Sedimentary Rock

Sedimentary rocks are formed when sand, silt, and mud (which are all small particles) are deposited on the floor of an ocean or lake. Then, when the water recedes, the sediment is left behind. The sediment is then compacted and cemented together by pressure and time to form sedimentary rock.

Sedimentary rocks on Mars

Sedimentary Rock Facts for Kids:

  • Sedimentary rocks are formed when the remains of living things are compacted and cemented together, over millions of years.
  • The remains of plants and animals fall to the bottom of a body of water and get buried by other sediments.
  • The pressure and heat from the weight of the layers above them cause them to change into sedimentary rock.
  • The sedimentary rock cycle is a process in which sediments are deposited, compacted, transformed into rock, and then eroded to form sediments again.
  • It is a cycle because the same materials (sediments) keep going through the process again and again.
  • Erosion is when pieces of rocks break off and fall down into the water or onto land.
  • Salt that we eat is made from sedimentary rocks. It comes from a chemical sedimentary rock called halite.
  • Most sedimentary rocks come from minerals that are left when water evaporates.
  • Limestone is used to build roads and for other construction.
  • Sandstone is solid and is used in buildings.
  • People use mudstone to make pottery.
  • Flint has been used for millions of years to make tools.
  • The White Cliffs in England are made from millions of animals that have died over the years.  These cliffs are full of shells of dead animals.
  • Animals that die in swamps do not decay but turn to peat.  Peat when it is under a lot of pressure for a long time turns to coal.
  • A rock that is a conglomerate rock is a sedimentary rock that is made by sand that has hardened.
  • When a rock changes from sediment to stone it is called lithify.

Where Are Sedimentary Rocks Made?

Sedimentary rocks are made in places such as:

  • Rivers
  • Lakes
  • Seas
  • Deserts
  • Oceans


Sediment is bits and pieces of debris and dirt that make up sedimentary rocks.  Sediment is formed when bigger rocks get broken down by erosion and other things.  Sometimes rocks break into pieces from wind, rain or even from snow.  When this happens, the rocks become sediment.

Some kinds of sediment include:

  • Pebbles
  • Mud
  • Sand

Different areas that are lower such as oceans and lakes collect the sediment.  When other materials lay on top of them, they get pushed together and they form rocks.



People that study rocks are called geologists.  The study of different rock types is geology.  These people look at different kinds of rocks such as:

  • Fossils
  • Mountains
  • Soil
  • Rocks
  • Crystals
  • Minerals

Geologists try to figure out why mountains were formed or how crystals were formed.  They also study the different minerals and figure out how they were made and how long they have been around.

The stratified remains of the Puʻu Mahana cinder cone

Most Common Rock

The most common rock that we see on the surface of the Earth is sedimentary rock.  This happens because the crust of the Earth has a lot of sediment.  Compared to igneous and metamorphic rocks, the surface of the Earth has more sedimentary rocks.

What Causes Sedimentary Rocks?

When there are mountains, they are usually millions of years old.  Most mountains are tall and are made of different sizes of rocks.  Some of the rocks stick out and are jagged while others are flat and close to the mountainside.

As time goes by, they get older and older and many things happen around the mountain such as wind erosion, water erosion.  Water runs through the mountains, snow falls, the wind blows.

Mountains begin to crumble and break a little bit at a time.  When the small rock that breaks off falls, it falls into a river, ocean, or stream.

These rocks that fall are called sediment.  Eventually, the water slows down and the layers of rock that have fallen in the water fall to the bottom of the river, lakes or streams.  Over time, more and more rocks stack on top of these and they form layers.

The layers of rocks eventually form sedimentary rocks.

How Can You Tell if a Rock is a Sedimentary Rock?

You can tell a sedimentary rock by the layers that it has.  These rocks can only be formed over time as layers pile on top of layers.  Some layers become hard beds of cliffs or strata.  Strata mean layers.

What Can a Sedimentary Rock Tell Scientists?

Scientists study rocks to learn information about an area.  They can tell a lot about an area when there are sedimentary rocks such as:

  • What kind of conditions existed when the rocks were deposited in the area.
  • What kind of organisms or minerals made the sedimentary rock.
  • Things about the past because of fossils.
  • How the sedimentary rocks go where they are.

Types of Sedimentary Rocks

There are six main types of sedimentary rocks including:

  • Sandstone
  • Limestone
  • Shale
  • Gypsum
  • Fossil Fuels
  • Greywacke
  • Chalk
  • Coal
  • Claystone
  • Flint
  • Mudstone

Clastic Rock

When a rock is under a lot of pressure and when the material dissolves and crystalizes, this is called clastic rock.

When this happens, the rock under the top rock becomes more compacted or stuck together and it makes it to get harder.  This is how sand becomes sandstone.

Consolidated Rock

When the rock is under pressure for a long time it becomes consolidated.  This means that it becomes solid.


Sandstone is a sedimentary rock that is made when sand gets stuck together.  This type of rock is usually soft and usually has different layers of different colored sand.


Limestone is a rock that is made of calcium carbonate.  Most limestone has small shells in it.  This type of rock forms when there are layers and heat and pressure are causing it to fuse together.  Marble is a type of limestone but is also a metamorphic rock.

Limestone is where most fossils are found.


Shale is a sedimentary rock that is made of clay that became hard.  Most shale can break apart in sections that are flat and not round.


Gypsum is a sedimentary rock that is made of minerals.  This is usually white and is sometimes used to make plaster and cement.  Gypsum usually has round rocks with pebbles stuck together in it.  It has different patterns.

Breccia is a type of gypsum that has little rocks that are stuck together.

Fossil Fuels

Fossil Fuels are natural gases, oil, and coal that are formed from organisms that have died and decayed.

Coal is a type of fossil fuel that is more than 300 million years ago.  Coal can turn into diamonds if it has enough pressure for a long time.  Coal helps to make electricity.

Fossil Fuels are not good for the environment because they are non-renewable, which means once they are gone, they can never come back and because they release greenhouse gases when they are burned.

Greenhouse gases are not good for the environment and cause the ozone layer to get smaller.

What Did You Learn?

  • What are sedimentary rocks?
    Sedimentary rocks are rocks that are formed when sediment or dirt or rocks fall and pile up.  As they pile on top of each other, they make pressure and this pressure forms them into a solid and this is a rock.
  • What are usually found in sedimentary rocks?
    Most fossils are found in sedimentary rocks.
  • What causes fossil fuels to form?  Fossils fuels form from animals that died and decayed over time.
  • What is the main cause of sedimentary rocks?
    Most sedimentary rocks come from erosion of water or rain.  When mountains lose bits of rocks or sand gets compacted together, this can cause sedimentary rocks to form.
  • What type of rock is most seen on the Earth’s crust?
    Sedimentary rocks are the most known rocks on the crust of the Earth.