What do the circle of stones represent?

Solution 1:

If you dig them out you will find stuff. Likely gold, treasure and remains. You don't need to dig terribly deep either, I usually excavate everything "two steps" which is about to the point where from inside the ring the ground outside is approximately shoulder level.

Solution 2:

If you look closely at these formations, they aren't circles; they're elongated and have points, shaped like a longship, which is what they're meant to symbolize. What you've found is a Viking graveyard, and underneath it you can find skeletal remains and treasure.

This concept is taken directly from the real world, with stone ships like this scattered around Scandinavia and the Baltic coast, with some dating back to the Nordic Bronze Age, and a lot of the older examples found in Gotland, Sweden. Little concrete is known about their purpose as surviving records are lacking, but one hypothesis is they were intended to symbolically equip warriors in the afterlife as they were in life.

https://en.wikipedia.org/wiki/Stone_ship