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Types of Sedimentary Rocks

Sedimentary rocks show up when layers of sediment compress on top of one another over years of exposure to heat and pressure. Sediment consists of sand, mud, and pebbles. However, geologists further specify sedimentary rocks into three separate categories. They define these by how the rock formed.

An image of a wall of sedimentary rock.

Layered rock. Sediment compresses into layers of rock layers over time.

Clastic Sedimentary Rocks

Clastic sedimentary rocks are the most common type. They form out of broken down bits of rocks. Rocks can break down because of weathering or erosion, and then they become sediment. Over time, this sediment can compress and become clastic sedimentary rock.

There are several kinds of clastic sedimentary rocks. We categorized them by the size and shape of the small granular pieces that make up the rock. The order of the classification of grains starts from smallest to largest. Clay, silt, and sand are the smallest type of grains. Anything larger than 2 millimeters is a pebble. And the four main large clastic rocks are shale, siltstone, sandstone, and conglomerate.

An image of a potter working with clay.

Clay is the smallest type of granule that can constitute a clastic sedimentary rock.

Types of Clastic Rocks

Shale is a type of clastic rock that mainly consists of clay; it is the most common sedimentary rock. Shale can be different colors depending on what minerals mix with the clay. It mostly forms in lakes and slow-moving water where the clay content is high. 
Siltstone consists of particles that are bigger than those of shale but smaller than conglomerate and sandstone. These are silt-sized particles, thus the name. Silt is between sand and clay size-wise and made of quartz and feldspar. Siltstone is mainly used in architecture due to its ability to withstand harsh conditions without breaking.
Conglomerates contain rounded gravel deposited by any agent, usually water. Over time the wet pile of sediments becomes so heavy that the gravel gets compacted together. Then minerals in the water cement the bits of gravel together.
Sandstone appears when sand gets piled up by water or wind. Over time the piles get so heavy that the sand presses together. Then minerals found in groundwater cement it together.  There are some interesting things found in sandstone. Fossils are often found in sandstone.
Also, have you ever been to the beach or a lake and looked out into the water and noticed that the sand gets piled up in ripples by the motion of the waves? Sometimes the sand gets buried and the ripple pattern preserved. Over time the sand will harden into rock and the ripple marks will still be in the rock.

Chemical Sedimentary Rocks

Chemical sedimentary rock forms when minerals, dissolved in water, begin to saturate the substance and harden over time. This creates several different kinds of crystals.

To help you better understand this concept, think of salt water. The salt will dissolve in the water, but the water will evaporate over time. This leaves behind salt particles. As water continues to evaporate, the amount of salt will increase until there is too much for the water to dissolve. This will lead the salt to form a precipitate, which means it slowly becomes solid.

An image of a pile of salt crystals in a wooden bowl.

Salt is a type of chemical sedimentary rock, and its mineral form is called halite.

Organic Sedimentary Rocks

Organic sedimentary rocks form from biological debris, like leaves, grass, or other plant or animal material. Coral, for example, hardens over time and becomes limestone.

The breaking down of organic materials results in peat, which is sort of soil that has decomposed material. As peat crystallizes and forms into rock after years of heat and pressure, organic sedimentary rock forms.

An image of a wall built with limestone.

Coral hardens under heat and pressure and becomes limestone.