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For thousands, even millions of years, little pieces of our earth have been eroded--broken down and worn away by wind and water. These little bits of our earth are washed downstream where they settle to the bottom of the rivers, lakes, and oceans. Layer after layer of eroded earth is deposited on top of each. These layers are pressed down more and more through time, until the bottom layers slowly turn into rock.
The forces of weather break rocks into small pieces that are carried away and deposited elsewhere. These small pieces are often deposited in shallow seas or lakes as sediments.
Sediment –Material carried by water, wind, or glaciers.
Sedimentary rocks are grouped by the way which they form.
There are three ways (Clastic, Organic, and Chemical).
Clastic - One way is when sediment is carried by water, dropped and piled and piled on top of each other. The sediment piles, layer by layer. Slowly the layers are pressed together (by cementation and compaction).
Chemical - A second way sedimentary rocks are formed is when dissolved minerals sink to the bottom of a body of water and form layers. Sedimentary rock is formed when dissolved minerals sink to the bottom of a body of water and build up into rock.
Organic - A third way sedimentary rocks form is when the remains of plants, animals and other life-forms harden into rock.
nCoal – forms on land from plants buried millions of years ago.
Scientists can learn about the history of the earth through studying sedimentary rocks. They sometimes contain fossil remains of animals or imprints of plants and animals that lived on the earth many years ago.
Here are some common sedimentary rocks.
Shale - formed from mud and clay
Conglomerate - formed from rock fragments cemented together
Sandstone - formed from sand grains cemented together
Limestone - formed from tiny sea animals
Coquina - formed from shells
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