The Grand Canyon has nearly 40 identified sedimentary rock layers.
Sedimentary rocks are made up of many different types of rock fragments, minerals or organic matter, such as plants or animals, that have been fused together over time. The three main types of sedimentary rocks are based on the type of materials in the rock: clastic, chemical and organic.
Clastic Sedimentary Rocks
Sandstone, such as this formation in Arizona, is a medium-grained clastic rock.
As rocks weather, they break down into smaller rocks and particles. Through natural forces such as wind or water, rock particles, also known as sediment, move and settle into different locations. Layers of rock particles, or strata, are deposited on top of previous layers. Over time, these layers become compacted and bound together, forming new rocks called sedimentary rocks. This process is called lithification. Examples of clastic sedimentary rocks include sandstone, breccia and quartz gritstone.
Chemical Sedimentary Rocks
High evaporation rates expose rock salt in the Dead Sea.
According to the U. S. Geological Survey, chemical sedimentary rocks are formed from chemical precipitation and evaporation. Stalactites and stalagmites form when water travels through, dissolves and redeposits minerals. Rock salt is exposed when water evaporates from layers of inorganic sediment that were initially deposited underwater. Other examples of chemical sedimentary rocks are oolistic limestone, gypsum and calcite.
Biogenic Sedimentary Rocks
Coal is a fossil fuel made from dead plants and animals.
Biogenic sedimentary rocks, also known as biologic or organic sedimentary rocks, contain organic material such as plant or animal debris. Coal, for example, forms when dead plant material accumulates and is buried and compacted by additional layers of sediment. Other examples of organic sedimentary rocks are chert and chalk. Biogenic rocks often contain visible animal or plant fossils. Coral limestone, another biogenic sedimentary rock, is the fossilized remains of coral.
Clues to the Earth’s History
Each layer represents what was once on the Earth’s surface.
Geologists use sedimentary rocks to understand the history of the Earth and its inhabitants. Fossils found in the layers of deposited sediment act as a timeline for plants and animals. Sedimentary rock characteristics provide clues about how the rock was formed. For example, geologists determined that sandstone, which has small wave marks or grooves, is a result of sediment that was deposited in shallow moving water with waves.