A curious structure, in Howard County, Maryland

I found this small stone structure on the south side of the National Road, east of Lisbon, Maryland. At first I thought it might just be an ordinary springhouse, but it seems that there must be more to it than that.

Though the four stone walls are essentially square, the roof extends to one side, covering a doorway with nothing behind it, and with no evidence of a wall having every been there. Curious.

I welcome your thoughts on this matter.