Plumbob creating designs in sand?

[beaverflutes]

I was at a large craft convention in Asheville, NC a few days ago, and I saw somebody that had attached a plumb bob to a string, and had a hook-like thing on top of it sitting on a piece of wood, which allowed the string to just hang there and spin around the hook center. The plumb bob's tip was in a plate of sand, and throughout the day it would slowly made designs in the sand.

How does this happen?

 

[mgb_phys]

The pendulum swings back and forth, the Earth (and sand) turns under it.
http://en.wikipedia.org/wiki/Foucault_pendulum

 

[Andy Resnick]

The OP could also be referring to Lissajous figures:

http://en.wikipedia.org/wiki/Lissajous_curve