How do sediments form layers

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How do sediments form layers?
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How else can layers of sediment be formed?

• You have seen how sediments can form when wind or
  moving water slows down. But there are two other
  ways that sediments can be formed. One is call
  evaporation. The other is called accumulation.

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Evaporation

• Sometimes water gets trapped. This can happen in
  a lake. If the climate is dry, the water will
  evaporate. This leaves behind a layer of all the
  substances that were dissolved in the water. It
  is called evaporite rock.

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Accumulation

• Sediments may also form by the accumulation of
  dead organisms. Look at the picture. It shows
  microscopic sea creatures. Each of them is
  surrounded by a tiny shell. When they die, the
  shells fall to the bottom of the sea. Over
  millions of years the shells pile up on the top
  of one another. In time they turn into rocks
  like chalk and other limestones.

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The rocks are laid down in layers.

• The sediments are laid down in layers. When they
  turn into rock the layers are still there.
  Younger rock is formed on top of older rock.
• Look at the picture. It shows the rock at a
  cliff face. This rock was formed over millions
  of years under the sea. You can still see the
  layers in the rock.

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The layers

• Form a historical record over millions of years.
  Geologists can look at fossils trapped in rock.
  This tells them what conditions were like a
  million years ago!
• Fossils are a cast of where a dead animal or
  plant was squashed or the remains of it left
  after millions of years.

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Copy these sentences and fill in the spaces using
the words below

• bed evaporates shells
• Rocks are formed when trapped water______. Rocks
  like limestone and chalk are formed when the
  _______ of small creatures accumulate on the sea
  ______.

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Key Points

• Rocks can form when a trapped lake evaporates.
• Rocks can form when shells from small sea
  creatures accumulate on the sea bed.