Investigating Rocks and Fossils

Types of sedimentary rock

How is sedimentary rock made?
Sedimentary rock is formed over millions of years. It is made out of small pieces of different rocks known as sediments and happens through the following steps.
  1. Sedimentation: These pieces are carried away by rivers to a lake or the sea, where they sink down to the bottom. In this way, more and more pieces of rock build up into layers called sediments.

  2. Compaction: The top sediments crush the bottom layers, compacting them together. This slowly squeezes out the water caught within the layers.

  3. Cementation: At the same time salt crystals form, which glue the rock fragments together, cementing them.

Which rocks are 'sedimentary'?

The images on this page all show different types of sedimentary rock:
  • Clay: Layers of clay often form part of sedimentary rocks like sandstone and limestone.

  • Sandstone: Made up of a great deal of sand and so it is very easy to cut and shape. This means that it is often used as building material.

  • Limestone: Made up of billions of tiny sea creatures which have been pressed together and compacted below the ocean floor. Limestone is often used in building, lime production, glass making and in cement. Chalk is a type of limestone.

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