Actually, that blocked level downhill might be a tad too low to be related to the coffin level in SoL - that terminates below another shaft hillock downslope from the entrance shaft. The total depth of these workings is no more than 25m from surface. Though of course if there was a lower level (or more), which I'm convinced there was, it would still work. The accounts refer to 'raising the upper gate to Oakden Level' - which implies that both an upper and lower gate were
lower than the canal. There's enough height difference between that and the 'water table' (resurgence altitudes is all we have) for it to work. I'm not even sure if there
is a water table here, in a practical sense - the dip of the reef is N/NE here, and about 45°, so water would just as likely run down it into the valley, possibly even under the shale margin, or at least around its perimeter - i.e. the fields next to the road. Unless there's a vertical shaft/aven nearby of course, which would take it all down to the basement immediately.