By a pond

I poked my head into a little patch of woods at the southeast corner of a pond and found a large, low, almost triangular rockpile:Here are views of its sides, moving clockwise:
There appears to be some kind of a low ramp spiraling up the sides of the thing. It looks like the pile is triangular although I was not struck by that at the time. Where have I seen something like that recently? Once maybe in Weston, maybe a similar but pentagonal one from Acton.