Tidal Flats - another intersection between arrowheads and rock piles

I was hunting for arrowheads on the tidal flats of Barnstable MA and saw a couple of stone structures while I was at it. One was a modern arrow pointing to a peace symbol and a heart symbol - all made from piled cobbles below the high tide line. But I thought this, in the foreground, was a familiar pattern from the ancient world:

You see an outline with a larger rock at one end. 

BTW, hunting for small bits of flaked rock against this visual background is exhausting.