Rock Piles at Egg Rock Concord

I went arrowhead hunting at egg rock because I have heard stories of people finding them there, at the confluence of the rivers, "in the gravel". But I did not expect to find anything and I did not bring my camera along. It is too bad because I saw a stone wall in there on the peninsula between the rivers and immediately hopped across to see if there were any structures beyond it, on the down slope.

First I saw a low mound, later a rock with an outline.
Then I went on with my arrowhead hunt. A wonderful moment of wading around in the Assabet. The water was warm and I soon got over my sense of "icky" from the uncertain footing. Did not find anything.
On the way back, I took another look at the low mound and discovered it has a smaller secondary one next to it, buried and nearly invisible. Also two or three small "U" outlines nearby. Also a thin trace of wall perpendicular to the main one, leading towards a split rock. I tried to add them in gray on the map above.