Pair of elongated rock piles

The largest site I know of near here was found by Bruce McAleer in Harvard, MA. Whenever I explore woods adjactent to this major site I discover new clusters of rock piles. Yesterday I went out to explore one fringe area like this and I kept finding things as I explored until I got back to parts of the site I had already visited. So anyway here is an interesting feature I had not seen before:
Two elongated rock piles, lying side by side. Here is a view from one end:
Look closely and you see a little white spot on the right hand side, a a few feet back from the end of the pile. Here is a detail:
If we look at the far end of the other pile we see something similar:

Again there is a white spot. Here is detail:Very symmetric.