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What are Sedimentary Rocks?

sedimentary rocks in Death Valley with a clear blue sky as the backdrop

Sedimentary Rocks Form in Layers

Although sedimentary rocks can form on dry land most form beneath water.  They form from larger rocks which over time have broken down into very small pieces.  The resulting sediment which is a mixture of sand, mud, gravel and clay is then carried away by the elements. 

Where the sediment settles on water it sinks to the bottom.  As the pile increases in size it gradually becomes heavier which squeezes out the water.  This stage of the process is known as compaction.

In the final stage of the process minerals in the water surrounding the mass precipitate and begin cementing the layers into rock.  Precipitation is a process whereby dissolved minerals crystallise as they come out of water.  An example could be a salt water solution.  Once the water evaporates just the crystallised salt is left.

The breaking down of rocks and minerals on Earth's surface is known as weathering.  Over thousands to millions of years huge mountains can be reduced to sediment.

Erosion is the process of carrying the sediment away.  Wind, rain, ice, acidity, salts and changes in temperature all contribute to weathering and erosion.

The sediment often includes living organisms which become trapped in the layers.  For this reason sedimentary rocks often contain fossils.  The layers which are correctly known as beds or strata tend to be very distinctive.  The layers at the bottom will always be the oldest.

Some of the most common types of sedimentary rock include sandstone which is made up of minute grains of sand, mudstone which is made up of muds and clays and limestone which consists mainly of fragments of shell and skeleton from dead marine organisms.  Chalk is a white type of limestone.

The oldest sedimentary rocks in Greenland which are 3.8 billion years old are known to have formed from sediment which precipitated from sea water.

Article Photo

Photo is the Last Chance Range in Death Valley. The image is clickable and redirects to the original photo. The page includes information about these sedimentary rocks. Courtesy of Ron wolf.

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