There edges of the sediments act similar

There are three types of rocks and one of them is sedimentary. In this lesson, we will learn about the compaction and cementation processes creating sedimentary rocks and see some examples of sedimentary rock types.

Forming Sedimentary Rocks

A sedimentary rock is one formed from the particles of any type of rock that has been eroded. Rocks erode due to water action, wind and other processes whereby the grains are carried away by rivers or streams. Eventually, the sediments are deposited into lakes or oceans. We will learn the two steps that form sedimentary rocks, and then learn about some specific types of sedimentary rocks. Let’s first discuss how sedimentary rocks are formed.


After sediments have been deposited in lakes or oceans, they get buried under other sediments. The increasing weight starts to compress the sediments and the edges of the sediments act similar to sloppily-made puzzle pieces and almost interlock.

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Since the grain particles are solids, there is a limit to how much they can be compressed. This leaves tiny amounts of space between the grains where water can enter and fill the gaps.

This leads to the next step which is the cementation of the grains.


Any water source in nature is not pure water; it is a homogeneous mixture which is combination of water and dissolved minerals evenly mixed throughout. This is called hard water. Certain areas have harder water than others.

You might be able to tell if where you live has hard water by looking at your water faucets. If you see a white-yellow powder or crust forming around your faucets, you probably have hard water. The dissolved minerals commonly found in water are calcium carbonate (CaCO3 ), silica which is silicon dioxide (SiO2 ), or hematite (Fe2 O3 ). When the dissolved-mineral-water solution becomes supersaturated (too much dissolved mineral per amount of water), the minerals come out of the solution and crystallize. The forming of these crystals can be thought of a special type of glue cementing the grains together.

Types of Sedimentary Rocks

There are a wide range of sedimentary rocks formed from the cementation process. When particles are deposited in lakes or other bodies of water, the particles get roughly sorted according to their size with the largest grains towards the bottom. The size of the grains is a major factor in determining the type of the sedimentary rock. Let’s look at a few types of these rocks starting with the rocks with the smallest grains.


Shale is formed from the smallest particles called clay with a particle size around 1/256 mm. It is a laminated rock meaning it has very compact, thin layers. Shales are often a rock layer that traps oil and natural gas.



Siltstones are sedimentary rocks of varying colors and are formed from particles of silt which are from clay up to 1/16 mm in size.



Sandstone is a particle with a size between 1/16 mm and 2 mm. Since there is such a wide range of sizes, there is a qualitative system categorizing them from very fine to very coarse. Sandstones make up many of the dramatic rock formations worldwide including some in the Grand Canyon.


The term conglomerate kind of sounds like what it is – a glob of poorly sorted particles. Technically, it is a sedimentary rock formed from gravel-sized grains of 2 mm or greater.


Lesson Summary

Sedimentary rocks are formed from different sized grains stemming from the erosion of all rock types.

The process starts with erosion followed by the transport of grains by streams or rivers and deposition into lakes or oceans. The grains are roughly separated by size when they are deposited, and they get buried by more sediment settling on top of them. The next two processes actually make the rock.1.

Compaction occurs when the overlying sediments’ weight compacts the grains as tightly as possible.2. Cementation is the process whereby dissolved minerals in the water between the grains crystallize cementing the grains together.Sedimentary rocks are largely classified by the size of the grains in the rock.From smallest to largest grain size:

  • Clay forms laminated shales.
  • Silt forms siltstones.
  • Sand contains a wide range of sizes forming sandstones.
  • Gravel-sized particles make up conglomerates.

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