Hey Greg, you're right in that there is a shaft beneath the tailings pile, however, that shaft is not filled in. When we had the opportunity to enter the mine, we walked to the end of the adit and came to a vertical stope.
At the top of this stope we noticed a plug made of tailings. So it looks like the shaft is'nt filled, its plugged. Be careful around the tailings pile, less you fall down into the stope. There's also a winze with a windlass still intact in this mine, as well as some very good timbering. You can see these here:
http://www.abandonedmines.net/wallkill.htm
The pathway and distance from the adit was matched up on the surface by Miner Bob, so it appears to me, that the work was started via the adit, then as they hit the mineral vein which was vertical, they mined upwards, eventually reaching the surface. The adit is not driven in a direct line, it sways and curves in different ways. If it was a drainage adit, they would have gone directly out. There are very few shafts in the area.
there's actually another shaft in wallkill you may have missed