Source Advanced Player’s Guide
Like caltrops, marbles can be used to slow down opponents. One 2-pound bag of marbles covers an area 5 feet square. A creature entering a square with marbles scattered on it must make a DC 10 Reflex save or fall prone (the creature’s stability bonus to trip applies to this save). Any creature moving at half speed or slower can pick its way through a square of marbles with no trouble.