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Types of Sedimentary Rocks

Clastic sedimentary rocks are sedimentary rocks formed out of broken down bits of rocks. There are several kinds of clastic sedimentary rocks; they are categorized by the size and sometimes the shape of the pieces that make up the rock. The kinds of clastic sedimentary rocks are called: Conglomerates, breccias, sandstone, mudstone, siltstone.

Conglomerates are made from bits of rounded gravel that were deposited by the agent, usually water.Over time the wet pile of sediments becomes so heavy that the gravel gets compacted together, and minerals in the water cement the bits of gravel together.The only difference between conglomerates and breccias is: Conglomerates are made from rounded gravel and breccias are make from angular gravel.

An illustration of conglomurate rock

Conglomurate rock

An illustration of breccia rock

Breccia rock

 

Sandstone is basically the same. Piles of sand are piled up by water or wind and over time the piles get so heavy the sand is pressed together and cemented by minerals found in ground water.There are some interesting things that can be 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 is piled up in ripples by the motion of the waves?Sometimes the sand will get buried and the ripple pattern is preserved.Over time the sand will harden into rock and the ripple marks will still be in the rock. I have found this “ripple marked” sandstone. Somewhere in my house I still have some.

Sometimes sand dunes will become so large they harden into rock preserving the hump shape of the sand dune. There is a very famous fossil that was found in China in one of these preserved sand dunes. The fossil is called the “fighting dinosaurs.”Paleontologists think a velociraptor and protoceratops were fighting and were trapped by a collapsing sand dune.

The Fighting Dinosaurs.  Photo: David Clark

 

Mudstone is the same as sandstone except the particles of rock that make up the mudstone are too small to be called sand.As the name sounds, the rock used to be mud that was buried and hardened into rock. Siltstone is made from even smaller particles than mudstone. Fossils can also be found in mudstone and siltstone. These types of rocks are sometimes called slate. In the United States, the national monument named Fossil Butte is near mudstone and siltstone formations that hold hundreds offish fossils.

Like “ripple marked” sandstone, mudstone can have marks made by water on it.Sometimes when mud hardens the surface cracks; these cracks can be preserved and harden into rock.Even more rare, the marks made by falling raindrops will be preserved in the rock. I have found the mud crack-marked rocks, but not the rain-marked rocks.I’ll have to keep looking.

This arch is made from sandstone

Sandstone

 

Biochemical sedimentary rocks are formed from the debris of life. For example, limestone is formed from out of decayed animal shells. Animals use calcium to form their shells.After the animal dies, the shell falls apart and the calcium combines with other elements and minerals and hardens into rock. Fossils are very common in this type of rock.I have found fossil trilobites in this rock.Another example of this type of rock is coquina. Coquina is formed from pieces of seashells cemented together.My geology professor had some, it was really neat to look at. Coal and chert are also common biochemical sedimentary rocks.

Chemical sedimentary rocks are formed when water evaporates and leaves behind minerals that harden into rock.A great example of this kind of rock is salt.So remember when you’re eating salt, you’re eating a rock.It’s a rock called halite.

Work at a salt mine

Work at a salt mine.

 

Salt mineral

Salt mineral.

 

The other category of sedimentary rock is very rare,usually caused by an impact like an asteroid.The asteroid hits so hard it breaks up rocks and fuses them together into a new sedimentary rock.Another example is hot rock spewed from a volcano,but not hot enough to melt the rock it touches; it hardens around other bits of rock forming a new sedimentary rock.Like I said, this is very rare.

Sedimentary rocks only form about 8 percent of the rocks on Earth that cover the other types of rocks like a thin coat of paint. Even though they are only a tiny percentage of the rocks on Earth, they are very important.They tell us a lot about the history of life on earth because sedimentary rocks are the only type of rock that can hold fossils and they are formed in layers with the oldest rocks on the bottom and the newest rocks on the top.

Written for the KidsKnowIt Network by: Brandon Guymon