Sedimentary Rocks

Sedimentary rocks fall into two different categories, chemical and clastic, basing them on their origins. Chemical sedimentary rocks are formed when the dissolved materials precipitate, the term used by chemists from falling out of a solution as a solid material. The most common chemical sedimentary rocks are the dolostone, the limestone and the carbonate rocks, they are composed of of fine-grained calcium and magnesium carbonates often derived from the shells of sea creatures. Chert is chemical sediment which is made up mostly of quartz. Iron formations are composed of silicates, carbonates, sulphides and iron oxides.

Chemical sediments can also be formed where ocean water becomes isolated from the ocean. These rocks which have the name of evaporites can contain gypsum (a calcium sulphate), halite (rock salt), and sylvite (a common potash salt).

Clastic sedimentary rocks are consolidated fragments of eroded rock, brought together by the action of wind, water and ice. In a conglomerate, large rounded rock fragments are held together by a groundmass of finer fragments. Sandstone, as the name implies perfectly, is made up of sand grains, even though this can also contain finer material. When most of the material is fine grained, the rock is called shale or siltstone.

One of the most common characteristics of most sedimentary rocks is the bedding. In these types of rocks, it is usually possible to notice the individual of material as they were laid down over time.

Beds start as flat layers, but they can be overturned or folded by later rectonic forces. Coarser particles typically form a distinct band, whereas finer particles form progressively higher beds or bands. Nonetheless, the individual particles in sedimentary rocks have no way of showing the same interlocking features of igneous rocks. The individual grains in sedimentary rocks are actually cemented together.