- A grade scale for classifying the diameters of sediments. Particles larger than 64 mm in diameter are classified as cobbles. Smaller particles are pebbles, granules, sand and silt. Those smaller than 0.0039 mm are clay. Several other grain size scales are in use, but the Udden-Wentworth scale (commonly called the Wentworth scale) is the one that is most frequently used in geology.