Awesome stuff, Jeff. A great mystery indeed. I am eager to get back there when the bugs die off and the foliage clears up. One thing to consider is that the spot along the river where you found the slag could have been sumberged for some time under the lake that once served lower forge. Just to put some perspective on the timeline we are dealing with. How close were the timbers to the slag? How closer were the pits? Also, did you perchance grab a piece of slag as a souvenir?
I have not seen a map depicting the forge pond for lower forge, but I would be surprised if it extented that far upstream. The timber in the water was about 15-20 feet upstream from the slag. The pits were on the western shore directly across from the slag, and then one slightly upstream, and one slightly down stream. But in close proximity. I didn't collect a souvenir.
I want to follow the cord road to the batsto to see if there is any evidence of a landing there.
Jeff