The stuff we're trying to figure out is the very metallic looking, very porous looking chunks of rust colored stuff that breaks apart very easily. From what I'm reading, both here and in books, it might be slag with a high bog iron content. Also from Iron In The Pines... "That so much ore was left in the slag testifies to the low efficiency of those early furnaces."
I just find it odd so much is left around if there was still content left in it to be recovered. I'm going to try and get out this weekend to shoot a photo of just what I'm talking about. There's a lot of it out at the RR at Carranza.
I believe what you are seeing is sandstone.
Guy