A "D" shaped space enclosed by stone walls

This is an unusual feature visible from the road in Carlisle. Let me describe what we are looking at. This is a little valley with a water course going off to the left towards a wetland. On the far side of the water course a stone wall follows the side of the little valley. Roughly in the center of the picture there is a semi-circular stretch of stone wall that forms a "D" with the straight stone wall. The semi-circle opens towards the water.
If you go over to explore it (it is on private property but I had permission a couple times, and not a couple of other times), you find that there is no opening in the wall, no entrance. So this is an unusual feature. The comparison may be absurd but this "D" always reminded me of a miniature of the Poverty Point earthworks.