How long was the adit at Seathwaite Tarn Mine?

 

In his book Mining in the Lake Counties, W T Shaw explains that you can guess the length of a tunnel from the volume of rock that has been removed. There is an impressive spoil heap at the entrance to the now blocked up adit - Wainwright's chapter on Grey Friar has a picture of what it used to look like. I'm estimating the spoil heap to be 10m x 10m x 5m meaning 500 cubic metres of rock. If the tunnel was 1.5m high and 1m wide, that would mean a tunnel 300m long. I have been unable to find any information about this mine because it is not mentioned in Shaw's book.