Old, old rock. With art?

This is my sister's rock, with markings similar to those on my rock. (See post from a few days ago.) They both came, probably, from the Backbone mountain region of western Maryland. My dad took them to staff at the Univ of MD, archeology dept most likely, and was told the markings were probably human-made but the rocks are not datable by any methods they had. Kathy's rock has been buried under a lot of snow. More this weekend no doubt. Just think what these rocks must have seen.

Ferns, moss, and stones.

The stone with the subtle cross-hatching came from my parents. I think, but am not sure, that it originally came from my great-grandmother's property, in northwestern MD. Kathy? My sister, Kathy, has a similar rock in her yard.