Sedimentary rocks are composites of pre-existing rocks that have been weathered and eroded, or pieces of once-living organisms. Most fossils are found in sedimentary rocks of which there are three main types: sandstone, limestone, and shale. Most of these rocks started as sediments carried in fluid, that were deposited as the current slowed. When the sediments are buried and dried, they become cemented to form rock. Most sedimentary rocks become cemented together by minerals and chemicals or are held together by electrical attraction; some, however, remain loose and unconsolidated.
Sedimentary rocks often have distinctive layering or bedding like those visible at the Grand Canyon. The layers of sediments are normally parallel or nearly parallel to the Earth's surface when they are formed. If they are found at high angles to the surface or are twisted or broken, some kind of movement must have occurred since the rock was formed. Many of the picturesque views of the desert southwest show mesas and arches made of layered sedimentary rock, such are those in Arches National Park pictured at right.
Although sedimentary rocks can form today, it is believed by creation scientists that the majority of the Earth's strata was formed catastrophically during the Biblical flood of Noah. On the otherhand, uniformitarian geologists hold to the view that most sedimentary rocks were formed slowly and gradually at current rates of erosion and deposition.
Clastic sedimentary rocks are the group of rocks most people think of when they think of sedimentary rocks. Clastic sedimentary rocks are made up of pieces (clasts) of pre-existing rocks. Pieces of rock are loosened by weathering, then transported to some basin or depression where sediment is trapped. If the sediment is buried deeply, it becomes compacted and cemented, forming sedimentary rock.
Clastic sedimentary rocks may have particles ranging in size from microscopic clay to huge boulders. Their names are based on their clast or grain size. The smallest grains are called clay, then silt, and sand. Grains larger than 2 millimeters are called pebbles. Shale is a rock made mostly of clay, siltstone is made up of silt-sized grains, sandstone is made of sand-sized clasts, and conglomerate is made of pebbles surrounded by a matrix of sand or mud.
Types of Clastic Sedimentary Rocks:
Biologic sedimentary rocks form when large numbers of living things die, pile up, and are compressed and cemented to form rock. Accumulated carbon-rich plant material may form coal. Deposits made mostly of animal shells may form limestone, coquina, or chert.
Types of Biological Sedimentary Rocks:
Chemical sedimentary rocks are formed by chemical precipitation. The stalactites and stalagmites you see in caves form this way, so does the rock salt that table salt comes from. This process begins when water traveling through rock dissolves some of the minerals, carrying them away from their source. Eventually these minerals can be redeposited, or precipitated, when the water evaporates away or when the water becomes over- saturated with minerals.
Types of Chemical Sedimentary Rocks:
- Sedimentary Rocks by the U.S. Geological Survey, last updated on 01/13/04.