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.

Sediment particles 0.00016–0.0024 in. (0.004–0.06 mm) in diameter, regardless of mineral type. Silt is easily transported by moving currents but settles in still water. An unconsolidated aggregate of silt particles is also called silt, whereas a consolidated aggregate is called siltstone. Silt deposits formed by wind are known as loess. Sediments are seldom composed entirely of silt but rather are a mixture of clay, silt, and sand. Clay-rich silt, upon consolidation, frequently develops parting along bedding surfaces and is then called shale. If parting does not develop, the massive rock is called mudstone.

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