I have no idea what it is, but it has the perfect dimensions for a grave.
Bing Maps shows the site superbly. The ellipse encloses the area of higher land, 'a' is the approximate position of the building, 'b' is the depression. To the west lie the lands of Glendrian Moss, drained early in the 18th century, while the worked lands to the east, to the north of the burn, are those of Glendrian.
A history of Glendrian is here.