35. Pah. You are too good for me. Took you less than an hour.
An oddity nevertheless. Just off the A5025 in North Anglesey. Stated by AJ Richards in the Gazeteer to be: Pit working owned by the Llangristos proprietors, unsuccesfully developed by the 1874 Llanfflewin Slate & Slab Quarry Co. Remains: Deep sloping cutting to access pit, fine quarry-waste formation intended to carry a tramroad across Llyn Llygeirian.
Looking at the Geological Map, it's in undifferentiated Ordovician, so could correspond to something on the Mainland, maybe Cwm Eigiau Formation, or could be anything at all!