The gravel used consists of varying amount of crushed stone, sand, and fines. Fines are silt or clay particles smaller than , which can act as a binder. Crushed stone, also called road metal, is used because gravel with fractured faces will stay in place better than rounded river pebbles. A good gravel for a gravel road will have a higher percentage of fines than gravel used as a subbase for a paved road. This often causes problems if a gravel road is paved without adding sand and gravel sized stone to dilute the percentage of fines.