what it is but in the 2nd photo you can see the junction of this material with something more like a mudstone.

I rather like the following material - but don’t really know what it is. I have named it “tapestry band granite”. Found on track up to Cwmorthin, but since it only seems to come in agregate size lumps it may have been imported from elsewhere. Has lovely green crystals most noticable when wet.

The same material as above and found on the track as before. A dolerite or a Gabbro?

Lovely volcanic tuff with layers of mudstone found in the Cwmorthin opencast – have mentioned these before.

Below is an attractive material in layers with specks and blebs in it. There is a lot of this below Cwmorthin, lower down the track, and I think it’s peeled off the large oucrops of rock next to the track - love to put a name to this, but some sources describe the outcrops in this vicinity as rhyolitic tuff. This is a reasonable size. First photo shows speckles though a layer

Next photos show the layer cake effect. All layers contain speckles Sample is about 1 foot approx.

Next is a fine grained light coloured very compact rock from the tip. Don’t know if it’s some kind of rhyolitic material. (size is about 1 foot). Right is a closeup showing texture.

Lastly a rock found on the track up to Rhosydd. Lots of variations of this material up there, but curiously not that common in vicinity of Cwmorthin.

Footnote

The very nice layer cake material with spots and blebs I have since found out is spotted hornfels which may have started out as mud stone. It has been altered by the nearby Tan-y-Grisiau microgranite intrusion - if that had extended its influence further there may likely have been no economically valuable slate to be had.