Sedimentary rocks are made of minerals and debris that have been weathered or eroded from rocks over long stretches of time. This sediment can be laid down by water, by the movement of earth or glaciers, by evaporation or by biological processes. Over 75 percent of the Earth's crust contains sedimentary rock.
Sedimentary rocks begin as sand, mud, clay or even organic material; the type of material determines the specific type of sedimentary rock. The material breaks away through weathering as erosion carries and deposits the material in another location. Over time, these materials pack together and pressure builds. With pressure comes low-level heat, approximately 200 degrees Fahrenheit. The combination of pressure and heat cements the material together to form solid rock. Sand forms sandstone while clay forms shale. Limestone originates from the remains of dead sea life.
There are three basic types of sedimentary rock: clastic, organic and chemical. Clastic sedimentary rocks form from eroded material from the Earth's crust. Sandstone and shale belong to this group of rocks. Organic sedimentary rocks originate from the remains of living things, both plant and animal. Some rocks form from the leftover shells of marine organisms. Others, like coal, come from layers of dead plant matter. The settling of minerals in shallow water results in chemical sedimentary rock. Some limestones and cherts form in this way; rock salt and gypsum are also chemical sedimentary rocks.
After they are laid down, water is compressed from the sediment, and other processes change it into hard but brittle rock. Some types of sedimentary rock form in the ocean and are known as marine deposits, while those that form on earth are called terrestrial deposits. Clastic rocks are grouped by the size of their grain, while biochemical and chemical rocks such as limestone are classified by their chemistry.