Caudale Quarry: Stone down a mine

We visited this quarry in Caudale above Brothers Water. We wanted to know how deep the shaft was so we timed a couple of stones down. The equations of uniformly accelerated motion give height h = 1/2*g*time squared. I got nearly 1.8 seconds which is 16 metres. We repeated the measurements. Second time it seemed to bounce down much deeper. I looked it up and was taken to one of my favourite blogs http://becausetheyrethere.com/2009/11/12/hartsop-hell-%E2%80%93-and-a-tribute-to-the-caudale-quarrymen/ You can read about the history of the quarry. He's even been down this hole and quotes it as 30 feet, which is about 10 metres.