From what I can see in the pics I believe they are all natural rocks not altered by man. I hunt on the sassafras River in Maryland and there's rocks everywhere that look just like them. It's hard to believe the shapes some of them are but at the end of the day they are just natural rocks.
i have some of those triangles too...part of the reason they are identified as natural is that you found more than one on the same day, coupled with the fact that they serve no purpose.