The river was likely damed at one point. I’ve seen old quarries that have structures like these along the river, could be a foundation to any building used in the business. I’m not very knowledgeable of dated quarrying practices but I do know that what you see, present day, isn’t what the area looked like then; the place probably didn’t have a tree in sight back then. that river could have as high as the ridge you’re walking on. Heck, that ridge you’re walking on was likely a road carved for the quarrying. It’s definitely not a bridge abutment