World Builders™                                                                             Session Two  --  Geology 

How Are Sedimentary Rocks Formed?

Erosion breaks rocks down, transports the fragments, and sorts them by size in rivers and lakes.  Over many millions of years, the fragments are semented together and become sedimentary rock.

badlands with layers

       Sedimentary rocks are formed from pebbles, sand, silt, or clay. They are made from the end products of erosion, which breaks down rock into tiny pieces.

        Streams carry the tiny eroded fragments of stone to lakes and oceans, or spread them out over the flood plains of of rivers. Layers of these materials are built up over millions of years.  More and more sediment is added. Dissolved minerals in the water are gradually precipitated, filling in the spaces between the rock fragments.  The layers of sediment turn into rock strata.

       Sedimentary materials are are also deposited by wind or glaciers.

       The strata of sedimentary rocks are often colored by minerals. A reddish color, as in the picture above, is caused by iron oxide, also called rust. Notice the more distant hills: some of their strata are colored by other minerals, such as manganese.

       The formation of sedimentary rocks is a cool process compared with the formation of igneous rocks. Igneous rocks begin with molten material. Sometimes sedimentary rocks are heated up when they are deep in the earth, and this can help to fuse the particles of sediment together. Such rocks retain their layered structure.

This is how sedimentary strata are formed:

lake bed


A stream is carrying silt and clay into a large lake or sea.


Millions of years have gone by and there are many layers of deposited sediment now. Some of the sediment granules have been cemented together by minerals that were dissolved in the water.  They have turned into layers of rock.

This part of the earth has been sinking, and the sea has become much larger.

sediment buildup


More sediment has been deposited. The layers on the bottom have a lot of weight on them. They will heat up from the pressure.

bending strata

Pressure from underneath the sediments is causing them to bend. The rock is so hot that it is somewhat plastic, and so will bend slowly if enough force is acting on it.

fault blockA big crack has occurred in this huge block of sediment. One part of the block slid downwards past the other part.

This crack is called a fault.

The sliding produced an earthquake.
You can see the layers are no longer lined up.



tilted layers

       Additional earth movements stretched and bent the rock layers even more. One end of the layers is above ground, and it shows the stripes of  the very tilted layers.

       One can see examples of bent and tilted layers in many places in the world. Sometimes the strata run straight up and down. Scientists learn about the age of the earth from studying these strata.

      Fossils are found in sedimentary rocks.  Scientists find skeletons, leaves, footprints, and shapes of animals and plants that lived long ago.  Think about it!  Igneous rocks would burn up these organic remains, but they can be fossilized if they are buried in soft mud.

     Like pages in a book, the stratified layers of rock hold information about the past of our planet and its life forms.

Geology Information Menu

Design Your Geology

Top of Page 

                  Photograph from a Corel CD-ROM : for viewing only, not for downloading.   More Information.
Copyright © 1999, 2003.   Elizabeth Anne Viau and her licensors.  All rights reserved. This material may be used by individuals for instructional purposes but not sold. Please inform the author if you use it at