osprey - I know exactly where you are talking about. I've run across them while hiking with my family a few years ago and notice them when I fish up through there too. In the same area is an old concrete structure on the stream. Unfortunately, I don't know what the foundations used to support. That area was certainly mined but the pit and the forge were upstream from there. An interesting side-note about a historical "artifact" nearby that stream is the huge pile of iron furnace slag that is piled by the stream. It has been noted that the stream has an acidification problem, and while it's purely anecdotal, I've observed a higher concentration of fish in the stretch directly downstream from the slag pile. Since limestone was one of the raw materials used as flux to remove impurities from the iron ore, the resulting slag contributes a basic solution to the stream, and I had a wild conjecture that in that section, it helps neutralize the acid, which in turns supports a better fish population.
Sandfly - are those photos by William T Clarke? One of the neatest exhibits I saw was at the Pennsylvania State Museum, I think in 2011. It was of a number of his photographs of old logging camps and activities. If they ever published a book of his photographs, it would definitely end up in my library.
wildtrout2 - I definitely recognize the spring house/explosives storage house/whatever-it-is. I have a photo of it somewhere, but I believe it may be from my 35mm camera days. On the pipelines, I believe many of them are natural gas. The ANF is littered with them.
Skip16510 - seems the other old dynamite factory that I know of went boom. There's one located near me in Shenk's Ferry. Blowing up might be the ultimate fate for most dynamite factories..
Unfortunately, the photos I do have are but a fraction of the stuff I've found over the years. Like many others, there are things that I've found that I wish I had taken pictures of, and most of the time, it was because I was in too much of a hurry to stop and pull out the camera or phone and snap a shot. I definitely have taken more historical photos in recent years, so maybe I've turned a corner. I still have to go through my 35mm prints; I didn't start taking digital photos until about 2004.