Sedimentary rock classification is based their method of formation, on their grain size, and on their composition. The table below gives only those sedimentary rocks commonly used as aggregates.
| Coarse Grained | Fine Grained | |
| Clastic Sedimentary Rocks | conglomerate, sandstone | siltstone, (shale) not often used |
| Chemical Precipitates | limestone, dolomite | limestone, dolomite |
Sedimentary rocks are formed by either precipitation from seawater, or by gradual accumulation of sediment brought in by rivers into shallow seas. These are collectively known as marine sedimentary rocks. The overlying sediment compresses the earlier deposited sediment, and the combination of pressure and dissolved minerals in pore solution solidify the rock.
Another major mode of origin, mainly in the SW US, are sandstones derived from sand dunes. After burial, the grains became cemented by mineral rich solutions into rock.
Sedimentary rocks, especially sandstone, limestone, and dolomite (dolostone) are among the most important aggregate sources. Because their mode of formation, their characteristics and properties are highly varied, as is their durability.
Shale and siltstone are less durable, and used mostly in fills.
A more thorough discussion of sedimentary rocks can be found at this site.
Ardmore Limestone quarry, Indiana
