Clastic rocks
Shale: no grains are visible, or perhaps only a few muscovite crystals are seen on bedding surfaces. Shales are mostly made out of clay, a diverse group of very tiny mica-like minerals. Some shales are very soft and fall apart in water, others can be quite hard. Shale is the most common sedimentary rock type on earth.
Siltstone: grains are barely visible with a hand lens or low power microscope. Grains are too small to identify except as indicated by dark or light color. Quartz and clays are the most common minerals.
Sandstone: grains are visible and many types can be identified with a hand lens or low power microscope. The grains in this sample are moderately well rounded and well sorted.
Arkose: a sandstone having a lot of feldspar. Grains in arkose are usually angular, indicating a short transport distance from the source area. This sample happens to bee poorly sorted, though this is not a characteristic of most arkoses.
Conglomerate: a rock containing >50% clasts larger than 2 mm (the limit for coarse sand), though conglomerates people usually think about usually have pebble size grains or larger. Clasts may be any rock type, and are commonly a mixture in any one outcrop.
Limestones
Coarse-grained limestone: this rock clearly has cleaved calcite crystals in it, and some fossils are identifiable in some parts (not easily in these images). Notice the single crystal crinoid stem fragment in the bottom left of the enlargement. This grain is nearly rectangular, 4 mm wide and 2 mm thick, and has cross-hatched calcite cleavage cracks.
Fine-grained limestone (chalk): in this rock most grains are too small to see with a hand lens. In thin section microfossils are visible. As with all limestone, the principal mineral is calcite.
Examples of sorting and rounding of grains
This sample has grains that are well sorted and most are well-rounded.
This sample has grains that are poorly rounded (angular), and this sample is poorly sorted.
Remember that sorting and rounding are independent characteristics. Sorting depends on the population of grain sizes available to make a rock, and the hydraulic environment at the time of deposition. Rounding depends on the amount of physical abrasion the grains have experienced, which depends on grain hardness, time exposed to abrasion, and the vigor of the abrasion processes. Having one doesn't imply the other.
Some other sedimentary textures
Breccia: large, angular, broken fragments of rock usually transported only a short distance, such as to the bottom of the same cliff from which they formed.
Fossils: this brown shale has fossils of leaves and other plant fragments.
Oolites: these were originally sand-size particles made of calcite, with concentric layers of calcite mud built up around a seed grain. They are common in tropical water carbonate banks. In this case, the calcite has been completely replaced with hematite.
Rock gypsum: this rather featureless rock is made almost entirely of gypsum, colored with a trace of hematite dust. Gypsum beds can be extensive, and represent evaporites left from the evaporation of saline water.
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Schenectady, NY 12308 U.S.A. |