Serpent Mounds of Stone

A short stretch of stone wall ending with a large "head" boulder, looks like a serpent, and may have been made to represent one. Since Larry Harrop posted a nice example, Tim MacSweeney located pictures of another, and I found a few examples from my area. So how about a comparison?

Here is a "serpent mound" from Sarah Doublet forest in Littleton, originally made famous by John Hanson Mitchell in the book "Tresspassing". The "head" is to the right. Here is a closeup:

Here is one from Tyngsboro:

Here is one from Michael Hoye (via Tim MacSweeney) (I think this is the Breakneck Brook Wildlife Management area):

And here, again, is Larry's photo: