Seems like this info has slowly made it around the community of trail users looping around Chestnut Ridge.
The area was once mined for iron ore. Once you consider this, the trail on the west side of the parkway spur starts to take on a different look.
As I find info on it I’ll try to post it here or to the Frozen Toe page. But John Carlin did a good write up here – http://johncarlinsvirginia.blogspot.com/2011/11/of-giant-holes-and-iron-mines.html