This is an example of an "intermediate stone" that marks the border between Maryland and Pennsylvania. It displays an M on the Maryland side of the stone and a P on the Pennsylvania side. It also bears a "triangulation station" disc used in the US Geodetic survey.
This marker is located several feet off to the side of a rural unpaved road, and is not protected.
Wikipedia article:
http://en.wikipedia.org/wiki/Mason_dixon_lineCategory:
penn mason line dixon calvert shield coat ----