Swamp Mat

Swamp Mat is a grassy plant that grows out from solid ground, over water, mud, and quicksand, forming a raft about six feet wide and up to hundreds of yards long. It traps mud in its leaves and draws much of the water from it, so that the top of the Swamp Mat is hard to distinguish visually from the trails it links to. Those stepping on it can easily tell the difference, however; Swamp Mat cannot support anything heavier than a small rodent.

Source

 * 1: Warhammer Fantasy RPG 2nd ED: Knights of the Grail
 * 1a: pg. 81 - 83