[photo by Gary]
This is a "crown stone" placed every 5 miles, meaning that bears the crests of the Calverts and the Penns in lieu of the usual "M" and "P", respectively. The north-facing crest visible here is that of the Penn family of Pennsylvania. For scale, the stone is about knee-high.
Immediately behind the stone (this view is facing south) the pasture in inhabited by...llamas, unknown in these parts in the 1760s:
This is a great site for on-the-ground stone location information. I've found it very useful and specific.