Of the three classes of rock, sedimentary rock is the least plentiful, although the majority of surface rocks are sedimentary. Sedimentary rock is formed near the Earth’s surface by the accumulation and cementation of fragments of earlier rocks, minerals and organisms. They are the product of the erosion of existing rocks. Eroded material accumulates and then erosion and weathering transport materials that are deposited as sediment. Pieces of rock or organic particles settle and accumulate either in the sea or on land. This particulate matter undergoes compaction and cementation in a process known as diagenesis. Sedimentary rocks are deposited in horizontal layers and are sometimes referred to as stratified rocks. Often sedimentary rocks contain fossils.
There are three major categories of sedimentary rocks: Detrital, Chemical, and Biological.
Detrital rocks or clastic rocks consist of fragments of pre-existing rocks that have been weathered away, transported and deposited by clast or particle size. Detrital sedimentary rocks are those for which the material has been transported as solid materials. Mineral grains or rock fragments formed by weathering are called clastic or detrital material. The fragments of pre-existing rocks or minerals that make up a sedimentary rock are called clasts. Clasts indicate that particles have been broken and transported. Clastic sedimentary rocks are primarily classified on the size of their clasts from boulder and very coarse gravel down to fine sand, mud and clay.
Chemical rocks form as water evaporates from a lake or ocean. Minerals form due to precipitation and grow attached to previously formed crystals. Rock is composed of intergrown mineral crystals. These sedimentary rocks occur when minerals or mineraloids (without fixed composition) are precipitated directly from water, or are concentrated by organic matter. Components have not been transported prior to deposition and therefore no clasts are present. An example of a non-clastic texture would be crystalline material.
Biological rocks are formed by the compaction of organic materials. Coal has high carbon content and amber is formed from an ancient, hardened tree sap.
The twelve most common sedimentary rocks are listed below and are organized by the three major categories.
Clastic (Fragmental) Sedimentary Rocks
- Sandstones (includes graywackes & arkoses & Flagstone)
- Siltstone (includes mudstone & argillite)
Chemical Sedimentary Rocks
Biological Sedimentary Rocks
Sedimentary Rock Building Materials. For the purpose of masonry, only half of the sedimentary rocks in the above list are commonly used as building materials.
Conglomerates and breccia are clastic rocks with large particle sizes. The texture gives them decorative interest. Sandstone is a clastic rock with medium sand particles sizes. Sandstone is quarried all over the world and comes in a tremendous variety of strength and color. It includes Flagstone and also a variation known as Greywacke. Limestone can be clastic or chemical and also comes in a huge variety. Travertine is a variety of limestone. Dolomite is very similar to limestone with a slight chemical difference.