World Builders™                                                                           Session Two  --  Geology 

 

Sedimentary Rocks

Sedimentary rocks are made of particles of minerals that have been collected by water or wind.

cliff with sedimentary rock layers      Look at these rocks.  They have stripes!  These rocks were once sand and mud at the bottom of a shallow lake or sea.  Rivers brought the little bits of rock down from the mountains and spread them in neat layers under the water.  

  The rocks that make up this cliff were once sand, silt, or clay.  Water captured them by erosion and carried the tiny particles into a lake or shallow sea.

Over millions of years these sediments were cemented together by dissolved minerals or fused into rock by the pressure of many more layers of sediment above them.  

Layers     

Layers are a sign that you are looking at sedimentary deposits.

sedimentary layers       Look at this little hill. You can see that there are layers of different colors. At one time this whole area was under water. Streams brought silt and clay into the still water where it settled in layers as the years went by.

      Layers are laid down as the seasons change. Wet seasons carry more sediment than dry ones, so the result is a layered effect, like a pile of papers. Some layers are very thin, some are thick. The colors of the layers are caused by minerals: for example, reddish rocks have been colored by iron.

     Sometimes changes in the colors of the layers are caused by the streams bringing in a different kind of sediment. Geologists can sometimes find traces of ash from large volcanic eruptions in the rocks.

     Sedimentary rocks preserve information from the distant past for us. Sometimes they contain fossils. Fossils are the remains of animals and plants that lived on earth many millions of years ago.

     The layers in the rocks tell us about long-ago changes in the landscape. Let's look at how these rocks form and at what happens to them.

   How Are Sedimentary Rocks Formed?


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© 1996,1997, 1998, 1999, 2000, 2002, 2003.   Elizabeth Anne Viau. 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 eviau@earthlink.net .