Betws Garmon iron mine it is aka Ystrad/Silurian/Garreg Fawr. The outcrop pops out and mined in a number of places in the area - Betws Garmon, Llandegai, Ogwen and Aber. Rather than bringing the ore down this side of the hill and onto the Welsh Highland Railway - instead, they used an aerial ropeway to take it up over the shoulder for shipping out from the banks of Llyn Padarn in Llanberis.
As well as the scar going up the hillside, there are a couple of rather damp adits not far from this structure.