The drill holes and the timber plugs in this underground drainage adit are clearly original mid-19th century. The nails still in some of them? Probably original, but I'm not sure.

I'm assuming, as these plug are consistently in the centre of the adit roof, that they were for keeping the adit on the correct heading.

If you know better, or can confirm my thoughts, please let me know, especially if you know what you're talking about!

Context: north Wales slate area.