I think I found an unmarked grave

I was deer hunting on my family's woodland property this morning and stumbled upon this in a swamp. I thought it might be part of an old wall, but there was only trees around. It's too purposeful to be random and the right size for an adult. The land has been in our family for 3 generations now, so I have to ask my father. In the meantime tell me what you think. Is it a grave or something else, because I can't stop thinking about it.