Pignut and Bitternut are difference spp of hickory. I am not sure it is either(at least to me) I cut and split a lot of bitternut this fall and the bark doesn't quite look right to me. N. Red Oak has a distinctive bark that looks like ski tracks going down it. Not sure if it is that either. I split that also this fall and the heartwood doesn't look dark enough to me plus I saw some stringy parts to that split piece you had. Red oak seems to split fairly easy for me. My guess would be a hickory but I am not sure what spp. I don't think shagbark, shellbark, or mockernut either. I don't see butternut much, could it be that?