Snow sand is one of the three terrors of fire swamps.
It is a drier and quicker form of quicksand. Buttercup unwittingly steps in some and is immediately immersed in it. Westley hacks a vine off of a nearby tree and dives in after her so he can pull them both out.
In the book it is called snow sand. The "lightning sand" moniker is said in the book to be a misidentification, as lightning sand is supposedly quick and moist, with snow sand being quick and dry.