siltstone
siltstone, hardened sedimentary rock that is composed primarily of angular silt-sized particles (0.0039 to 0.063 mm [0.00015 to 0.0025 inch] in diameter) and is not laminated or easily split into thin layers. Siltstones, which are hard and durable, occur in thin layers rarely thick enough to be classified as formations.
- Related Topics:
- mudrock
- clastic rock
Siltstones are intermediate between sandstones and shales but are not so common as either. They contain less alumina, potash, and water than shales but more silica; in addition to mica, they may contain abundant chlorite and other micaceous clay minerals. Although many shales contain more than 50 percent silt, not all are siltstones; siltstones differ from these shales in that they commonly are chemically cemented and show such features as cross-bedding (i.e., lamination inclined to the main bedding plane), cut-and-fill structures, and flowage within a layer. See also mudstone.