Looking at the pictures of the piles Geophile calls "fishtails", both hers and the ones from Norman Muller, remind me a bit of some piles I saw in Westford. Here are examples. They are not all triangular but have in common a characteristic flat face on one side.They were from a site with a number of other distinctive features, like small pile with a space and then a larger pile:
I'll say more about the site some other time.
Someone needs to hypothesize why some structures - piles and walls - needed clean faces on one side. That seems to be the main attribute of these "fishtail" piles.