Reader Vin writes:
Here's an interesting rock for ya.
This is setting in a stone wall in Acton, I removed it for this photo and
have replaced it.
The base is about 12" x 12".
The height is about 12"
The center 'column', which is shaped like the profile of an airplane wing,
must have been carved out by running water.
Next to this stone wall there is a trench, and on the other side of the
trench there are a couple 6 foot high ledges or mounds of buried rock, with
a smaller rocks piled on top.
It looks like this rock came from an underground cavern, and it might have
come from those 2 mounds (assuming there's a small cavern in there), or it
might have been carried and placed here.