silt, predominantly quartz mineral particles that are between sand size and clay size, i.e., between 1/16 and 1/256 mm (1/406 -1/6502 in.) in diameter. Silt, like clay and sand, is a product of the weathering and decomposition of preexisting rock. Hardened silt forms a sedimentary rock called siltstone, which tends to deposit in thin layers sometimes referred to as flagstone because it is hard, durable, and flat, breaking into nearly rectangular slabs.