Wingate Sandstones and Navajo Sandstones sculpted by erosion

Uluru (Ayers Rock) in Australia, the world's largest monolith, is made of sandstone.

The process of sedimentary rock formation involves a variety of sediment types. Some sediments are tiny grains of sand. Others include broken pieces of rock rolling along the bottom of a stream, fragmented seashells tossed upon a shoreline, or simply layers of mud. Sediments may also include plant or animal remains.

Based on their origin, sediments are categorized into three main types. Those formed from broken pieces of rock are called DETRITAL SEDIMENTS, while sediments deposited from the remains of plants and animals are called ORGANIC SEDIMENTS. The third sediment type is the CHEMICAL SEDIMENTS, which form when a solid chemical comes out of a solution of water. The most familiar example of this process is the evaporation of seawater, which leaves salt behind.

Lithification The process by which sediments become compacted and cemented together into a sedimentary rock.

How do sediments accumulate to begin with? As gravity, water, or wind carries them to a destination, sediments are deposited, usually in layers called STRATA. For example, many horizontal bands can be seen in the Navajo Sandstone Formation. Those bands are the strata of the rock, and each one represents a new layer of sand that was deposited.

Shale is a clastic sedimentary rock formed from clay minerals.

Sedimentary rocks are classified according to their sediment type. Detrital sediments form rocks classified as CLASTIC, while organic and chemical sediments form chemical, or NON-CLASTIC, rocks. Note that the term chemical is used to describe both a sediment and a sedimentary rock type.

A common clastic sedimentary rock is shale, the most abundant of all sedimentary rocks. Shale forms when mud and clay harden. Because the clay sediments are extremely small, they settle out slowly. In fact, shale formations can take about 5 million years to form.

Limestone cliffs

Sandstone, another clastic sedimentary rock, is composed mainly of cemented grains of sand. Its main mineral constituent is quartz, and it is known to be very hard and abrasive. Another common clastic rock, conglomerate, looks like a mixture of different-sized rounded pebbles cemented together. Conglomerate forms when rapidly moving water drops pebbles into sand at the bottom of a river or stream. Over time, the pebbles and sand become cemented together.

The most common chemical sedimentary rock is limestone. Most limestone rocks are organic — that is, they develop from the remains of organisms. Much limestone is made up of marine animals and contains pieces of shells, corals, and mollusks. Another chemical sedimentary rock is coal, which is often found in layers with other sedimentary rocks. As plant remains are deposited in layers and slowly alter into carbon over millions of years, they form coal.

Coal Tells Its Story Although renowned as a source of energy, coal is not usually thought of as a sedimentary rock. Let's take a closer look at the formation of this highly sought-after commodity.

Ripple marks formed in sand by wind

Sedimentary rocks often preserve some of the features of the depositional process. For example, little waves or ripple marks may develop in the sand on a beach. The rock that forms from that sand may have rippled lines. Another common feature is mud cracks, which develop when the sediment at a bottom of a river or stream goes through cycles of flooding and drying.

Sometimes, sedimentary rocks contain features called CONCRETIONS, hard, rounded objects that become enclosed in the rock. A special type of concretion is a geode, whose hollow interior is lined with crystals.

A Sequoia stem and alder leaf fossilized in sedimentary rock.

Sedimentary rocks are also the source of most fossils. Because older rocks contain simpler fossils, the age of a rock can be estimated by looking at any fossils in it. The color of sedimentary rock is another clue to the chemical composition of the sediments from which it formed. The bands of red and pink rocks in the Grand Canyon, for example, come from iron-bearing minerals such as hematite.

Sedimentary rocks cover almost all of the ocean floor and about three-fourths of Earth's surface land area. Like igneous rocks, sedimentary rocks may undergo changes that transform them into new, metamorphic rocks.

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